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

[Daniel Browning]

They'll be the same- if anything, the second will be a little noisier due to the light loss from the extra glass of the teleconverter.

You are right if we only consider photon shot noise, but when it comes to read noise, the TC will be superior. For wildlife, I'm always hitting read noise because of the fast shutter speeds and long lenses required.

One thing I often wonder about is how many abberations are introduced by the teleconverter. Teleconverters have a nasty reputation for introducing optical problems, but my guess is they mostly just magnify those that are there already.

I think you're right: a lot of their bad reputation comes from simply magnifying existing aberrations in the underlying lens. Thankfully, the worst part of the aberrations are some C.A. and distortion that are easy to correct.

Anyone done a test of this?

I'm sure there are some good tests out there somewhere for this, but I can't recall any. It would have to be certain that the pixel size is small enough to completely oversample the lens, so as technology marches on this test will get better with more and more lenses.