I recently read an interesting article at foto.no (in Norwegian) about ISO noise in digital cameras, and how to expose to best use the capabilities of your camera.

Basically (as I understand it) there is always more detail left in the RAW file when “exposing to the right”, ie. exposing so you just avoid burning out the hightlights. Feeding the sensor with more light can be more efficient than trying to shoot with as low ISO as possible. The examples in the article showed that instead of underexposing with a low ISO value, you’d get less noise if you’d overexpose on a higher ISO setting. I’m paraphrasing and simplifying, but the gist of it was that it’s better to lower your exposure in post-processing rather than trying to keep the ISO value as low as possible.

The test

I decided to try this out for myself on my Canon 40D. I’ve never been quite comfortable shooting with my 40D over ISO 400, as I’ve felt that the noise and loss of detail has been pretty obvious. I shot the scene of my poorly lit room with my Sigma 10-20 at f4.0 in Aperture mode, RAW. (Click images for 100%):

Both photos taken handheld with Sigma 10-20 at f4.0. Only adjusted white balance and exposure to match each other.

The first photo is taken with ISO400,f4,1/8 sec (no exp compensation, centerwheighted), the second on ISO800,f4,1/4sec (+1 stop exp comp). I only adjusted white balance and exposure to match each other in post – no noise reduction other than the default 25% color in Lightroom. Even at thumbnail size you can see that the ISO400 clearly is worse than the ISO800 shot.

There’s more detail, more even gradients, less blotches and smoother tones in the ISO800 photo. I even think the hightlights are better preserved with the “overexposed” ISO800 shot. It’s not stock material, but definitively useful and would stand up to more heavy post-processing.

OK, time for the 800-1600 battle. I’ve almost never used ISO 1600 on my 40D (and the expanded ISO3200 only a handful of times). This might change:

On the right (confusingly – sorry) you have your basic ISO800 shot with no exposure compensation. Just what the light meter in the 40D gave me. To the left the ISO1600 is clearly better, even though I did not do much exposure compensation (it might have been +2/3). I’d say that with a bit of careful noise reduction work and some heave white balancing I’d have a pretty descent, crappily lit indoor shot.

Conclusion

The high ISO settings on my Canon 40D don’t suck. By carefully adjusting exposure to keep your histogram to the right (and avoid burning out useful hightlights), you can get good, clean results on the highest ISO settings. I’ve always struggled to keep my ISO down, often resulting in the need to gain the exposure and getting lots of crappy noise, blotches and banding with it. I will definately work on my techniques when shooting low light stuff, and not worry too much about high ISO values. Now, how did I enable that ISO3200 setting again?