I am about to upgrade to a Canon 7D for my birding using my 400mm 5.6L lens. With the 1.6 crop factor sensor that gives (roughly) a magnification factor equivalent to my other option, a Canon 5D mkII with the same lens plus a Canon 1.4 extender.
Assuming all other things are equal and the cameras are set up and handled correctly, any suggestions as to which option is most likely to produce the best image quality?
I would choose the 7D for a few reasons:
The effective maximum aperture of the 5D Mark II combo will be f/5.6 X 1.4 = 7.84, nearly f/8. This will somewhat cancel out the light-gathering advantage of the full frame camera.
You will still have a bit less effective reach with the full-frame camera, even considering the small pixel-count difference and even if you crop. Cropping always degrades image quality as well.
While the 400 f/5.6L is not a bad prime by any means, its image quality will noticeably suffer when using the 1.4X teleconverter (mouse over the image to see the differences), further reducing any seeming advantage of the full-frame setup. You would not much like the results with a 2X TC, let alone stacking TCs.
If 560mm is not enough reach, you will have "nowhere to go" with the full-frame setup, whereas on the 7D you can still slap on the teleconverter and likely get a much better result than you would by cropping the full-frame image. (And, remember, the 7D without the TC will have a bit longer effective reach than the 5D Mark II with the TC.)
Optical differences are hard to assess without comparing both in the field and filling the frame with the intended subject. Note however that besides the optical impact of the TC noted above, the 7D + naked 400 f/5.6L would be expected to have more consistent edge and corner performance than the 5D Mark II + 400 f/5.6L + 1.4X TC. Balancing this "sweet spot" effect of using a crop camera is that the lens will be challenged a bit more by the higher pixel density of the APS-C camera. Still, with such a high-quality prime, you will still get quite excellent results on the 7D wide open.
The 7D offers other advantages in autofocus, frame rate and build.
You may also note that the pixel-count difference here is nearly negligible, only about 300-400 pixels in each dimension. For all these reasons I'd lean toward the 7D, at least without a chance to field-test them both.
Using an extender will usually result in less image quality, all else being equal.
One situation where all else isn't equal will be low light. You can't automatically assume full frame will be better. That would depend on the sensor.
If you have plenty of light and you are limited by glass, crop sensors deliver better images based on my experience.
The lens plus converter is f8—won't focus on a 5dII. I think the only body with f8 AF is the 1dx.
Even if it did focus, being a stop slower mostly eats up the advantage of FF.
Converters are never optically perfect. They WILL soften your image. But often not enough to matter.
The only way a converter makes sense over a crop sensor is if it gets you significantly more pixels in the final image. The 5dII only has a 3mp advantage over the 7d, and even that's mostly lost to the 1.4x vs. 1.6x difference.
The one reason I'd go with the 5dII-plus-converter is a history of AF issues on the 7d. But if your AF is behaving well there's no advantage to using FF in this particular situation.
I'm not a bird photographer and I never had such a long lens, but as I understand the Full Frame 5D mkII should give you much better quality in terms of light, considering that you're using a f/5.6 lens. but on the other hand the 7D is a much faster camera, with 8fps, and it should be very useful for fast moving subjects.
5Dmk2 is a 21MP camera, that's 3MP more than 7D, so you can always crop the images and it should cover your need of a longer lens.
