ISO 1600 vs correcting exposure in photoshop

[Jon Ruyle]

So then it's strictly thermal noise that the long-exp. NR function addresses?

Yes. Well, almost. I believe it just takes a dark exposure of the same length and subtracts. This is primarily to reduce thermal noise, but it also must mitigate read noise as well (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)

So with a tripod, is there ever a reason to boost ISO beyond 100, short of long (5-min.) exposures? (or even then?)

If the subject isn't moving and the tripod is sufficiently steady, then I don't think so. Thermal noise is less than linear, so longer exposure means better snr.

Although come to think of it, even at ISO 1600, what was a 5-min. exposure at ISO 100 is still going to be pretty long...something like...let's see, ISO 100 to ISO 1600 is 4 stops, so 5 minutes plus 4-stops = 18.75 secs. (cross fingers once again) Is that correct?

That's what I get []

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.)

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

[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.

[Jon Ruyle]

Thanks for the clarification, Daniel.

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

Iris does it I think, and it supports 5DII raws.

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.

That absolutely sucks, and is a deal breaker for me since I like to take multiple exposures with a timer. Glad I decided to pull my 5DII out of the trash []

[Daniel Browning]

Iris does it I think, and it supports 5DII raws.

Yeah, but it's not the kind of popular I was thinking of. It's also kind of basic on other features. Not a very advanced demosaic, etc. What I would really like is if programs could open raw files, make adjustments to raw values, and re-save a Bayer raw DNG file. I would love to be be able to:

• Bias frame and dark frame subtraction in an IRIS batch script
• Lens corrections in DxO Optics Pro (vignetting, distortion, CA, deconvolution, etc.)
• Demosaic and everything else in RPP

I can dream.

Glad I decided to pull my 5DII out of the trash [img]/emoticons/emotion-1.gif[/img]