(click image to view larger)
That means, on a digital camera at least, noise. The limits are constrained by the conditions under which you are shooting and, using the 90mm, the lowest speed with which I can now (usually) get a sharp shot hand-held is 1/125th. So, with the lens wide open at f2.8, the only option I have left is to increase the senors sensitivity. Push that too far and you get digital noise showing in the shadows and eventually over the whole image.
However, because we are used to film being grainy, this can sometimes be an advantage pictorially. Unfortunately, digital noise doesn't behave visually in the same subtle way as film grain. Film grain varies in depth and intensity proportionate to exposure and development and blends into highlights and shadows in an attractive way. Digital noise doesn't. It's uniform and the human eye recognises this uniformity all too readily. It's not generally very attractive and is usually worth avoiding at any cost. But sometimes you have no option ...
These two shots show how the Leica performed recently for me under very poor conditions. It's surprisingly successful in monochrome giving a fair 'filmic' quality (to the monochrome image at least). The colour shot is the unworked whole frame and the cropped area the tweaked final image. The result is acceptable I think and shows surprising good detail despite the noise which looks more or less like film grain.
Shot at 125th second, f.2.8, at 1600 ASA. That's three stops faster than normal. Actually, because the image was underexposed even then, I pulled the exposure in RAW so the sensitivity is more like 3200 ASA.