ISO 1600 vs correcting exposure in photoshop

[Daniel Browning]

Where do these statements come from?

3 stops was chosen because that's a typical amount of headroom for the autoexposure system to meter for.

Are they arbitrary figures, just so you can illustrate the differences? Or is "f/2.8 ISO 400: 3 stops..." (and the others) simply true, all by itself?

It's arbitrary in the sense that I chose the number that Canon uses for its AE system. But raw photographers are free to choose any amount of headroom they need for the shot, from 1 stop to 7 stops. The AE system will tend to choose between 3 and 4.5 stops, depending on the camera, settings, and other factors.


What's not arbitrary is the *relative* amount of headroom. When exposure is reduced by a stop, without changing ISO, headroom increases exactly 1 stop, every time. In a fixed exposure, ISO 800 always has 1 stop less headroom than ISO 400. That's why it went down from 4 to 3 in my example. But if I had chosen a different headroom to start with, say 5 stops, then it would have gone from 5 to 4.

Aren't headroom values dependent upon the dynamic range, which obviously vary from scene to scene? Or are these statements "constants" in some way?

You've got it. Headroom is chosen by the photographer based on the scene. A black lab against a black wall holding a black ball will need far less headroom than a white dress in sunlight against a black wall.

Sorry if I'm complicating this. I always think of Headroom as a safety margin of sorts, a tonal range that is available in the development process, existing only because the photographer didn't clip the HLs. How much headroom is a function of the exposure (and the scene's avail range of course) and then the process in PP (the creative intent).

I don't think that's the best view of headroom. I'll try to explain. The camera just captures linear values from clipped to noise. It knows that 1 stop less light means it is "darker". But it has no concept of "black". Black is just the point where we, as humans, say "this is too noisy, I'm going to call it black."


Raw files have no concept of "middle gray", either. Middle gray is just the point where humans say "make *this* certain brightness into the midtone". That could be 7 stops below clipping, which would result in a *lot* of highlight headroom. Or it could be 1 stop below clipping, which would result in very little highlight headroom. Cameras tend to meter for 2.5-4.5 stops of highlight headroom, and most raw converters are tuned for this as well. It can be quite difficult to get raw converters to use a different value. Film had 6 stops of highlight headroom.

I mean, I see the trend - higher ISOs squeezing the HL headroom - but how or why does it happen?

I don't know exactly what goes on inside the camera. I just know that it clips it.