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Help with ISO speed

Just Jas 19 26.3k 1 England
8 Feb 2008 12:07PM

Quote:All I can find Ian is the GN meaning a coverage of 12m at ISO 100, which isn't a great help as the camera doesn''t have ISO 100

If you know the guide number at ISO 100, say for example it is 20, then the guide number at ISO 200 is 1.4x 20 = 28, at ISO 400 is 1.4x 28 = 40.

Where the 1.4 is the square root of 2 (1.414) rounded off.

Each doubling of the ISO is multiplied by 1.4

Hope this helps.

csurry 19 9.2k 92
8 Feb 2008 12:38PM
Aha, I knew there was a formula, but could not find it.

Well done Jas.
Nick_w Plus
13 4.3k 99 England
8 Feb 2008 5:11PM
I wonder if we are all still thinking in terms of film, where the ISO had a direct relationship to the cristal size of the emulsion, which menifested itself in terms of grain.

If you look at the reported noise levels of say Canon and Nikon the figures for ISO 100 & 200 are very similar, well within experimental error.

It makes me think that the sensors are calibrated to record optimum light at ISO 200 and that the signal is surpressed (rather than amplified) for 100. The D3 starts at 200 - and you have to use "extend" it to get down to 100 - just makes me think that is what they have been doing all along just labelling the setting as 100.
Just a thought

Just Jas 19 26.3k 1 England
9 Feb 2008 2:33PM

Quote:.....and that the signal is surpressed (rather than amplified) for 100.

If you mean less than unity gain for the amplifiers then I wouldn't think so. That would a very low output for the processing to work on.

It might well be, however, that the sensor/amplifying / processing chain produces optimum results at a gain equivalent to ISO 200.

Carabosse 18 41.5k 270 England
9 Feb 2008 2:45PM
I did read somewhere that where DSLRs offer ISO 50 that is achieved by suppression.

Which suggests that amplification takes place at maybe ISO 200 or above with the neutral point at 100.

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