I often hear photographers talk about “getting it right in-camera”. This is because real photographers don’t need Photoshop, as their great skill produces perfectly-exposed images. Besides, if shooting JPEG, any exposure correction will exaggerate the format’s limitations. But, if shooting raw (like some real photographers do), you could be making a big mistake by getting it right in-camera.
One of the main points of shooting raw is that you preserve the maximum amount of data captured for processing. Most cameras capture more than 4,000 levels of red, green, and blue, in raw format, and if your brightest image zones are 1 stop below full exposure, you’ve just wasted 2,000 of them.
You could argue that the difference between 2,000 and 4,000 is the same as the difference between a lot and loads, but we must also consider what happens at the other end of the tonal range: the shadows. Underexposing a dark subject by 1 stop could reduce its tonal resolution from 32 levels per stop to 16. You see, a darker stop always has half of the tonal resolution of the lighter one above it.
What this effectively means is that the photographic image is painted with an increasingly coarser palette in darker areas. It means that any digital noise becomes correspondingly more apparent. It means that your post-processing has less data to work on, and it makes bigger mistakes.
Raw-shooting photographers recognised this problem and developed a technique called “Expose to the Right” (ETTR). Simply-put, you expose your scene so that the brightest zones of your image are very close to maximum exposure, regardless of the rest of the image. You ETTR by making sure the image histogram sits as far to the right as possible, maybe even just touching the edge. This ensures that you are using the maximum possible tonal resolution of the raw format—improving image quality, noise level, and post-processing capability.
Unfortunately, most cameras’ histograms aren’t very good at representing the raw data they capture, and it can be difficult to tell if those blinking highlights are accurate. Cameras calculate their histograms based on their on-board JPEG processing, which has a tendency to artificially boost contrast. So you might end up having to guess how close you are to full exposure to ETTR.
You can work around this by setting your camera to give a flat, neutral AdobeRGB image, with no adjustments to contrast, saturation, nor sharpening. It makes your LCD look a bit dull, but your histogram will be a lot more representative of your image’s raw data, and ETTR will be easier. But you don’t care, if you’re a real photographer!
Keith Nuttall, 2011