ASA and digital cameras

[David Haas]

I understand the relationship between shutter speed and aperture but what
happens in a digital camera when you change ASA readings. Does it actually
increase sensitivity as in film? Exactly what does it do?

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D. Haas

[Don]

Yes and no. The higher speed films have larger grains, which make them more
likely to receive the required number of photons to sensitize the grain, but
also more visible. In a CCD, the "grain" (charge bucket) stays the same
size, but the signal is amplified more before it is digitized. Since the
primary noise source is the shot noise of this amplifier, the noise is
amplified more.

That's a simplified explanation. It's quite difficult to assign an ISO
(ASA) rating to a digital camera that makes it comparable to the same rating
in a film camera. There is currently no accepted standard, although I've
heard that ISO is currently working on one.

Don

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