An excellent explanation, Guy, and I mostly agree with it.
One exception re sensor resolutions and and print sizes, though. There are cameras that make fine looking 8 X 10s that cannot hack larger sizes. In practice, people don't always back up to keep photos occupying the same fraction of their visual field until the photos start to get really large. The rule of thumb that seems to work fairly well for me is that for a print up to about 12 X 18 to be viewable even at near distances and look nice, it needs a minimum of about 200 dpi from the camera. For an 8 X 10 print, allowing for borders, that means that a 3 MP camera should be adequate; 1500 pixels divided by 200 is 7.5" for the narrow axis. In practice, I have found that with some subjects you can go down to about 130-150 opriginal pixels per printed inch and have a print that looks good as long as you use high quality interpolation, like Genuine Fractals or a program that does Sync or Lanczos interpolation. These don't add information, obviously, but do a better job of keeping the illusion that there is more information there.
The result of all this is that it is entirely possible to make nice 11 X 14 prints from 3 MP images. It depends on the subject--I have a few landscapes taken at 5 MP that don't make nice 11 X 14 prints, because of the problems Guy mentioned with leaves and pixels. On the other hand, I have some macros of frogs taken at 3 MP that make beautiful 11 X 14s. Noise is also important; when it is low coming off the sensor it seems to be possible to make larger prints from a given resolution. If heavy NR is needed, however, the maximum print size drops, and it is often better to allow some noise than to lose lots of detail.
I do suspect that, as Guy also said, somewhere around 10-12 MP is practically enough--12 MP is good enough to make a 12 X 18 or even 15 X 20 print of a landscape that looks good with your nose touching it, and anything larger would be ridiculous to view from that close, so no more resolution is needed.
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So it all depends on what you want to do with the images. If only
ever looking at images on a computer monitor then all you need is a 2
or 3 megapixel camera, but they don't make those any more.
Really, for today's monitors anyway, all you really need is a 1.2 MP camera, or 2 MP even for HDTV type monitors. That really does limit you, though, the biggest decent looking prints you can make are about 4 X 5 inches.
If only ever printing to postcard size then about 3 megapixels is
fine, but they don't make those any more. Heh, heh, I've got a 3
megapixel Olympus C-730, slow to use but excellent quality.
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See above; I think 3 MP is major overkill for postcard (3.5 X 5") size, it gives you about 500 dpi on paper, which is 2-3 times as much detail as people can see even from very close up.
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Ross Alford
http://www.pbase.com/northqueenslandphotos