Re: What do you considering the upper limit for ISO?

Floyd L. Davidson wrote:

If the amplifier is set for 2x gain, the ADC will get
that same 1 volt signal at a brightness level that
produces only 0.5 volts from the sensor. That's ISO
400. If the amplifier is set for 4x gain, the 1 volt to
the ADC occurs for a 0.25 volt signal from the sensor,
and that is ISO 800.

There is a practical limit to increasing the amplifier
gain however. Noise from the sensor gets amplified too.
And indeed, noise from the amplifier increases also. At
some point there isn't enough difference between the
noise and the signal (Signal to Noise Ratio) and the
image becomes less than useful. At that point, psuedo
ISO increases are used in some cameras.


To a point that's true. But in another sense, no.

That point is reached when the amplifier gain is high
enough, and the S/N high enough, that it can reliably
detect the actual number of electrons, which then get counted.

This point has been reached for small numbers of pixels,
but not for camera-sized sensors, especially ones with
respectable dynamic range.

Doug McDonald
.

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