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General photography questions and answers: Discuss Did you know.......The predicted number of shots (at least on a 20D maybe others) varies according to the iso? On large jpeg ...
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Old 13-11-2005, 17:04   #1 (permalink)
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Did you know....

The predicted number of shots (at least on a 20D maybe others) varies according to the iso?

On large jpeg at 100 iso it says 543 possible images on my 2Gb CF
On large jpeg at 3200 iso is says only 387 only.

So why is that then?

I would have thought it was the other way round

Any more useless facts out there?
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Old 13-11-2005, 17:19   #2 (permalink)
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High ISO == more noise (in theory) == less compression possible == bigger image files
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Old 13-11-2005, 17:25   #3 (permalink)
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What SammyC said.

JPEG compression works by picking out blocks of the same colour in images. So instead of storing each individual pixel as you would in BMP format, it goes "oh right, so there's a 60x40 block of colour - I'll just store that information instead of the pixels themselves" if you get what i mean.

If that block of solid colour is broken up by noise/different coloured pixels, then almost every pixel would need to be saved separately, thus making the file much bigger.

This is also why you sometimes get jpeg artifacts or a 'chequering' effect on highly compressed Jpegs.

Confused? You will be.
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Old 13-11-2005, 17:34   #4 (permalink)
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I think GfK is correct with his explanation.
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Old 13-11-2005, 17:42   #5 (permalink)
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Obvious really - I had noticed the available images count going up as well as down during camera outings... just thought it was guessing strangely
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