ISO 1600 vs correcting exposure in photoshop

[Daniel Browning]

You're welcome.

So the 10 stops of DR you speak of - is that what digital sensors are considered capable of capturing?

Close to it. Engineers measure dynamic range from the clipping point down to where SNR reaches 1:1, which can be 11.5 stops on many cameras. But 1:1 is *very* noisy. Noisier than many photographers like to use. So how much dynamic range someone will use depends on their personal standard of how much noise is acceptable, especially pattern noise. You can find the "engineering" dynamic range for many cameras here:

http://dxomark.com/

And when you say "you can put middle gray anywhere you want" and then later say "It's up to the raw converter" I'm not sure how those 2 statements go together.

It's both. You choose where to put it by changing the parameters of the raw converter (if it lets you).

The first one I figured meant when we meter a scene, we're choosing the gray point. But if it's "up to the raw converter"...?

Basically, it's best to "expose for the developer". In other words, think ahead to how the shot will be converted, then expose for that. If you know that you can move middle gray down, so that you get 2 stops more highlights for a certain contrasty shot, then you'll "underexpose" (compared to the AE meter) by two stops, but in reality you'll get just the exposure you wanted (perfect exposure).

Doesn't our metering system take care of dividing the dynamic range? I thought that's what they all did, establish the 18% gray point (actually closer to 13% from what I've read) and then the 1/4 tones, 3/4 tones etc. fall where they may.

That's right.

Yes. Well, almost. I believe it just takes a dark exposure of the same length and subtracts.

I think so too.

This is primarily to reduce thermal noise, but it also must mitigate read noise as well

It does help the "fixed pattern" part of the read noise, which is very beneficial, and it also takes care of the hot/dead pixels, but the random noise actually gets a little worse, because the read noises add in quadrature. If Canon used the technique you mentioned, it wouldn't add noise:

(though if one wants to reduce read noise in a low signal image, more standard practice is to take several very fast exposures, combine eg by averaging, then subtract)

Unfortunately, none of the popular raw converters support dark frame or bias frame subtraction, so I tend to just recommend the long-exp NR.

I assume the canon long exposure nr just takes a dark exposure of the same length and subtracts. Does Nikon do something different?

They have two forms of long exp NR. One is the same as Canon (dark frame subtraction), and can be turned off. The other is an algorithm that looks for hot pixels, and cannot be disabled. This is of course bothersome for astrophotographers, since stars tend to look a lot like hot pixels. There is a cumbersome workaround, though: if you *enable* long exp NR, then turn off the camera in the middle of its dark frame, it will cancel the dark frame subtraction, cancel the hot pixel killing software, but still save the light frame. Turning off the camera after every frame is too much work for me, though.