Quote Originally Posted by peety3


Go read this: http://www.lensrentals.com/news/2009.01.06/teleconverters-101


Or, if that's too much to read, here's the bottom line: "The end result of all this is that the best teleconverters are ‘tuned’
optically to work best with supertelephoto prime lenses. They are OK
with telephoto lenses, and not very good with standard range lenses."


What does "tuned" mean? They don't correct for aberrations of specific lenses, do they? Aren't they just diverging lenses?


I believe that teleconverters work best with supertelephoto lenses, but I doubt it is because they're somehow specificlly designed to work with long focal lengths or with specific lens designs (though if I'm wrong and someone would be kind enough to educate me, I would appreciate it)


Also, even if true, the statement does not answer the question "which is better, to use a teleconverter or crop?"


In my mind:


Crop advantages:


1) No additional glass to degrade image (TC's probably add CA and other abberations, but I dunno how much. Probably not much.)


2) autofocus doesn't get super slow


3) More flexible... you can "extend" not just to 1.4x or 2x but by whatever factor you want. Or, if after the fact you decide you want a wider picture than you thought you would, you can just not crop it (or not crop it as much)


Teleconverter advantages:


1) You don't throw away pixels


2) Less read noise because you're shooting at a higher ISO