Having a brain fart with ISO

[andnowimbroke]

I get that high ISO increases noise (especially in black areas). Most people even shoot with an increase in exposure compensation to help avoid shadowy area (rather than just balancing exposure because of what's in the scene) but not too much to blow highlights out really bad. Doesn't raising your exposure comp justchange one of your settings (A/S/ISO speeds)? If your in a dark place where you are already playing with your limits (100mm lens @F4 set to ISO800 with 1/75sec shutter speed, how is adjusting the comp going to keep down high ISO noise in the shadows? The Ex.comp is going to want to make it brighter by adjusting something which might lead to a softer, blurry, or noisy picture. I read this thing twice and it makes more sense in my head than in writing. If you understood it, help me out. I know I'm missing sumthung.


Greg

[PaulM]

I don

[Daniel Browning]

I know I'm missing sumthung.

You're not missing anything, Greg -- you've got it right.


The people who say "increase exposure compensation to get less noise at high ISO" are really just saying "use a lower ISO to get less noise", though most of them don't realize it.


If you take an "ISO 100" raw file that is 2 stops underexposed, then boost it +2 in post, the resulting JPEG file is actually ISO 400 (according to the ISO standard).


If you take an "ISO 1600" raw file that is 2 stops overexposed, then cut it -2 in post, the resulting JEPG file is actually ISO 400.


In other words, the ISO setting of the raw camera file and the resulting JPEG file can be two totally different things.


The best result is to ETTR then ITTR: expose to the right, then ISO to the right. That is, first increase exposure as much as possible (while avoiding unwanted motion blur, thin DOF issues, etc.), then, if you still have room on the right side of your histogram, increase ISO as much as possible (up to the highest useful limit of the camera, which is 1600 on most Canons). The first part is most important, but the second can have slight benefits too (especially from ISO 100 to 400).

[HDNitehawk]

You're not missing anything, Greg -- you've got it right.


The people who say "increase exposure compensation to get less noise at high ISO" are really just saying "use a lower ISO to get less noise", though most of them don't realize it.


If you take an "ISO 100" raw file that is 2 stops underexposed, then boost it +2 in post, the resulting JPEG file is actually ISO 400 (according to the ISO standard).


If you take an "ISO 1600" raw file that is 2 stops overexposed, then cut it -2 in post, the resulting JEPG file is actually ISO 400.


In other words, the ISO setting of the raw camera file and the resulting JPEG file can be two totally different things.


The best result is to ETTR then ITTR: expose to the right, then ISO to the right. That is, first increase exposure as much as possible (while avoiding unwanted motion blur, thin DOF issues, etc.), then, if you still have room on the right side of your histogram, increase ISO as much as possible (up to the highest useful limit of the camera, which is 1600 on most Canons). The first part is most important, but the second can have slight benefits too (especially from ISO 100 to 400).

Daniel


If I am understanding you right. If I am in AV or TV mode and I set the exposure compensation a bit higher or lower, that it is adjusting the ISO settings to over expose or underexpose. For some reason I thought it was adjusting shutter speed. Since the change is displayed like the shutter.


Rick

[Daniel Browning]

If I am understanding you right. If I am in AV or TV mode and I set the exposure compensation a bit higher or lower, that it is adjusting the ISO settings to over expose or underexpose. For some reason I thought it was adjusting shutter speed. Since the change is displayed like the shutter.

That's not exactly what I meant -- I'll try to clarify [].


There is a difference between the ISO "setting" on the camera (and raw file) and the actual ISO of the JPEG after it has been through post production. When you set the camera to ISO 400 and Av 4.0, the exposure compensation will not change the ISO setting. But if you set EC to +2, then do -2 in DPP, the resulting file *will* have a different ISO than the one on the camera. See what I mean?



<meta content="text/html; charset=utf-8" http-equiv="content-type" />
Camera settings (ISO 100, Av 4.0, EC +0) + Post production settings (0 EC) = actual ISO of the JPEG file (ISO 100)


Camera settings (ISO 400, Av 4.0, EC +2) + Post production settings (-2 EC) = actual ISO of JPEG file (ISO 100)


In those two examples, the camera ISO settings were different (one was ISO 400), but the true ISO of the JPEG file is the same ISO 100 for both.


As another example, ISO 1600 with +2 EC has *less* noise than ISO 800 with 0 EC, when both are printed at the same brightness. But that's because using ISO 1600 with +2 EC results in an exposure index that is actually ISO 400.


Hope that helps,

[andnowimbroke]

Hope that helps,

LOTS! Thanks to you and the others that responded. It is greatly appreciated


g

[peety3]

I get that high ISO increases noise (especially in black areas). Most people even shoot with an increase in exposure compensation to help avoid shadowy area (rather than just balancing exposure because of what's in the scene) but not too much to blow highlights out really bad. Doesn't raising your exposure comp justchange one of your settings (A/S/ISO speeds)? If your in a dark place where you are already playing with your limits (100mm lens @F4 set to ISO800 with 1/75sec shutter speed, how is adjusting the comp going to keep down high ISO noise in the shadows? The Ex.comp is going to want to make it brighter by adjusting something which might lead to a softer, blurry, or noisy picture. I read this thing twice and it makes more sense in my head than in writing. If you understood it, help me out. I know I'm missing sumthung.

I think a lot of this depends on your approach and objective. If you start from scratch and decide "I want to take this shot at f/4.5 because that gives the DoF I want", a truly regimented photographer might set their camera for Av f/4.5 and ISO100, then take a test shot. At this point, we have two variables we need to adjust: exposure comp and ISO. Theoretically, the initial preview should be sufficient to adjust exposure comp to preserve shadows while not sacrificing important highlight detail. That leaves only one adjustment, ISO, and the ISO will likely be increased until you achieve a non-shaky shutter speed, a stop-action/artistic shutter speed, a combination of both, or a reasonable tradeoff given the realities of physics and what's in your hand.


If you look at it from another angle, although increasing exposure comp might lead to a softer, blurry, and/or noisy picture, it's extending the shutter time (if everything else stays the same and you're in Av), and that's going to decrease noise in the shadows (you're giving the sensor more photons courtesy of more time).