I'm thinking of buying myself the 400 f5.6 prime for Christmas. I already have a 2x TC but it'd reduce the aperture too much on the 400, and I wouldn't be able to afford the 1.4x TC for a while.


But that got me thinking. As far as I understand it, a teleconverter just magnifies the existing image, right? And you lose a stop or two of light because the light coming in from the front of the lens is, effectively, being spread more thinly over the sensor?


Surely, assuming that loss of resolution isn't an issue (for argument, using an ∞MP camera), the image degredation (CA etc) will be the same as cropping? In fact I'd expect a cropped image to look better, as it's not being sent through any extra glass.


In my experience with using my zooms on my 5DII, image degredation becomes an issue with cropping long before loss of resolution does; and besides, a 1.4x crop is still using more of the sensor than an APS-C camera. I realise that resolution is more likely to become the limiting factor for cropping with primes, but the 5DII has so much to play with that even then I'm not convinced it will be an issue for day-to-day print sizes.


What's the opinion of the experts? On today's modern high-res sensors, should we be using TCs at all?