"Roger N. Clark (change username to rnclark)"
wrote:

Bill Crocker wrote:
As a rule, extenders do a lot to degrade the image. Added to that, Tamron
leaves a lot to be desired. There is a reason why Canon's cost more.
Consider using a Canon 1.4x, if you have to use one at all. A 3x is almost
a joke.

This is incorrect. While an extender magnifies the image of a lens,
and therefore any defects in the image, a good sharp lens
will do well with a good sharp extender. Here are some lens tests
with and without extenders (kenko pro 300 extenders):
http://clarkvision.com/imagedetail/r...lens-sharpness

I use Kenko pro 300 extenders and they are very good. I even stack
the 1.4 + 2x on a 500mm f/4 IS L lens and get excellent
results and still get autofocus with a Canon 1D Mark II.

Yeah. The main problem when I stack the 1.4 and 2.0 Canon converters
isn't the image quality (focus is a problem though with my gumby 10D),
but camera support. One needs telescope-class stuff, or draw a large
sample of images.

Get the "right angle finder C" as it magnifies the image and will
help you manually focus.

I was less than impressed with this implement. It does magnify, and
it is somewhat useful for focusing with the 2.0 alone or 2.0+1.4
stacked, the optical quality is pretty awful. (Quality in the sense
of viewing quality -- of course it doesn't affect captured image
quality). It would also have been nice if the magnification "switch"
was parfocal. These are fairly obvious, irking misfeatures for the
high price one pays...