http://www.nikonchat.com/d90-nikon/
For point and shoot the lowest ISO is 64. So why start the Nikon D90 200?
Does this have anything to do with the size the sensor? If so, an ISO 200 DSLR (Nikon D90) would be on par with 35 mm 25 ASA, or AAS 64 in terms of resolution? However, the value found was lower than 64, may be there is less.
ISO (which is essentially the same as ASA) has nothing to do with the decision – there is no a correlation (almost). D200 starts at ISO 200 (ISO 100 is also available as "Low" or similar I think) because the sensor is good enough (which has to do with size, but also the design / production quality) to perform in this great sensitivity. The p & s especially older and / or cheaper can perform horribly at ISO 200, so its lowest ISO is set to 64, which may be difficult to use anywhere but on the outside bright (or flash), especially taking into account the slow lens that are usual for this type of cameras, but when used correctly yields decent results. The resolution is a very subjective term, is not related to ISO, however as they meet with the ISO, you get more grain, so you can claim poorer resolution. All in all resolutions of the current digital camera and D90 is no exception, is generally lower than a resolution of quality good movie. What does it mean for you – that, with equivalent optical (of the same lens mounted on a body of a film or digital) would probably be able enlarge, if you shoot with film. If the lens is good quality – we're talking poster sizes as 24×36 inches or larger. Up to 20×30 you can easily make with most current dSLR with proper techniques of shooting and not be too high ISO to avoid unnecessary noise. And of course the film – usually more under ISO (ASA), the highest resolution. Except for the noise, the resolution dSLR reality remains the same – either ISO 100 or ISO 3200. LEM.
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