More about ISO

Started Jul 19, 2013 | Discussions thread
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bobn2
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More about ISO
Jul 19, 2013

The recent train of discussions on 'ISOless' method included much discussion on what ISO is, and one subthread raised the Noise based method of ISO exposure index determination (within ISO 12232). This method rates the ISO according to the exposure for a SNR of 40 (excellent) or 10 (very good) according to the formula Snoise = 10/H, where H is the exposure for an excellent (40:1 or very good 10:1) SNR. One problem with the noise based speed is that the SNR is a pixel level SNR, so it does not relflect the effects of resampling to output at a common size. The original SNR's were worked out by consideration of visually acceptable levels viewing a 70 pixel per cm image from 25cm, which with a standard (50mm equivalent) works out at an image of 17x12cm and about 1 MP, so I have normalised the SNR to 1MP. So, here is the Noise based ISO exposure index (excellent, 40:1) for three cameras, plotted against the nominal ISO setting:

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Bob

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