A typical test image looked like this:

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."

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:

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:

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:

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.

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:

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:

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.

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:

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:

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:

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:

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.

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.

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:

Here's my conclusions: