High ISO is bad right?

One of the mantras you hear over and over is that to avoid noise, you should shoot as as low an ISO as possible because "high ISOs cause noise". However, this is based on a misunderstanding of how the physics of these sensors work. It turns out that actually, for equal exposure (i.e. shutter and aperture), you want to use as high an ISO as you can. The noise will be almost identical but you will be more evenly using the analog to digital converter. I know this sounds heretic, but it is very easy to prove. I took a set of shots all at the exact same exposure but with changing ISOs from 200 to 1600. I converted them in Lightroom 2.4 using default settings, but with the blacks set to zero and the noise reduction and sharpening turned off. I had to add exposure compensation to the low ISO shots in order to bring them up to the same level as the ISO 1600 one. Mouseover to see the other image (first time this might take a few seconds as the alternative image will have to be loaded).



D300, 50 mm/f 1.8, 1/5 s at f 3.2.

So can you guess which one is which? The slight color difference is coming from the hue shift inherent in the Adobe profiles (Adobe standard in this case) and are not real. You can get rid of those by untwisting them. But the noise is basically identical and the detail in the image is identical. The answer is that the mouseover image is ISO 200.

Of course, if you correctly expose the ISO 200 image, you will get far lower noise. Right now, it was 4 stops underexposed. This has nothing to do with the ISO, but everything to do with the physics. Correctly exposing at ISO 200 means exposing 4 stops longer. This means 2⁴ more light hits the sensor. Since incoherent photons follow Poisson statistics, this means that the noise increases by a factor 2² (=sqrt(2⁴)), so the signal to noise ratio increases by a factor of 2⁴/2²=4. What matters in modern digital cameras that have little read noise and thermal noise (as long as you're not doing very long exposures) is simply the amount of light hitting the sensor. To get lowest noise, you should expose longer or increase the aperture. At the same time, to avoid blowing out the highlights, this means lowering ISO. Some folks make the conclusion from this that this means that lower ISO means lower noise or the inverse that high ISO means higher noise. However, as I showed above, this is simply not true. For a single camera, it is simply the increased exposure that you need at lower ISO, not something inherent to low ISO itself. So if you have a fixed aperture and fixed shutter, it is probably better to increase the ISO until you get correct exposure, which in digital often means avoiding blown highlights that you care about, than it is to underexpose at lower ISO.

ETTR?

Conversely, if you only care about noise and have ample time, you should choose your lowest native ISO and increase your exposure until you start blowing out highlights that you care about. This is the ETTR mantra that you often hear (but that people often get the physics off wrong). See here for the canonical article that is right about the outcome, but wrong about the reason for the observation. It has nothing to do with digital binning steps, but everything with photon shot noise as I explain above. Note however, that if you are in a large dynamic range situation such as a setting sun on a landscape, you are far better off bracketing a few exposures and combining them afterwards or using a graduated ND filter than using the ETTR technique. In such a setting, ETTR usually means increased noise. This is because you have to expose to not ruin the very important highlights (e.g. the mountain illuminated by the setting sun), which means underexposing the darker foreground considerably. This induces lots of shadow noise. Using a graduated ND, or exposing separately for the highlights and the foreground solves this problem.

EDIT: I learned a lot by reading an article about this a while ago. So I spend a few minutes trying to tease it out of Google again. Here is a link to the pertinent part. The author gives the correct explanation behind ETTR and reaches the same conclusion as I reached above. To quote this excellent article:

Bottom line: Read noise at high ISO is much smaller than read noise at low ISO, in terms of the error in photon counting that it represents. Thus, better image quality is obtained for using the highest ISO for which the signal is not clipped.