ISO settings on digital cameras

[potok]

ok i am still confused how does camera makes changes when i change ISO

from 100 to 1600. What happens inside of that little magnesium alloy

body.

 

Does anyone know simple explanation. I know basics of film ISO. Can

you do digital "push" processing? To me it still looks like when i

change ISO to higher, camera generously gives me few more stops and

then makes the images blurier.

 

Sometimes I fell like that is function from film cameras/film we are

used to, so it was incorporated into digital cameras just to give us

the good old feeling for next few years, or is it more complex process?

[mendel_leisk]

I think there is still no free lunch. As you raise the iso, the image get's "noiser", almost like grain, though it's gradual. Do some test shots at various iso's and compare. I've been meaning to try that, as well. I'm a complete newbie with a 20d.

[potok]

i am just wondering what actually happens, and why? how is that noise created?

[ajweiss]

CMOS (and CCDs) are analog devices. There is a set of amplifiers between the chip (or on the

"out" side of the chip) and the digitizer. By changing the ISO, you are changing the gain of

these amplifiers.

[jbq]

What Adam said. There's an analog amplification stage between the pixels and the ADC(s), so that the entire range of the ADC is used.

[jimstrutz]

To further the explaination, "By changing the ISO, you are changing the gain of these amplifiers." It's like turning up the volume on a cheap radio. Yeah, it get's louder, but also adds more static. Someday we may have perfect amplifiers for each pixel, and no noise will be introduced. But maybe not.

 

Amplification is not the only source of digital noise. There is some within the pixel itself, and even perfect amplifiers would amplify the pixel noise along with everything else.

[jim_larson1]

Note that ISO settings of 100, 200, 400, 800, 1600 are holdovers from film settings.

 

It is kind of like having a stereo knob with numbers like 1, 2, 4, 8, 16.

 

I would not mind a camera capable of being set to ISO 300. (or, in my dream, have a Av mode where you fix the shutter at (say) 90, and then the camera calculates a suitable ISO).

[bobatkins]

I believe Nikon DLSRs have an ISO mode where you set shutter speed and aperture and the camera picks the right ISO.

 

Or at least so I've been told...

[jayhai]

TO answer the second part of the question, as far as i know you can not "push" the

exposure like film, because with digital you are just selecting the film speed, not the

actual film and then the iso setting.

[athinkle]

Also, both ambient and localized heat increase noise in these sensors. This is why some MF digital backs move the LCD assembly off the camera back.

[potok]

thank you for all your help, now i understand how digital ISO works. well partialy but enough

[raul_trifan]

Well, after reading all of your responses, I've got another rhetoric question: does the increasing of ISO speed setting could be harmfull to CCD or CMOS sensor? Can ISO 800 affect somehow the life of CCD or CMOS and in a few months the number of dead/hot pixels to increase for that reason?

 

Thank you.