ISO 1600 vs correcting exposure in photoshop

[Daniel Browning]

So headroom really isn't some fluctuating range of tones, dependent upon the scene. It's really best defined as what the recording medium is capable of. Is that the right way to think of it?

In the case of film, that's correct: the headroom is built right into the recording medium. For example, a certain emulsion might have 6 stops above middle gray (headroom) and 6 stops below middle gray ("footroom") and that's it.


But with digital, it's a little different. You can put middle gray anywhere you want. So with 10 stops of dynamic range, you can have 3.5 stops above middle gray (headroom), and 6.5 below. Or 8 stops above and only 2 stops below. It's up to the raw converter. In practice, raw converters tend to put out 3.5 stops above, and 4.5 stops below, and stop there (8 stops).

And so Canon can choose to make f/2.8 have 3 stops at ISO 400, or it could've chosen something different. The way it is, with RAW converters tuned similarly, is no accident...

Precisely.

On a related note - does any of this discussion apply to shooting on a tripod?

Yes. Usually when you're on a tripod it's possible to "ETTR" so that the histogram is as far to the right as you want it. So there's no need to move on to the next step (ITTR).

Mr. Busch's book says, "Another way of adjusting exposure is by changing the ISO sensitivity..." and so on.

That's disappointing.

I've never used the long-exp. NR on my 40D. Do you think it's worthwhile using it? According to David Busch's book it's good for shutter speeds slower than 1 second, although you'll trade off some detail if you use it.

Yes, I think it's great. It reduces hot pixels and pattern noise, but it increases random noise a little bit. I can't think of why it would trade off any detail. (Maybe he's thinking of Nikon's long exp NR, which does eradicate small details.)

Would I have done better with a higher ISO and a faster shutter speed?

No, you did it right. Always do "ETTR then ITTR", never the reverse. Doing ITTR first would result in a lot more noise. Exposure is more important than ISO.