Sunday, September 27, 2009

Why "Expose to the Right" is just plain wrong

A recent discussion over on the Leica User Forum got me to finally finish writing this. The discussion in question was largely around how to best use Capture One, but it demonstrated (again!) the almost unthinking acceptance in various parts of the photographic community that "expose to the right" (ETTR for short) is the right way to set exposure on digital cameras. In fact in some corners of the web - and I won't point to them here - ETTR is practically religion.

What I'll be showing in this post is that ETTR is at best wildly over-hyped, and at worst will give you a less satisfactory end result than just exposing normally. I'll be doing that two ways. Firstly, by showing some practical examples of images using ETTR versus images normally exposed, but secondly showing that even in theory, ETTR is flawed under most conditions.

What is ETTR?

What ETTR says is in essence that the best way to set exposure on a digital camera is to place the highlights on the right-hand side of the histogram, hence the "expose to the right" name. This is in contrast to the "classic" exposure techniques which involve either an average exposure, which effectively places the mid-tones of the scene in the middle of the camera's range, or variants of the zone system, which says that you as the photographer need to take a conscious decision about where to expose particular tones in the scene.

ETTR was popularized by (and maybe invented by, depending on who you believe) Michael Reichmann, the publisher of the The Luminous Landscape, in this article. Michael credits the original idea to a comment by Thomas Knoll, the chief architect of Adobe's Camera Raw. What the comment amounted to is that because camera sensors work in a linear space, while we see images in a gamma space, you maximize the signal to noise ratio of the sensor by exposing as much as possible "to the right", where there are more A/D converter codes available. (Read the LL article if you want more detail). As you would expect from Thomas, that's 100% right. And thus was ETTR born.

Note that there are a bunch of different variation on what ETTR really is, and religious wars to go with those variations - some variations focuss on exposure adjustments upwards, some downward, and some in both directions:
  1. "Overexposure ETTR" works by overexposing low contrast scenes. What this means is that noise is reduced, and you get the benefit of the greater number of A/D codes to the right. This is the "classic" version of ETTR that the Reichmann article focuses on.
  2. "Underexposure ETTR" works by underexposing on high contrast scenes, placing the highlights on the right, but driving shadows down to the left. This preserves the highlights, but at cost of generating greater noise in the shadows.
What I'll show here applies to ETTR adjustments in either direction, although I'll focus on the "classic ETTR" as described in the Reichmann article referenced above.

So, given I just said "100% right" to the proposition that ETTR maximizes sensor signal to noise ratio, why do I say ETTR is plain wrong? Simple. What most of the proponents of ETTR forget, or perhaps don't understand, is that maximizing the signal to noise ratio of the sensor is absolutely a good thing, but the sensor is only a small part of the image processing chain that gets you from pressing the shutter button to a print. What I'll show below is that ETTR's benefits are actually minimal except in one very specific situation, but that ETTR is actively dangerous to the rest of the image processing chain pretty much all of the time.

The test conditions

Those of you that took a look at the LL article may have noticed a critical point. No images demonstrating the improved end results from ETTR. Just theory, but without any practice. Which should have set alarm bells ringing right there. So, what we're going to do is to look at some images. First, here are the test conditions:
  1. I've used my Canon G10. The G10 is useful in this situation, because being a 14 Mpix small sensor camera, it delivers fairly clean images at low ISO, but really noisy images at high ISO, so we'll be able to look at images at both ends of the spectrum. If ETTR is going to work, it should work at one or the other end of the G10's spectrum of sensitivity.
  2. To ensure consistency, I've used images of a Gretag Macbeth 24-patch chart, shot on a tripod. I've masked the brightest neutral patches off to give a low contrast test image. Shots were in diffuse daylight.
  3. Exposure values were determined such as to ensure that no channel went into saturation - one of the practical problems with ETTR is that it's very easy to blow the red channel, and end up with color shifts because of that alone. However, that's not what we're investigating today, so histograms were carefully monitored.
  4. I've used Capture One 4.8.3 and Lightroom 2.4 for these tests. All setting were default, other than white balance, which I set to Daylight for all images (as shot was 5000K), and whatever adjustment to the exposure slider was required to compensate for the degree of ETTR push or pull I applied. In all cases, "correct post ETTR exposure" was to be close to the theoretical l value of 50.867 (in L*a*b values) for the CC22 patch - the lightest patch I left uncovered. I made no changes to any other setting (contrast, black point, brightness, curves, etc) - all were left at their defaults for the particular program.
A typical test image looked like this:


Sample test image full size

Testing at low ISO

So, let's get to the images. As a reference point for some low ISO tests, we'll use a ISO 200 image. I've selected 200 so that we can compare to a ISO 100 image later, and we will be using Capture One. Further, we'll be looking just at the intersection of the CC4, CC5, CC10 and CC11 patches (the "foliage", "blue flower", "purple" and "blue green" patches) at 100% scale, as shown in the next image."

ISO 200, 1/15 sec, f/4.5 crop - normal exposure
Capture One V4.8.3, G10 Generic profile

So what happens if we "expose to the right" by one stop - aka overexpose the image by one stop (so +1 stops), then bring it back to the same exposure as the first image by adjusting exposure by -1 in Capture One, so getting us back to 0:

ISO 200, 1/8 sec, f/4.5 crop, +1 stop ETTR exposure
Capture One V4.8.3, G10 Generic profile

So, on an immediate glance - well, not much. Let's overlay the two images to get a better idea of the differences. Here what I've done is to overlay the center part reference image with the "ETTR image" - I've offset them slightly so that its easier to see where the ETTR image starts and stops:


Outer: ISO 200, 1/15 sec, f/4.5 crop - normal exposure
Inner: ISO 200, 1/8 sec, f/4.5 crop, +1 stop ETTR exposure
Capture One V4.8.3, G10 Generic profile

Taking a look at the images overlaid you can see that the inner, ETTR exposure really is just a little better; there is less noise, more or less as ETTR promised. (ETTR fanatics should stop reading now). But that's not the end of this. I choose ISO 200 for a reason, let's take a look at a 100 ISO image, superimposed on our better quality ISO 200 ETTR image:


Outer: ISO 200, 1/8sec, f/4.5 crop, +1 stop ETTR exposure
Inner: ISO 100, 1/8sec, f/4.5 crop - normal exposure
Capture One V4.8.3, G10 Generic profile

As you can see, there's no real difference. So, in this example, all the benefits that you can get by one stop of ETTR can also be obtained by just adjusting the ISO setting down by notch.

This isn't some kind of an aberration that's specific to the G10 or these ISO setting either - there's a good theoretical reason. Take a look at the actual exposure values of each image - they're the same - 1/8sec, f/4.5. Now sensors, either CCD or CMOS essentially work by accumulating electrons in the sensor well; the more electrons, the higher the brightness of that pixel. In this case, because the exposure was the same, the number of electrons was the same. But noise in sensors, counted in terms of number of electrons, is relatively fixed. So for the same exposure, regardless of ISO setting, you will tend to get roughly the same amount of visible noise. The reason why noise increases with ISO setting isn't actually because the amount of noise increases on an absolute scale, it is because as you increase ISO setting, the white point goes down, so the same number of noisy electrons become a larger and larger part of what you see. For example, at ISO 200, you might have 10000 image electrons, and 100 noise electrons, while at ISO 100 you would have 20000 image electrons, but still the same 100 noise electrons. Overexpose the ISO 200 image by 1 stop, and you double the electrons to 20000. So you're right back to the noise level of your ISO 100 image. Only you have to adjust the camera, and adjust back in post, all to get exactly the same result as just changing the ISO setting.

High ISO Test

Ok, the low ISO tests suggest we're wasting our time with ETTR. But let's try that at high ISO, on a really noisy image. First, lets compare a normally exposed ISO 1600 image (outer) to a ETTR exposed ISO 1600 image:


Outer: ISO 1600, 1/125sec, f/4.5 crop - normal exposure
Inner: ISO 1600, 1/60sec, f/4.5 crop, +1 stop ETTR exposure
Capture One V4.8.3, G10 Generic profile

Again, as we'd expect, the ETTR exposure shows less noise. However, again, lets also take a look at the ETTR exposure versus just reducing ISO by a notch and exposing normally:


Outer: ISO 1600, 1/60sec, f/4.5 crop, +1 stop ETTR exposure
Inner: ISO 800, 1/60sec, f/4.5 crop - normal exposure
Capture One V4.8.3, G10 Generic profile

Interesting - the normally exposed inner is actually a bit better than the ETTR equivalent. Why? - because the G10 applies its own internal noise reduction algorithms, based on ISO setting, and Canon's engineers, as you might expect, know a few things about their sensors. So here what we have is that the results as delivered by the camera, exposed normally at a lower ISO, are better than using ETTR. In other words, here ETTR gave a worse image than just exposing normally, and letting the camera do its stuff.

The One Situation where ETTR does work

What the tests above establish is that as a general rule, ETTR is no better than just adjusting the ISO setting down, and in some situations, e.g., where the camera does its own noise reduction, is worse than just exposing normally.

But there is one situation where ETTR can help - when you're already at the lowest ISO setting you camera offers. Take a look at the next image overlay:


Outer: ISO 100, 1/8sec, f/4.5 crop - normal exposure
Inner: ISO 100, 1/4sec, f/4.5 crop, +1 stop ETTR exposure
Capture One V4.8.3, G10 Generic profile

While the G10 has a ISO 100 setting it doesn't have a ISO 50 setting. (It does a ISO 80 setting, but I'm ignoring that as it's too close to 100 to make much of a difference.) So what ETTR is doing here is allowing us to synthesize a lower ISO setting, and hence a better noise performance, than the camera actually has. The disadvantage of course is that the camera's dynamic range is reduced by one stop, but if you have a low contrast image, that might be a price worth paying. But that's not the only disadvantage of ETTR:

The real problem with ETTR - color and tone curve shifts

We've established that outside of one situation - the situation where you're already at the lowest ISO setting you camera has - ETTR offers no practical advantage, and in some situations such as high ISO, may be an active disadvantage as regards noise performance. But now we get to the unpleasant part - color shifts. I already mentioned in the "test conditions" section that when using ETTR it is easy to blow the red channel, and get color shifts as a result. However, what I'll show in this section is that even without blowing a channel, you can still get color and tone curve shifts. Just to emphasize again - none of the images in this article have blown channels.

Firstly, take a look over the previous sets of overlaid images. If you take a close look, and some of you with sharp eyes may have noticed this before, there are some subtle color shifts, especially in the lower right green-blue patch.

However, the issue becomes a lot clearer when you look at how Lightroom responds to ETTR. In this case, we'll use a 2 stop ETTR to make the difference clearer:


Outer: ISO 100, 1/15sec, f/4.5 crop, normal exposure
Inner: ISO 100, 1/4sec, f/4.5 crop, +2 stops ETTR exposure
Lightroom 2.4, Adobe Standard profile

Notice how much more difference there is between the inner and outer parts of the image. To some extent, that's because of the 2 stop difference in noise. But also take a look at two things:
  • The color difference in the lower right green patch;
  • The difference in brightness between the border areas - the normal exposure is darker than the ETTR exposure; in fact, of all the overlay images, this is only overlaid image where you can see a distinct difference in the border.
And the difference isn't just random - when you look at the numbers, a pattern emerges:

What you can see is that for each individual ETTR value (each setting of the exposure slider, -1, 0 and +1), the values are consistent, but the L value for the border area, and the b value for the green patch change as ETTR changes between those values. In other words, the color and tone curve is consistent between the ISO 100 and ISO 200 images, but inconsistent between the images with different ETTR values.

So what's happening here? Two things. Let's go back to the theory - Firstly, all raw developer programs apply a tone curve to raw images. In Lightroom, this is just called "Brightness", in Capture One it is explicitly called a "Film Standard" curve, and in Aperture it is called "Raw Boost". However, in Capture One, the curve is applied before the tone curve, while in Adobe products such as Lightroom, the curve is applied after. So Lightroom shows a shift in brightness with changes in Exposure setting, Capture One doesn't.

The second thing that's happening is "hue twists". Hue twists are deliberate changes to colors in an image to give a more pleasing result. So in current versions of Lightroom, you can choose between a number of different profiles, e.g., "Adobe Standard", "Camera Neutral", "Camera Portrait", etc. Each of these is designed to give better colors under specific circumstances. So, for example, a landscape profile will take a light sky blue, and make it a darker "more blue sky" sort of a blue.

So, when you offset exposures by using ETTR, what you are doing at the same time is to completely upset a whole bunch of carefully calculated tweaks to make your images look better. For example, that slightly overexposed blue sky is now way overexposed, and as a result the profile will tweak it to somewhere entirely not like a blue sky. The result is an image with a sky that looks just slightly not right. And you get to spend a lot of time in post trying to sort out subtle color and tone shifts that aren't obvious, but just make your image look slightly wrong.

For those interested in more details on tone curves and hue twists, I've blogged on them in more detail here and here.

Making ETTR work

So, ok, in practice ETTR is only useful under one very specific circumstance - when you want a lower ISO than your camera has, and you're willing to sacrifice both dynamic range and color reproduction to get improved noise performance. But ETTR does have theoretical advantages. Is there any way to get the advantages without also getting the disadvantages?

The answer is yes, but only partially, and at a price. The price is the cost of a "Gen-2" Nikon DSLR, and Nikon's NX2 software. The way this works is as follows. Newer Nikons, e.g., the D3x, have something called Active D-Lighting. In its normal variants, D-Lighting is just some optimization in software, but when set to "Active", D-Lighting actually automatically does ETTR for you. However, I say partial because it only does an "under exposure" ETTR for you; in other words it preserves highlights in high contrast scenes, but doesn't increase exposure in low contrast scenes. The clever bit however is that the camera then encodes the D-Lighting data into the raw image. Then NX2, Nikon's raw developer, can correctly adjust exposure prior to applying any tone curves or hue twists, and without you having to play with sliders. So no tone curve or color shifts. Magic. There's only one problem - this works only with NX2; Nikon have not disclosed to any other raw developer producer how the D-Lighting data in the raw file is encoded, so you only get the magic if you use a Nikon camera and NX2.

All those extra A/D converter codes

I've given a bunch of examples of how the only visible improvement that ETTR gives is as a result of lowered noise, but what about the argument in the original Reichmann article that the advantage was in more A/D codes. Well, I can't prove a negative. But the evidence on this is pretty overwhelming:
  1. There is no sign of any visible improvement from additional codes in any of the test images shown here.
  2. In the Nikon community, many DSLRs can shoot either compressed or uncompressed raws. The compressed raws have 683 codes versus the 4096 to 16384 that uncompressed Nikon raws have. Ever since Nikon cameras came out that could shoot both compressed and uncompressed, people have been trying to show a visible difference. They never have, at least without doing really heroic post processing, things like 4 stops of shadow recovery. And bear in mind, ETTR requires careful exposure; it's easier to get the exposure right in the conventional sense than it is to apply ETTR without blowing channels.
  3. In the Leica community the M8 compresses down from 16384 codes to only 256 codes. Similarly, there has been a lot of testing done to try to demonstrate a loss in image quality from that compression. Including by me - see here. Likewise, nobody has succeeded in showing any difference under normal conditions.
So, unless and until someone can show me an image, normally processed from a real camera that shows a visible advantage that can't be duplicated by switching to a lower ISO, I don't see any evidence to suggest that the theoretical "more levels" advantage translates into any kind of a practical image quality advantage.

And in conclusion.......

Here's my conclusions:
  1. There is no advantage to image quality from ETTR that can't be duplicated by selecting a lower ISO, if a lower ISO setting is available. In some situations, such as where there is in-camera noise reduction, ETTR actually increases noise. That's what the practical tests show, and the theory of the case confirms the practical results to be correct.
  2. The only situation where there is an advantage to ETTR is if you're already at the lowest ISO setting your camera, and you use ETTR to synthesize a lower ISO. However, given the noise performance of most modern cameras, that advantage is often very small. The test I did here - a small sensor high pixel count camera - is the best possible scenario for seeing an improvement. Using a modern DSLR, the improvement would be marginal at best.
  3. Any kind of ETTR brings significant disadvantages in the shape of color and tone curve shifts that will have to be repaired in post processing. While these shifts are small, they are easily the equivalent in effect of changing profiles. So, in effect, ETTR negates the advantages that modern raw developers such as Lightroom bring with them.
Bottom line - ETTR offers improved image quality in only one specific situation - where you can use a lower ISO setting than your camera has. In all other situations, ETTR will only ever decrease image quality.

Update : see my later post here as well, as well as the subsequent two ETTR posts, the last one of which adds another situation in which ETTR may be useful. For some cameras, if you're willing to overexpose by four (yes four!) stops.

36 comments:

fefe said...

Interresting analysis. What about the case where a normal exposure will blow highlights at min iso (high contrast scene)?

I am not sure this is part of the ETTR definition, but this is the only case where I pay attention to the right side of my histogram. I end up underexposing by just the right amount to not clip any channel.

I then have to change the curves especially to push shadow and midtones, which might induce color shifts and increased noise, but at least I kept the information in the highlights.

Sandy said...

Yes, if a normal exposure is going to blow highlights, then you have to take a decision as to what to do. Nothing wrong with that.

ETTR, in its "religious" form anyway, is about always exposing to the right, not just if you have a blown highlight problem. And that I do have a problem with!!

Sandy

Simon said...

I arrived at your site after encountering dcpTool, which I found quite useful. However, I must say I don't quite agree with this post.

First, I don't believe that the color shift you report is due to exposing to the right or the profile. Most likely, it is caused by the 'blacks' slider in Lightroom/ACR.

The blacks slider determines the offset that is subtracted from all channels before any further adjustments are made - this messes up the equivalency between your examples. The default value is 5, which is much higher than the actual offset in most raw files. If you decrease this to a value between 0 (no offset) and 2, you are likely to get a much better result.

Also, if I'm not mistaken, the profile is only applied *after* the black point, exposure and white balance corrections. Therefore, I wouldn't expect any effect of 'hue twists' in your examples. [Hue twists probably do affect local exposure corrections; haven't checked this yet] Only nonlinearities in the sensor should lead to small deviations.

Finally, I do believe that in most cases it is beneficial or at least not detrimental to expose to the right as far as possible, even if it means bumping the ISO. If this were not the case, it would perhaps be best to lock the camera at ISO100, even if in low light, and push-process in post.

This also follows from the dynamic range measurements that are reported on http://www.dxomark.com/index.php/eng/Image-Quality-Database
The dynamic range curves often consist of a plateau for low ISO values and a steady descent for high ISO values. In the plateau, it makes sense to push the ISO as far as it will go without blowing highlights. On the slope (1 stop ISO=1 stop dynamic range), it makes no difference at all (as in your example).

Simon

Sandy said...

Simon,

I'm glad that dcpTool is working well for you.

However, I don't think there's any doubt that the color shifts shown are largely due to the profile. It's not correct to say that the profile is applied after the black point - the way the Adobe have implemented their profiles allow for hue twists to be applied either before or after basic exposure adjustments (which includes the black point), by using either the LookTable or the HueSatDelta table. Eric Chan, who is an engineer on the Camera Raw team, confirmed that this is the case on this thread: http://forums.adobe.com/message/1209891#1209891, and also stated explicitly that is due to the profile (see post 38 in the thread). Finally, dcpTool, which does indeed work, works by modifying the profile.

As regards the usefulness of ETTR - well, we're going to have to agree to disagree on that!

Sandy

Simon said...

Sandy,

What I meant was that the current profile with the 'hue twist' should not affect the color when you use the exposure slider to compensate for exposure differences between shots. You said so yourself in post #72 in the thread you refer to (case 1a). This is exactly the situation that applies when you expose to the right, hence the presence or absence of a hue shift in the profile should not matter.

What does matter is the black level compensation. This is done before the exposure compensation and can lead to large deviations when exposing to the right.

As an example, let's assume that a certain exposure leads to 500 electrons being counted, and the sensor/ADC has an offset of 50. However, the default black level correction in ACR is too high at 100, so the output is 450 (in linear space). Now suppose we overexpose by one stop, giving us 1000 electrons+50-100 = 950 in the output. If we apply a 1 stop correction in software, we get 475, slightly more than the 450 we got in the reference case.

This discrepancy is ultimately caused by the mismatch between the real offset and that used by the raw converter. The default value of blacks='5' is too high for this kind of 'scientific' usage. The resulting changes are entirely consistent with the slight brightening you report when exposing the color checker to the right.

On a related note, I am not sure whether the black level compensation is performed before or after the HueSatMap correction. It is definitely performed before exposure compensation, though, as some exerimenting will show.

Simon

Sandy said...

Simon,

Well, rather than debate the issue in theory, I did the experiment, which is to say that I looked at the various images I shot with the black level set to zero. The color shift is still there.

That's in any event consistent with my understanding of the Adobe processing pipeline, which is that all of exposure, contrast, black level and the recovery controls are "basic exposure controls", and so will be processed before the HueSatTable, thus potentially leading to color drift if they are adjusted.

Re the post 72 you refer to, the point I was making was a generic one, which is that however you structure the location of hue twists, before or after exposure controls, there will always be a some situation in which you get color shifts that are likely to surprise the user.

I agree with you re the black level being to high for "scientific usage" - the issue is that ETTR is in a sense scientific usage, while you can get around the color shift, etc issues, it comes under the category of "don't try this at home". Most people will end up spending more time untangling black levels, exposures, color shifts, etc than could ever be justified.

Sandy

Simon said...

I remain confident that there is nothing in the current ACR/Lightroom workflow that interferes with the concept of exposing to the right, at least in theory. Hence I strongly disagree with the assessment that exposing to the right "is just plain wrong".

However, it may be slightly less trivial to do it right than it is often made out to be. That much I can agree with.

Cheers,
Simon

x y said...

The Luminous Landscape...Michael Reichmann...

I used to read it when I started out with photography.

Then at some point I realized Michael Reichmann belongs with Ken Rockwell...

Check out his "reviews" of new cameras. Ohhhh....the wonderful colors...oohhh...the fantastic resolution...

Also check out his "Photoshop techniques": "my friend X, the notorious expert in Y, showed me how this is the best way to do Z, and IT IS TRUE!"

Please...

Dorin Nicolaescu-Musteață said...

Sandy, you have to set the Black level higher for the ETTR+2 image. ACR sets the black point before Exposure. So, what's a good 5 level for the normal image, is too low for an ETTR+2 image.

As for color shifts, my tests (Nikon D80) show no shifts at all. That's because the hue shifts (LookTable) is done after Exposure adjustment. That's the reason for the color shifts people complain about. And Eric explained in Adobe Forums why this isn't done before exposure - to keep color in images with different exposures the same - you must remember, you've participated in those discussions too.

Regarding, no ETTR advantage at higher than base ISOs... Sandy, who the the hell is increasing their ISO to get better noise? Off course, this whole ETTR thing is about base ISO. That is, when your limited by your lower ISO - can't go lower - and you're essentially get a synthenitc lower ISO.

Your 1/4 vs 1/8 at ISO 100 example is the only valid case for ETTR. And no wonder it's the only case when the system works.

Sandy said...

Dorin, the black levels are indeed identical in all images, and yes, you can do better - but actually, you make my point for me. The point being that to even get an ETTR exposure looking as good as as a normal exposure takes a lot of playing around in post.

As regards the D80, it's quite possible that it shows no color shift. Some of Adobe's profiles use LookTables, some HueSatDelta tables, and some both, as I discussed in this blog post from 2009: http://chromasoft.blogspot.com/2009/02/adobe-hue-twist.html

However, there are always the tone curves, as I discuss in the blog post, and those will cause you trouble with midtones with ETTR exposure offsets, even if the profiles for your camera doesn't show hue shifts. Yes, you can change the tone curve in post, but why bother? The gains from ETTR are somewhere between zero and minimal even under the best of circumstances.

In regards of high ISO - well, actually a lot of people use ETTR at high ISO in the hope that it's a "magic bullet" of sorts. And that's really the point of the blog - and I think we agree on this - ETTR can give a marginal improvement under the right circumstances, but it's not a magic bullet that will somehow save you from the laws of physics.

Rich said...

An interesting article, and some things to keep in mind when shooting.

One concern is the testing with only ISO 100 and 200. It's rare that I find myself adjusting exposure by a full stop at a time. The alternative to adding 1/3 or 2/3 of a stop of exposure to push to the right is changing ISO to 125, 160, or the like. Unfortunately, Canon is knows for their "synthetic" in-between ISO's, which mean that going to ISO 160 instead of adding 1/3 of a stop of exposure at ISO 200 will have worse noise characteristics.

I think this testing points out the need to fully understand your gear, your software, etc., as well as test with your own specific setup to find what works best.

The results here show what someone shooting with a G10 should look for, but a different camera may have different results, so the generic "this only applies in one situation" has the same issues as "always ETTR"

STRIKE said...

1. Learn how a digital sensor works.
2. The idea behind this article was quite simple, you just needed 2 or 3 sentences to explain it. I found no evidence on it to say "ETTR is plain wrong". Plain.
3. Ettr does work, when you know how to use your gear. Plain.
4. Nobody said, never, to blow the highlights, just to bring your histogram to the right. Does signal to noise says anything to you? Hello?
5. If you have a high contrast scene, then you should look for reducing the contrast on it by adding light to your shadows. Just as you learnt when studying photography, right?
Unless you are photographing a landscape. In wich case you can use hdr technique.
6. Filling the shadows means extra light, or gear. Digital is good in many aspects, but nobody said, never, that it was perfect.

Laur said...

Thanks for this post. It raised a great question of whether doing ETTR is indeed providing an advantage over shooting at correspondingly lower ISO values and it shows that for some cameras there may not be any advantage.

But I repeated the experiment for two of my DSLRs and the results have been contrary to those presented here. In my tests, ETTR did provide modest to obvious improvements in noise reduction, depending on the camera and ISO used.

My experiment and comments around its results is here:
Testing exposure to the right.html

Bruno Degazio said...

You;'re confusing noise performance with luminance/color resolution. "Expose-to-the-Right" (ETTR) is meant to enhance the latter, not the former.

I don't believe the the original article said anything about improving noise, so I'm not sure why you;re testing it.

If you want to test ETTR you need to have color or luminance gradations in your shot, not flat panels. The improvement will EASILY be evident then.

Joe said...

This analysis is flawed in several ways, as others have pointed out, but what I found most peculiar is the notion of using a higher ISO in order to Expose To The Right.

The whole point of ETTR is to capture as much light as possible without blowing out the highlights (provided that motion blur is not an issue), which yields better use of A/D encoding. This implies taking a longer exposure or using a wider aperture.

Using a higher ISO simply increases the signal gain of the sensor rather than capturing more light, which may superficially yield better use of A/D codes but will of course amplify any noise, and will not reduce posterization within the shadows.

Comparing numbers of electrons is plain wrong - it's photons that we're after. And comparing ISO 1600 to ISO 800 isn't 'interesting', it's a straw man argument!

Perhaps if the author had focussed on performing a more scientific analysis of the efficacy of ETTR, rather than attempting to prove that ETTR is 'wildly over-hyped', 'practically religion', they may have retained some credibility.

George said...

Great article, and a super detailed argument. I've been sceptical about ETTR for a while, and this really helps put into words, what I had also concluded in a far less formal manner.
Cheers.

Oji Valencia said...

Can you explain further why using a Canon G10
is the proper medium in demonstrating this experiment?

Sandy said...

I used the G10 because it is a small sensor, high pixel count camera with relatively high noise. So it will show any benefits from ETTR more clearly than e.g., a DSLR with a larger sensor and lower noise.

Alexander S. said...

But isn't this technique best for use only in RAW pictures, where you can change the exposure in post? Isn't it a benefit? Wikipedia has this explanation: "This technique is only relevant for use when shooting in a raw image format then processing in a raw converter before exporting the file to a raster graphics editor. If the technique is used with JPEG files (the default on most consumer cameras) it will not work as intended and may result in overexposed pictures for low-contrast scenes and underexposure for high-contrast scenes. Exposing to the right by increasing the ISO setting (in digital camera systems) will not work as intended and may result in increased noise. The practical usefulness of this technique in many circumstances, especially in conjunction with modern cameras and modern photo editing software, has been disputed."

Sandy! Please, comment.

Sandy said...

Alexander - If the question is whether the images I show were shot raw, then the answer is yes, of course.

Alexander S. said...

You mean, you shot RAW files on Canon G10? I thought, only EOS models shoot RAW.

May I ask about such a feature as exposure compensation setting in a camera? When to use it, do you know?

Sandy said...

All of the Canon G series can shoot raw, so far as I know.

Exposure compensation? There are lots of reasons why you might want to use that - e.g., you think the camera's meter will get the exposure wrong, you want to create a particular effect, you're bracketing, etc, etc

There are a lot of tutorials on the net about how to use exposure controls......

Alexander S. said...

Thanks! I've read about it and figured another question. I'm shooting RAW with Canon's Neutral picture profile, then I bring the photo to Adobe Lightroom. It automatically assigns the Adobe Standard Profile. I've read your experiments on these profiles, but could't find the solution - what's the better profile - Adobe Standard or Camera Neutral to develop the photo. They all seem to change hue in the picture, so I can't figure out which one is the most true and natural. What profile do you use?

Sandy said...

Alexander,

To be honest, I use whichever profile looks the best for a particular image. E.g., lanscapes versus portraits, etc - it's really whatever gets the result you want.

olamid said...

I appreciate the blogger's perspective and think it is good stuff to know. But many people overgeneralize these results.

For me expose to the right is about maximizing dynamic range. Today's cameras are getting better dynamic range, and I see that the blogger intentionally chose to work with cameras with high dynamic range for the test.

The fatal flaw in this test was shooting only low dynamic range subjects. An evenly illuminated matte paper color target simply does not have anywhere near the dynamic range that a real world scene with uneven lighting, specular highlights, glossy black hole blacks, and shaded areas can have.

Sure, if your scene will easily fit into the camera's dynamic range then you can shift the image around within that dynamic range like the test does (even underexposing or overexposing) without much degradation, and the curves of the processing systems become the dominant factor in determining image quality.

But if your image has more dynamic range than your camera can capture, then I don't think anyone rational could argue that exposing to the right won't minimize shadow noise.

The devil is in the details, and one does have to be careful to avoid blown highlights when exposing to the right. But this article didn't even examine the issue of underexposed shadows because the matte black paper was always within the dynamic range of the camera during the tests.

So although this is a useful test, it has unfortunately led some people to generalize the results to real world situations, such as the author of the line in the current version of the wikipedia ETTR article which brought me here:

"Exposing to the right by increasing the ISO setting (in digital camera systems) will not work as intended and may result in increased noise."

If the intention is to reduce shadow noise, then exposing to the right by increasing exposure (longer shutter speed or wider apertures) most definitely will help.

I would even argue that exposing to the right by increasing ISO would be helpful in most cases; again the test above did not cover the case of shadows because the target was evenly illuminated.

Sandy said...

Well, I'm afraid that that's not an argument that even most of the proponents of ETTR would agree with. "Maximizing the dynamic range" sounds good, but the question is, "maximizing the dynamic range" of what?

Yes, if you do ETTR, you can increase the dynamic range of the raw image. Problem is, that gets you absolutely nothing, in the shadows or anywhere else on its own because to get back to a normal exposure, you have to dial out whatever amount of ETTR you applied. So at the end of the image processing chain, you've done nothing for the dynamic range of the image.

The ETTR argument is that even after you've adjusted back, you get a better result, either because of noise or number of levels.

Irving Weiner said...

"So at the end of the image processing chain, you've done nothing for the dynamic range of the image."
This is exactly my thoughts--instead of increasing the 'gain' to ETTR, let's move the ADC range to the LEFT!. Reducing the full scale 'ref' of the ADC to the max level of the scene permits a greater portion of the scene levels to be contained in the ADC's window. Zone's 0-1-2 will now contribute an increased quantity of 'black' data with finer resolution-reducing posterization.Noise(ADC&Input)should remain 'unchanged' as no signal'gain' is employed to ETTR.
I used this approach in an earlier life (EE) to permit selectable input scales to be chosen. Yes, noise of the ADC may be greater than the smallest dark scene levels--but we are now the darkest darkest black levels!

How to implement? Software, the microP reads the scene levels, selects the peak value and alters the ADC ref.
Since the scene level data has not been 'ETTR'd' no need for additional scaling when imported to PSE,CS5...just continue with your 'normal' PostProcessing.

irv weiner

RobStanfield said...

When I read Michael Reichmann's article Expose (to the) Right, it seemed to make absolute sense.

When I tried the techniques, I wasn't disappointed. I use the latest versions of DxO, DPP and ACR and I've never noted a shift in colors or hue that offended me. I print lots of shots on a HP Z3100. Color shifts because of ETTR doesn't seem evident on prints.

Coming from an audio recording background, the intention is to get maximum signal to noise at every stage of processing.

Having said that, a camera manufacturer has intimate knowledge of their sensors, and electronics, just like Microsoft has undocumented calls in their software.

It seems to me that companies like Canon would have considered the ETTR concept long before the users. They're not stupid.

So why doesn't the camera manufacturers make it easy to obtain the perfectly filled range on the LCD display?

I don't know. Did Reichmann patent his idea?

All I know is this, I come home often with more than 1 of the same image, and pick the one that nailed it... because sometimes the focus is off... or I selected the wrong aperture .. or

etc. etc.

Alex said...

I've recently found one camera body where ETTR works, even just by increasing the ISO and not actually letting in more photons: the Canon 5D Mark II.

I was blown away, upon purchasing this body, at how terribly noisy the shadows are. Even on a base ISO 100 exposure straight out of camera, I wouldn't call the shadows clean - but especially if you brought them up at all in ACR, I found my old 40D (surprising) and D7000 (not surprising) to be much better performers in this area.

Try exposing correctly at ISO 100 for a landscape image with dark foreground elements on the 5D2. Try bringing those foreground shadows up. They'll look HORRIBLE. Banding and noise all over, unusable in print.

Then try exposing to the right (even solely via ISO, surprisingly), bringing down your exposure so mids/highlights are where they would have been in a "correct" exposure, and bringing those shadows back up as much as you were trying to before. They're CLEAN. I believe that's because this sensor is optimized for highlight recovery. You can push it pretty far before blowing anything permanently in the highlights, but you can't push the shadows at ALL, even at ISO 100. Thus, it makes sense to expose for the shadows and bring it back down in post.

Look at this image I shot at ISO 1250 on the 5D2 recently (to freeze motion in the wind), which I brought down 1+2/3 stops in post (save for the sky exposure, which was at ISO 100): here. Then look at this 100% crop and tell me that looks the same as an ISO 400 (1250 minus 1.66 stops) exposure on the 5D2, especially given the fact that the shadows have proper detail. Nope, you can't even do that at ISO 100 on this camera. Straight out of camera at 100, properly exposed for that flower, those rock shadows would be black. Bringing them up to that level would make them unusable. If you need to beat up your files to control high-contrast scenes, ETTR rules - as long as you're capable of blending in lower exposures for a sky.

I never found I had to do it on Nikon, however - especially with the D7000 (also applicable to the new D7100, D600, D800, and D800E). On those sensors, Equivalence actually works and you can shoot everything at ISO 100 and bring it up later, with exactly the same noise as if you had raised the ISO in the first place. This is how you purport all sensors to work, which is simply not true. Different sensors have different highlight/shadow recovery abilities, and different noise floors/SNRs, so ETTR is necessary on some for clean results *if you process your images*. Straight out of the camera, yeah - they'll look similar. Just don't try pushing those shadows on Canon sensors, or you'll be sorry you didn't ETTR.

Regarding color shifts or the washed out shadows you display at the end of your article - those would only be an issue if you blindly used the LR/ACR defaults for every single image you processed. Just use the blacks slider and your eyes on the "offending" ETTR image, and you've got proper tones with less noise. LOOK at your hues and CORRECT them - they're never perfect SOOC anyway. The benefits of ETTR far outweigh the potential inconvenience of having to actually look at your images as you process them.

Maurizio De Cecco said...

OK, i am a background in audio DSP and not image, but some concepts are similar (even if you call them differently).

I have an objection/questions.

When you discuss and try ETTR you always move to the right by changing the ISO. Of course this raise the sensor noise level, and this defeat the idea of ETTR, because it does not raise the dynamic range (that is, use as much dynamic range as possible)

What about what happens changing aperture or time instead ? Yes, it is not the same photo, but the results you got would be different. How different, i do not know.

Maurizio

Sandy said...

Maurizio,

I'm afraid that you misread the article. The article looks at BOTH changing shutter speed/aperture and changing ISO, and there are examples of both.

Maurizio De Cecco said...

Sorry, i was too quick in reading; but your findings actually confirm what i was talking about.

When you compare two images, same ISO, one ETTR and the other not, results are equal or just a bit better for ETTR.

When you lower the ISO, instead of overexposing, you get the same result.

This is what you should expect, with respect to noise sensor; you get the same dynamic range in both case.

The point is, as you rightfully explain at the beginning of the article, is that ETTR is not (and cannot be) a technique to reduce noise sensor, but it is a technique to reduce quantisation noise (the noise introduced by transforming an analog signal in a digital one with a limited number of bits).

I actually have no idea if quantisation noise is really an issue, but i suppose it is lower but not too far from sensor noise (if the sensor noise is much higher of the quantisation noise the number of bits used is useless, if the sensor noise is much lower of the quantisation noise there is an opportunity to raise the DAC resolution).

Maurizio De Cecco said...

Sorry, what i wrote above contains many factual mistakes, this is what happens when you write too fast; so please ignore it, and I'll start from the beginning.

First, the point here is how to reduce the noise floor, and not how to raise the dynamic range. Lowering the ISO reduce the noise floor; underexpose in post processing reduce the noise floor; if you stay within the system linearity (ie. not blowing highlights or red channel etc) they are approximatively equivalent (the second is always worse, for the reasons you gave and probably others). So, your findings are expected and not surprising.
As you says, ETTR is useful to reduce sensor noise only at the base ISO.

But the initial motivation for ETTR is not sensor noise, but quantisation noise; this is what the sentence about using all these bits; the big difference is that quantisation noise is "post-ISO", while the sensor noise is pre-ISO; by this i means that the mechanisms that regulate ISO in the sensors amplify the sensor noise, so the noise floor is affected by the ISO settings, while the quantisation noise do not depends on ISO, but only on the number of bits in the output.
Quantisation noise is a complex beast, because that is actually a distortion that is transformed in noise using a number of techniques; this techniques usually try to produce a noise that is less visible to human eyes (or ear, for sound).

So, do quantisation noise affect the quality of modern digital camera ? I would suppose that on cameras like the G10 the sensor noise largely dominate quantisation noise, and that in order to me sure this kind of noise (and i says measure, not seeing), you need a very high quality sensor, APS-C or FF, where the sensor noise is low. But frankly, i have no idea if you can really see the artefacts of quantisation on a 14 bit raw.

OK, this time should be correct, :) i beg your pardon again for my previous silly posting.

Unknown said...

This article is "Just Plain Wrong Wrong Wrong"! First of all, ETTR is only for Raws. If you are shooting JPEGs, you should always shoot as close to the desired output image as possible. Secondly, the purpose of ETTR is to minimize noise in the deep shadows -- 5 EV or more below saturation. Taking a test image while obstructing the white squares, and then pontificating that you can't see a difference in squares that are 2-3 EV down -- what do you expect? This article should be taken down. It is too far off in every way to be fixed.

Sandy said...

For clarity, all image are, of course, raw.

Duca said...

Hallo everybody,
I think ETTR is a great resource in digital photography! But if you want to shot with high ISO, you are NOT a serious photographer and any other argument falls down.
My 2 cents...