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"Exposing to the Right" Exposed as Signal to Noise Ratio

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There's an absorbing debate about 'exposing to the right' going on at The Online Photographer right now (you can read the piece here). In light of the debate the article has started, I thought it would be interesting to look at the expose to the right technique, explain what it is and run some practical tests to see if it works.



Why isn't exposure easier?

Let's start by looking at why exposure can be so problematic. There's no doubt that arriving at the 'correct' or 'best' exposure is not always easy. That's why your camera comes with several exposure modes, so you can select the one that is most appropriate to the subject you're photographing and the way you work.

The best exposure depends not only on the amount of light illuminating the subject but the medium that you using to photograph it. Take film as an example. If you use slide film, you often need to underexpose it slightly to get the best result. If you use colour negative film, you normally need to overexpose by a stop or so to get the best negative to make a print from. If you use black and white film, you can use the 'expose for the shadows, develop for the highlights' technique to determine exposure. Three different mediums – each with a different ideal exposure.


Exposure and digital photography

The same applies to digital cameras. Your approach to exposure depends on whether you are using the JPEG or Raw format. If you're shooting JPEGs, you need to get the exposure as accurately as possible at the time you shoot. You can adjust exposure in post-processing, but thanks to the limited bit-depth of JPEG files it's best to think of that as fine-tuning rather than as a way to compensate for over or under-exposed images.

If you're shooting Raw however, the aim is slightly different. You want to create the best possible Raw file for post-processing, and that means one that contains as much information as possible that can be used by your Raw processing software. An important point about exposing to the right is that, for these reasons, it only works with Raw files.


Exposing to the right

So, where does exposing to the right (often abbreviated to ETTR) fit in? Exposing to the right is a technique for minimizing noise in your images. To understand what exposing to the right is intended to do, we need to understand where noise comes from.


Signal to noise ratio

All digital camera sensors have a certain level of background noise.The amount of noise you see in the end photo depends on several factors, including the signal to noise ratio. Signal, in this case, is light.

Lets say that you're shooting in bright light, and this gives you an exposure of 1/125 second at f8 at ISO 100. Now imagine that you are shooting the same scene but in much dimmer light and to use the settings of 1/125 second and f8 you need to raise the ISO to 1600. At this setting, the quantity of light hitting the sensor during the exposure is 1/16th of that in the initial photo taken at ISO 100. There is less 'signal', and the result is more noise in the final image. That's the basic reason why there is more noise at high ISO settings than low ones.


The histogram and exposing to the right


Take a look at the above histogram. It's a screen shot from the back of my EOS 5D Mark II. You can see that the histogram is bunched up in the middle, indicating that I've just taken a photo of a low contrast subject.

Exposing to the right simply means increasing exposure so that the histogram is closer to the right hand side of the graph. The increase in exposure means that more light, or signal, hits the camera's sensor during the exposure, reducing the noise levels in the image. Ideally you want to get the histogram as close to the right as possible, but without clipping any highlights. If the image is too bright you can darken it when you process the Raw file.


Now take a look at this histogram. It's the same subject as the previous one, but I've increased the exposure by a stop and that's pushed the histogram to the right. Twice as much light has hit the sensor during the exposure resulting in, according to the expose to the right theory, a processed image with less noise.


Does exposing to the right work?

How well does exposing to the right work? Now it's time to look at the photos to find out. But before we look at the results, there are a couple of points to bear in mind.

First, is that noise levels vary between cameras. Generally speaking, newer cameras are less noisy than older ones, and full frame cameras are less noisy than cameras with smaller sensors. If you have a Nikon D3s, for example, you're probably not too worried about noise, especially if you only shoot at the lower ISO settings.

Noise can also be treated in other ways in post-processing. I use Lightroom 3 and the noise reduction tools produce good results. There are also Photoshop plug-ins that help reduce noise. Exposing to the right is not the only 'treatment' for high noise levels.


Sample images


This is a comparison of the two images that belong to the histograms above. You're looking at a 100% magnification, and we can clearly see a big improvement in image quality in the photo that has been 'exposed to the right.' Both images were processed with Lightroom 3 at the default noise reduction settings. The right hand image has been darkened using the exposure slider until it matched the brightness of the first one.

Sharp-eyed readers will already have spotted that I took these photos at ISO 25,600 – the highest ISO setting on my EOS 5D Mark II. I used this ISO setting as I knew the difference would be quite marked. But I can tell you that from my experience, exposing to the right, with low contrast subjects like this, makes a visible difference to image quality at ISOs 1600 and upwards on this camera. At lower ISO settings than that, it's not really worth the effort.

Another factor to consider is that in order to expose to the right, you have to increase the exposure, usually by at least a stop (there's not much point in raising the ISO as that will increase noise). This means that you have to open the aperture or use a slower shutter speed, and this may not always be practical, especially if you are shooting in low light.


When exposing to the right doesn't work

We've seen so far that exposing to the right works well when you have a low contrast subject, you are using high ISO settings and you're shooting Raw. Is there a time when exposing to the right doesn't work?


This histogram shows a photo that I took at ISO 3200, again on my EOS 5D Mark II. The histogram is leaning to the left, which suggests that the image could benefit from exposing to the right. The background is quite dark in places, so I know that noise will show up there.


This histogram shows what happened when I increased the exposure by a stop. If you look closely, you can see that the right hand side of the histogram is cut off, indicating clipped highlights. This is confirmed by looking at the highlight alert:


Having said that, the histogram and highlight alert are generated from a JPEG file generated from the Raw file. JPEGs hold less highlight detail than Raw files, so it's possible that we can pull that highlight detail back in post-processing. Let's see what happened:


A close-up comparison, showing the result at Lightroom's default noise settings and darkening the brighter image, again shows an improvement in noise levels. I had to open the aperture by a stop to increase the exposure, and this has decreased the depth-of-field (which may or may not be a disadvantage, depending on whether you like the effect or not). But what about the clipped highlights?


This comparison shows that the highlights are clipped in both images, but the clipping is worse in the one that was exposed to the right. It's a little hard to tell because there is less depth-of-field in the second image, but a close look at both shows that the clipped highlights are larger in the second image. I couldn't recover the highlights with the recovery slider in Lightroom (which is normal with bright specular highlights on metal surfaces like these).

So, even though exposing to the right marginally improved noise levels it was at the expense of some highlight detail. Whether you would see that difference in noise levels between the two images in a print is debatable, to be honest I doubt that you would – but I'm sure that you would spot the difference in the highlights. In this situation, exposing to the right didn't result in a better quality image. In fact, the initial image may even have benefitted from some underexposure to improve the rendition of the highlights (one of the author's points in The Online Photographer article).


Test your camera

I recommend that you take some time to test your camera and see if exposing to the right makes a difference to the type of images you take at high ISO settings on your camera. This is the best way to see if the technique will benefit you. The photos in this article were taken on an EOS 5D Mark II, which produces images with low noise levels. If photos from your camera have higher noise levels than mine, you will see more of a benefit from exposing to the right.


Conclusion

I don't think it's very constructive to say that exposing to the right never works, or that it always works. As the photos in this article demonstrate, there are times when it could be a benefit and times when it won't.

The best approach is to carry your own tests, and decide whether exposing to the right is a useful technique for you. And don't forget, if you're photographing a static subject, like I did with these photos, it's easy to take a couple of photos at different exposures and then decide which gives the best result in post-processing.

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