Originally posted by northcoastgreg I'm hard pressed to notice any differences at normal viewing distances.
But here is the mistake you make.
AT normal viewing distance, the human eye can resolve 1.38 MP (which is based on the 1/1730 Zeiss formula used for depth of field calculations determining the circle of confusion). This is reflected in the true HD 1920x1080 specification which is 2 MP. Ok. Let's work with this then.
Now, there is a noticeable difference in acuity between an image at its native resolution and one sampled down to 1/4th its pixels, esp. with a Bayer filter and/or AA filters. So, let's increase the above size by 4x, leading to 8MP. This is what the 16,000$ Canon C300 does.
8MP is also what DxO uses to normalize their metrics for still photography.
Everything beyond 8MP is wasted. The *istD already had a nice balance actually.
10MP, 12MP, 14MP, 16MP, 18MP, 24MP and 36MP are all a waste of resources.
So, where do you stop? It is arbitrary if you didn't stop at 8MP.
My take is to not stop at all at any point below 100MP.
Beyond that, I am a long time advocate of zooming by cropping, what Nokia now did in the 41MP 808 Pureview camera. I rather see fast primes with outstanding center resolution than much heavier optical zoom only yielding the same performance with a fine pitch sensor. Or a combination of both where a single zoom like the 24-70/2.8G has enough center resolution to deliver the equivalent of 24-150mm 8MP photos. With good enough quality to make lens changing obsolete in many circumstances. That's what DX crop mode is really good for.