If you look at photos taken of the same subject, the SD500 shows more detail when viewed at full size.
That is the effect of 2 more mexapixel.
The lens quality appears the same.
The SD500 has a stronger flash. 16.4 feet as opposed to 11.4 feet for the SD400.
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Get with your friend and do some comparisons, but I would not expect any significant difference.
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I just bought an SD400 and my friend got the SD500...did I make a mistake...should I have spent the extra money?!?!

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