Canon is just trying to keep you on your toes.

There are differences between all three.
Sometimes you have to look close to find them.

Example...
the SD800 has a wide angle lens (28mm focal length)
the SD900 does not have image stabilization.
the SD900 has 10 megapixels.

Here is a link to a comparison chart:

http://www.dpreview.com/reviews/compare_post.asp?method=sidebyside&cameras=canon_sd700is%2Ccanon_sd800is%2Ccanon_sd900&show=all

Examine every line in the chart to find all the differences.

As far as image quality.....all the same....very good.

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