Chatting with a freind earlier this evening, I realised that I may have confused things slightly when talking about high ISO levels in my post explaining why not all megapixels are equal.
Higher ISO film has traditionally been used to take photographs at a higher speed or in poor lighting conditions (it’s not always possible, or desirable, to use a flash) although high ISO films typically introduce grain into an image. Similarly, using a higher ISO setting on a digital camera can help in the same situations – albeit at the expense of introducing digital noise. That situation is changing as modern DSLRs such as the Nikon D3 are reported to take acceptable images at very high ISO levels (e.g. ISO 6400 – that’s six stops faster than standard daylight film used by most of us a few years back and four stops faster than the film that many consumers would have used for “action” shots).
For those of us who can’t afford a D3, it’s worth noting that squeezing more and more megapixels onto a tiny sensor will increase digital noise. For the reasons I described in my original post (the type of sensor, the technical differences between pixels and photosites, the firmware and software supporting the imaging chip and even the size of the pixels) the only real answer is a larger sensor, which is why a full frame DSLR will produce appreciably better low-light images than a digital compact camera or a cameraphone and why my Canon Ixus 70 produces terrible night-time shots on its high ISO setting.