To crop or to use an extender with the 5DII?

[Daniel Browning]

why didn't we have a 21MP full-frame sensor in the days of the 30D if the technology was there??

Frame rate. The more pixels you have, the slower the readout (for a given cost and technology). On top of that, the DIGIC processing would slow way down too. So they could have made the 5D Classic with 21 MP, but it would have been like 0.5 FPS instead of 3 FPS. That's too slow for most of the market, and it would have sold even worse than it did.

But if we then take the 7D as an example, with its considerably
higher pixel density, then I take it that the gap between cropping and
extenders is at least narrowed?

Yes. The 7D has 49% higher linear resolution than the 30D, so there are some lenses that no longer benefit from a 1.4X TC, particularly the lowest quality zooms. (Good zooms, like the 70-200, still benefit from TC on the 7D.)

[Feanor]

Yes. The 7D has 49% higher linear resolution than the 30D, so there are some lenses that no longer benefit from a 1.4X TC, particularly the lowest quality zooms. (Good zooms, like the 70-200, still benefit from TC on the 7D.)
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Hmm. You've got me wondering again whether the 5DII is the right camera for my interests - after the mass bemusement in a thread suggesting I swap a while ago I shelved the idea but I'm really not that interested in landscape photography. The AF points and extra reach would really be a big boon. But then again shots that I do manage to take on the 5DII with the 400mm - assuming I can get close enough and assuming the AF points don't let me down - promise to be stunning.