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I have a Nikon D40. The ISO ranges from 200 to 1600. Why isn't ISO 100 (or smaller) available? Is this common in other cameras? I understand that there are technical limitations to provide higher ISO values, but are there reasons why they must start at 200? And is the fact that the smaller ISO is 200 a problem?

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The camera is a bit 'elderly' these days, but the ISO range is usually determined by the native base of the sensor itself. However, most current cameras will do ISO 100 (and the odd one even less), but the limits are usually based on the technology at the time, cost factors, etc.

For a more technical description, the ISO of a digital sensor is determined by the signal gain of the sensor and these are matched up to be similar to film. At the low end of the gain, you get low ISO, so the lowest gain of the sensor matched up to film determines that end as well as the far end. ISO 12232:2006 gives guidance to camera makers on how to actually line these up, and the options vary quite a bit.

Now, if you want a really detailed article on film speed that covers film through digital, Wikipedia has it. It's an interesting read.

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