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There is some loss surely, but it may be hard to see...but I can't imagine a case using extenders to lenghten a lens when using strobes...Canons are designed for use with prime lenses over 300mm and one of the 2x under 300mm...normally for sports /wildlife
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Yep,...there is loss of image quality, that is true... the 1.4x is not too bad, but the 2.0x is much more noticable on prints larger than 5x7.. but,...what are you going to do if you don't have enough reach..
They also decrease depth of field, wich is good for me,....and decrease auto focus sensitivity....much like any f4.0 or f5.6 would in comparison to an f2.8 lens..
The list (that I am familiar with are:
135mm f2.0 L
500mmf4.5L & f4L IS
I'm not so sure about the 100-400, 35-350, or 28-300.. perhaps someone else would know..
These shots were taken with the 135mm f2.0L & 2x....(because I didn't have my longer lenses with me when I saw this unfold right in front of me.., but I did have my extenders in the side pouch of my bag..
This shot was taken with the 400mm f2.8II & 2.0x = 800mm f5.6 and shot on neg film, Fuji 800iso..
This was taken with the 300mm f2.8 and 2.0x ...it is "acceptable looking" to me
Here is the 300mm w/o an extender,...in a very demanding situation... no way could the AF of the 10D have nailed this shot if I had either of the extenders..
In all honesty,.... the sharper the lens, the better results you'll get from using the extenders...and I don't think the 70-200mm L produces acceptable images with the 2.0x.. with the 1.4x, it's pretty good.. Keep in mind, a "420mm" focal length from a 300mm with a 1.4 on it is no substitute for a 400mm f2.8 prime..or a 70-200mm with a 1.4 on it to the 300mm f2.8 prime.. no way..