Russell, the calculations you made are mostly right except I think that
you can go down to fax resolution if you have to.

Also one item you do not mention is the lens distortion and depth of field
required to shoot a A4 document full frame. You will need extra lights, a
VERY steady hand, and achieve a perfect parallelism between the camera CCD
plane and the document (to avoid any part of the image being out of
focus). I have done this and I have a full photo studio's equipment to
play with and it is not easy. For example getting fine details in a
printed circuit board drawing with components etc elsewhere than in the
center of the picture is a major undertaking and requires special tweaking
on each take in despite of using a proper rock solid repro frame and
camera stand with proper photo lights and a full manual control lens (to
be able to menipulate the f number to remove shading and unsharpness on
the whole frame).

Due to the way most contemporary digital still cameras work you will have
to aim for a higher end one which allow at least full manual control of
all the lens parameters (esp. f number).

This rises the question whether you could not use a portable or
semiportable flatbed (not handheld) scanner for this at 1/10 to 1/100 of
the price of a 3 megapixel camera. I do not have a model number for you
but some portable flatbeds have a removable cover so you can put them on
top of what you want to copy instead of putting the thing to be copied on
the scanner.

Even a lowly $60 scanner will outperform the 3Mpxel camera easily for
contact copy (scanners can afford high resolution because they use a
single linear CCD so the price should be roughly the square root of an
equivalent pixel resolution CCD).

Also beware how the pixels are counted. A 1CCD camera with 3Mpixels does
not give the effective resolution you think it gives ...

hope this helps,

Peter

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