I've written about this fairly often,, usually suggesting that they aren't worth it, but then I do use them as well. My take is this. If you have zoom lens or a slow lens (anything slower than f4) forget it. depending on how strong a one you get they will use 1 to 2 f stops
of speed and you get out beyond the range where much works right.
The only circumstances I find that worthy of consideration is if you own an exotic high speed extreme telephoto.---and you get a really good converter.....
All the converter realy does is to crop the picture some. You can crop it in photoshop as well. I call it optical cropping vs. digital cropping. The significance difference is
that the sensor is infront of the digital crop and behind the optical crop.
In theory if your sensor is as good as your lense-----it won't make any difference which place
the cropping takes place. If you have a K5 and use it to the max you shouldn't and won't
see much benefit from the converter as opposed to digital cropping.
I do have some exotic long lenses, and (as listed in my signature line). I do consider
using the Pentax AF 1.7x adapter or the dedicated 1.4x-L TC with someof them, but even with the best of the best its hard to see much benefit. Mostly the TC's make you feel better
when you are using the camera, and the most significant benefit I can see--is that they help
you focus on the real target more accurately whether you are using Auto focus or manual focus because the real target is bigger in the viewer.
I have an additional article and a number of sample photos on a page in my website at:
http://www.vannattabros.com/photo9.html
(this is not a commercial site)
Last edited by rvannatta; 02-13-2012 at 10:23 AM.