DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
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bobn2 Forum Pro • Posts: 42,389
Re: ISO, noise, exposure, and total light

awaldram wrote:

This gain increases the noise for any given exposure solution.

It really doesn't.

Acording to ISO 12232:2006 it does

Please explain where. So far as I know, ISO 12232:2006 says nothing about 'gain'. Nor does it say that 'gain' will increase 'noise'.

and though not often used in consumer cameras two of the calibration methods do exactly that

Do exactly what? Are you talking about the two noise based methods? They still say nothing about gain or that noise is caused by gain. And as an aside, they are not 'calibration' - that is a misuse of that word - they are two methods for speed estimation which actually produce very different results from the grey value based ones.


Not sure what your saying here ?

ISO (loosely) is the gain applied to a fixed output to achieve a standard between imaging devices.

Not according to ISO 12232:2006 it isn't. In any case, that makes no sense, even if it were (probably why it's not what the standards says). You can't apply 'gain' to an output, and in anycase the input and output are different kinds of thing. The input is luminous energy measured via photon counts, the output is grey scale values, whose luminosity depends on the illumination used for viewing.

Whilst I understand what your saying I can’t agree 100%

ISO in a digital camera is calibrated acording to ISO 12232:2006

Again, misuse of the word 'calibrated'.

Main article: Signal to noise ratio (imaging) Digital noise at 3200 ISO vs. 100 ISO The noise-based speed is defined as the exposure that will lead to a given signal-to-noise ratio on individual pixels. Two ratios are used, the 40:1 ("excellent image quality") and the 10:1 ("acceptable image quality") ratio. These ratios have been subjectively determined based on a resolution of 70 pixels per cm (180 DPI) when viewed at 25 cm (10 inch) distance. The signal-to-noise ratio is defined as the standard deviation of a weighted average of the luminance and color of individual pixels. The noise-based speed is mostly determined by the properties of the sensor and somewhat affected by the noise in the electronic gain and AD converter

No camera uses the noise based methods, nor is allowed to use the noise based methods by CIPA. The noise based methods still do not say that 'ISO' is 'gain', they don't include 'gain' at all, no do they say that 'noise' comes from 'gain'. The noise based methods produce a very different estimation of 'ISO' to the tone based ones.

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