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Posted by Peter Cox on Tue, Feb 5, 2008, at 11:59 pm

Expose to the Right for Maximum Detail

When out with your digital camera, do you photograph the scene so that the image looks ‘OK’ on the LCD? If you take a photograph that’s a bit on the dark side, do you think ‘Never mind – I’ll just brighten that up later in Photoshop’?

Perhaps you are used to shooting slide film and have developed the habit of underexposing images slightly to ensure that your highlights don’t get ‘blown out’ (lose detail). In digital photography, blown out highlights are as much of a problem as they are in film, but overcompensating by underexposing unearths another problem that is specific to digital imaging.

So what’s the problem? Essentially, digital cameras don’t capture anywhere near as much detail in the shadows and dark midtones of an image as they do in the highlights and brighter midtones. To counter that, we need to ‘expose to the right’, such that the highlights in our image almost lose detail (become overexposed, or ‘clip’). This allows us to darken the image later and retain lovely detail in the shadows – detail that would be absent if we took a dark image and then brightened it.

That’s the advice in a nutshell. If you’d like to understand why, read on…

Understanding Why

First we need to understand how digital sensors record light, and how that’s different from the way we see the world.

Human vision is non-linear in its response to light. What does that mean? Well, our eyes exaggerate the differences between subtle tones in shadows, making the differences appear greater than they are.

In contrast, digital sensors are linear devices. They record the actual difference in brightness between tones, rather than the exaggerated difference we see. In order for a digital image to look natural, the shadows and mid-tones need to be stretched out using a curve (click to see a video tutorial explaining curves).

This is usually done automatically for you – either in the camera (if you’re shooting JPEG), or in the RAW converter (if you’re shooting RAW).

The image below appears very dark because it was processed from the RAW file without applying the standard curve. It shows the tones as the camera captured them.

No curve, tones as camera recorded them.
Gamma of 1.

The image below represents a normal curve and was processed to obtain the same result you would normally get from the RAW file if all settings were left at their defaults. This is how the image would normally come out of the camera.

Normal curve applied, tones as we would see them.
Gamma of roughly 0.45.

As you can see, the histogram of the ‘linear’ image at left has the tones all concentrated in the shadows, while the ‘non-linear’ image has a nice even distribution of tones all the way from shadows to highlights. In order to obtain the natural look, the camera (or RAW processor) has to stretch out the shadow tones to brighten them.

Implications

Your camera can typically record just over 4,000 tones, allowing it to present over 4,000 shades of gray between black and white.

This range of tones is called the camera’s dynamic range, and is measured in stops. A stop is simply a doubling or a halving of something. For example, changing your shutter speed from one second to two seconds doubles the length of time the shutter is open; you have lengthened it by one stop.

In terms of dynamic range, a stop refers to a doubling or halving of brightness. The chart below show an approximation of how the camera distributes the tones available to it. Digital cameras vary in how many stops of dynamic range they have. This example assumes 6 stops.

4096 brightness levels in total.
Native camera linear capture.

As you can see, the brightest stop in the dynamic range has half (2048) of the available 4096 tones. Each stop below that has half the tones of the one above, leaving the darkest stop with a meagre 64 tones. You’ll also notice that the tones seem concentrated towards the dark side of the strip due to the linear nature of the sensor’s capture of light.

The chart below has had the distribution corrected with a curve similar to that used in the photograph above, resulting in an even (to our eyes) distribution of tones from dark to light. This has had the effect of stretching out the tones in the darker areas, and compressing them in the lighter areas.

4096 brightness levels in total.
Gamma corrected.

Notice, however, that the amount of tones in each stop hasn’t changed – that darkest stop still only has 64 brightness levels. This means that the darkest areas of the image have very little detail compared to the brighter areas of grass and sky.

Now, this is nothing to worry about in this image, as the ‘normal’ version is bright enough. But what if the image was underexposed, and we wanted to brighten it?

The Dangers of Underexposing

The following images were taken seconds apart at different exposure settings, although ISO was left unchanged. They were processed from the original RAW files at the default settings. No clipping of shadows has occurred – there is no pure black anywhere in either image, and as the ISO was identical in both images, they have identical noise content.

Correctly exposed.

Under exposed.

Under exposed as above, but brightened in postprocessing

You can see there is a massive loss in quality in the last (brightened) image, both in loss of detail and increased noise, particularly in the basalt columns towards the right of frame.

So what happened? The shadows in the first image were brighter, and so fell into an area that had more tonal levels available to capture the detail in the rock. The shadows in the darker image were right at the bottom of the barrel in 64 tone territory, which means there was no real detail to bring out.

Noise is greater in the brightened image because there is so little image data in the dark shadows that noise becomes more obvious. When brightening the almost non-existant image data, we end up brightening the noise too.

How Does This Apply in the Real World

When photographing, the old adage is true – protect your highlights. If your highlights clip (go to white), then you lose that detail and the exercise is in vain – you will have lovely open shadows, but lost highlight detail.

Where possible, set your exposures so that the highlights go to the verge of (but not past!) the point of clipping. That way, your shadows will be as bright as they possibly can. This may look odd, even bad on the camera’s LCD, but when you get to the computer and darken the image to an aesthetically pleasing level, the quality will be superior.

Here’s an example. Both images were processed from the same RAW file. The one on top has the settings left at their defaults, so it is as the camera took it. The one on the bottom has been darkened in the RAW processor to a more appropriate brightness.

Image with no adjustments (except cropping) – direct from camera.

Image darkened in RAW processor – shadows darkened and overall brightness reduced.

The adjustments here took about ten seconds to perform, and the quality difference over a darker image will be evident in the shadows, especially if you decide to selectively brighten some shadows in the final result.

A Word of Warning

It’s important to realize that this technique is best performed on RAW files, not JPEGs. If you’re shooting JPEG, you have far less latitude to edit the image before it starts to deteriorate. RAW provides far greater latitude in this regard, both in the RAW processor where many of your edits are ‘free’ and don’t degrate image quality and when editing the image in Photoshop proper.

In Summary

The next time you’re out photographing, remember that what you see on the LCD isn’t the final image. If you follow the advice in this article you will end up with an image that looks too bright, and washed out. But as long as you were careful and the histogram isn’t flowing off the right hand side (indicating clipped highlights), you will be able to darken the image in the RAW conversion.

Not all scenes are suitable for this technique – those where the histogram extends all the way from black to white, for example. But where possible, following this advice will result in much cleaner and more detailed shadows, and correspondingly greater overall image quality.

I hope that you have found this tutorial useful. If you’d like to improve your camera or post-processing techniques, I run regular workshops at my studio in West Cork and in Dublin.

On top of that, I also run ten-day touring workshops. Contact me for details.

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