Very good comparisons here.
First and foremost, i'd like to say that all of these cameras are very, very good indeed and it's clear (as someone has already said) that prints of these will look identical. The full size pics (
not the crops) look very similar to my eye, perhaps very slightly different on colours on the 50D. It was also hard to tell which of the crops at the low ISOs was "better" than another. If I had to pick one, i'd say the D300 seemed sharper than all regarding the crops?
At higher ISOs up to 1600, they all look the same (again, slightly paler colour on the 50D here) except for the K-7 (the K20D is also softer but not as noisy as the K-7). As already discussed and documented, the high ISO on the K-7 just does not cut it. Once we're up to 3200, it looks like Pentax just doesn't cut it at all. That all said, I don't care for ISO 3200 and up on
any camera because I wouldn't use it. If I needed that sort of sensitivity to take photographs, i'd have bought a D700.
The big important factors here to the general public (in my opinion) are "what do you get for your money" and "what are you requirements as a photographer".
For me, the fact I can get a K20 body for £500 as opposed to the D300 at £1000 (and it takes awesome photographs up to ISO 1600), it just does not warrant double the money for a tiny increase in IQ and upwards of ISO 3200 performance. If I needed 6 fps and very fast/accurate autofocus, then it's a no brainer; the D300 would be my choice. How anyone can justify £1200 for a K-7 body is beyond me - I want to believe it's a good camera, but the price just isn't worth it to me as a Pentax owner - it's more expensive than the D300 and the IQ isn't up to K20D standards, let alone D300 standards. From what I've read in reviews, the K-7 autofocus speed isn't exactly competitive either (considering the K-7 is
more expensive than a D300).
I don't wish to sound like i'm bashing the K-7, I'm just so disappointed as a Pentaxian that there is no upgrade path beyond my K20D. If i'm spending £1200, I want the image quality to be at least as good as my existing camera, not taking a step back. If I move into doing sports photography, I know what camera I need.
I'll keep the K20D for my landscapes and canvas prints though
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For info, here is another thread on resolutions of these cameras (minus the K-7)
Canon EOS 50D Review: 30. Compared to (Resolution): Digital Photography Review