The Canon EOS and Photography Forums

My recent thinking about using a telextender on my 500 got me to thinking about the trade-off with simply cropping as I have been doing. As best I understand it:

Cropping an image from a 5dII has simple and known image degradation effects - calculate the size of the cropped area vs size of the original image. No optical effects to worry about and it can't be blurred any more than it is accidentally.

The telextender adds optical degradation due to the increased number of optical  elements (but will need less cropping) and the less than perfectly optimized design, plus there's the unknown amount of potential resolution loss due to an inadequate support and less than stellar technique - both of which are magnified by the telextender's multiplication factor.

Is there any objective evidence or well documented subjective opinion on the relative merits of these techniques?

Bob has a few articles here:
http://www.bobatkins.com/photography/tutorials/tc3.html
http://www.bobatkins.com/photography/technical/multipliers.html
http://www.bobatkins.com/photography/reviews/canon_EF_70-300_II.html

I

Keith has provided some good links there which address your question.

The bottom line is that with a high quality lens and high quality TC, you get higher image quality with the TC than by cropping the image.

However with a lower qualty lens and/or a lower quality TC, it's possible that cropping the image gives a better result than using a TC.

The best results are obtained using a prime (single focal length) lens, though high quality zooms also give good results with a TC (better than cropping). Consumer grade zooms rarely do well with a TC, though there are exceptions.

Bob:
switching to Pedantic mode, the aircraft you snapped in the 70-300 article is not an AWACS.  AWACS are Airforce and built on a jet-propelled platform.  What you saw was the E-2 Hawkeye:
http://en.wikipedia.org/wiki/E-2_Hawkeye