kapula:
Thanks Grajan!
That didn't quite do it for me though. I know how ISO works on conventional cameras. With higher ASA film the grains get bigger and less light is needed to illuminate them. But since with digital cameras there's no film and the ccd (the camera's "film") is not changeable wouldn't one get pretty much same results photoshopping picture afterwards? Ofcourse if you shoot in jpeg mode you loose some info as the picture is compressed. Basically is ISO changes done programmably by adjusting ccd's pixels bigger?
Actually you are correct kapula, if you look at the example on the link, on the digital pictures of the ISO 100 and ISO 800.
With film the only differences between ISO 100 and 800 films (2 separate films) is that the grain size or resolution on the 100 is much finer than the blocky ISO 800 here is the important issue: The colors stayed the same.
Look back on the pictures from the link: The digital ISO 800 is not only grainy but colors had really SHIFTED Badly (red dots)!
This is because, as you stated in the beginning, the digital camera never replaced the CCD sensor with a true ISO 800 sensor like in Film, with pixels that are MORE sensitive to lower light levels. The pixels in the CDD sensor is still rated for an ISO of 100 not 800. What they are doing is amplifying the signal, the problem with this is had the noise is also amplified!
Here is an example:
Let’s say for example a 1mm square silver crystal in film and a 1mm square pixel in digital needs 200 photons to properly get the correct exposure rating of ISO 100.
So we need at least 200 photons to make this work. What happens when we only have 25 photons such as in low light conditions? Well with film they made the silver crystal much larger so the film will be more sensitive to the lower amount of photons coming in and calling it ISO 800, the grain gets bigger but the color is still as true as the ISO 100.
Shifting to digital the 1mm square pixel is still the same size and is still rated at ISO 100! So now with only 25 photons are hitting a pixel that is rate for 200, what the camera manufacturers did is to increase the electrical signal. This is fine but since the pixel is not receiving the minimum amount of 200 photons the pixel is now GUESSING at the correct colors. Just like your eyes at night or in dimly lit rooms your eyes and your brain are guessing at the correct color.
This guessing maybe correct or maybe not, this is what the noise in digital is all about.
