ISO Misconceptions

Started Oct 7, 2008 | Discussions thread
Fred Briggs
OP Fred Briggs Senior Member • Posts: 1,023
Re: output vs input; sensor input drives ISO choice

John down under wrote:

Fred Briggs wrote:

Misconception 2: Increasing the ISO setting creates additional noise.

Hi Fred. I see what you're getting at, but I think you should be
emphasising that the sensor input level is what drives the choice of
ISO for a desired output level. ISO doesn't drive input levels.
It's the other way around.

I thought that was exactly what I was saying! That is, if you do not have enough light (sensor input level) you use a higher ISO setting. But low light is the illness, using a high ISO setting is just a symptom of that condition. It is the "not enough light" bit which degrades signal to noise ratio (SNR).

It just seems that High ISO settings and more image noise are so inextricably linked in some peoples minds that they ascribe a direct causal connection between the two.

Also, image noise is an output function. You could argue that the
image doesn't even exist until it becomes an output. Your frame of
reference in considering noise in an image should be a properly
exposed output image, not the input at the sensor. Higher ISO
creates more absolute noise at the output as it's amplifying noise
present at the input. For a properly exposed output image, that also
means more relative noise in the output image at higher ISOs.

Yes ISO amplification creates more noise at the output in absolute terms, but it also creates more signal, while adding very little extra noise relative to the signal it receives. Hence ISO amplification does not appreciably degrade SNR. Since it is SNR which largely determines how visible the noise component of the sensor's output is in the final image, you need to look elsewhere for the cause of the reduced SNR.

The main source of the SNR degradation, visible as greater noise in the image when using higher ISO settings, must therefore be a degraded signal to noise ratio PRIOR to ISO amplification, and this is largely due to the sub-optimal light levels being received by the sensor in conditions where high ISO settings are normally used.


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Cheers John - Adelaide Australia - - Canon 50D,
40D - Fuji F100fd

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