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 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.
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:
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.
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:
- 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.
- 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.
- 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.
- 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:
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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:
- There is no sign of any visible improvement from additional codes in any of the test images shown here.
- 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.
- 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:
- 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.
- 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.
- 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.
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