Pintography: exposing to the right and RAW

Raw files record lots of data. In fact, they record more data than current computer screens can display. In order to view the raw file, you have to tell the computer what bits of the RAW data you want to look at.

If a jpeg is like a pint glass, then a raw file is just a much bigger glass. You can still get a pint out of it, and you can decide whether to use the stuff at the top of the glass (highlights), or the bottom of the glass (shadows). Or a combination of both. The rest gets chucked away.

 

That's why your histogram doesn't tell you the whole story if you're shooting raw. The histogram gives you the preview for the jpeg, because it assumes that you want to keep the parts that the monitor can display.

The small histograms are for the jpeg. These are the ones that the camera will show you when you take a photo. The camera's meter decides which part of the histogram to look at, and makes it fit nicely into a graph. It does this by squashing the height of the histogram, showing the proportions of light and dark tones within the image.

The large histogram shows the correctly exposed raw file, taking in everything in the scene. Your camera can't show the whole thing, but it's still there.

The histograms are different widths, because our glasses (and image files) have different capacities. To compare how much data can be held, we use the term Dynamic Range (DR). A jpeg has a DR of 5 to 6 stops of light. A raw file has a much bigger DR (currently as much as 12-14 stops of light). A one-stop increase means that you've doubled the amount of light coming into the camera, so doubling for each stop means that a 14-stop DR needs a much bigger file than a jpeg.

A histogram shows the darkest areas at the left, where there is no light. A bigger DR lengthens the base line of the histogram to the right (since you can't get less light than no light). Therefore, the jpeg will become over-exposed before the raw file does, because it has less DR than the larger file (an under-exposed raw file will still contain enough information to produce a good jpeg).

So if you are going to try “shooting to the right”, and you're a raw shooter, learn to ignore your histogram just a little. Push it an extra stop past the blinkies, and see what you can do with your raw converter.

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