Why bother changing base ISO?

Started Jul 18, 2007 | Discussions thread
Muntz Contributing Member • Posts: 967
Re: Posterization

As I posted before, I experimented and found little difference. I suspect that as you crank up the iso, the tonal levels even in the highlights will diminish. I'd love for someone else to try it out and share their results.

I admit I also thought that increasing iso meant increasing sensor sensitivity and therefore allowing the capture of "faint electrons" that would simply not be captured at a lower iso. But it doesn't seem to be the case.

Hans Giersberg wrote:

Even assuming that bumping up an ISO 100 image doesn't result in more
noise than just shooting at a higher ISO to begin with (a dubious
assumption), you are still left with the fact that the darker tones
are represented by fewer tonal levels than the lighter tones. So when
you brighten up the darker tones, you'll have more and more
posterization, depending on how much you brighten the image.
Remember, the brightest stop on a 12-bit camera is represented by
2048 tonal levels. The next brightest stop is represented by 1024
tonal levels. Then 512 levels, and 256 levels, etc. On a D200, by the
time you are two stops below midtone, all of the information is
contained in only 128 tonal levels. That's OK if it's in shadow. But
when you brighten this, you'll start seeing the posterization.

You're basically saying you're too lazy to do it right. If so, expect
subpar results.

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