Limits of digital photography

Reading the constant complaints about the need for fast ISO speed on the camera forums, it is time to ask, what is the maximum useful ISO level anyway?

Well one way to decide would be to look at the light levels for the most extreme photographic condition, moonlight is 0.27 Lux.

The other two factors in the equation are film speed and lens speed. We might in theory want to take pictures at 1/8000th but this is not very likely. A more sensible limit would be 1/50th, the longest exposure for which we might take a hand held shot with a 200mm focal length lens with optical vibration reduction.

Lens speed is also somewhat arbitrary. Lets pick f/2.8.

So what ISO speed allows us to take pictures at 1/50th and f/2.8 in 0.27 lux?

Well doing the math from Wikipedia suggests that this would take ISO 174,000, or three stops higher than the top, pushed ISO speed on a Nikon D3.

In other words it should be just about possible today with a D3 at the absolute maximum ISO setting if you have an f/1.4 lens and are willing to accept a bit of noise and do some post processing.