I just compared most of my teleconverters, so I could
choose which ones to use with my 300/2.8.
The best two were fine wide open. They became excellent
one stop down from wide open.
The TC's were tested on a Tokina 300/2.8 AT-X manual
focus lens. Note: none of them except the Albinar
caused any vignetting or significant light fall-off in
the corners.
Here's my impressions:
Excellent:
Kiron 2x (MF)
Pentax 1.7x AF Adapter
Good:
Kenko 1.5x AF
Promaster (Kenko) 1.7x AF
Not so good unless stopped to f8:
Kiron 1.5x (MF)
Unacceptable (serious barrel distortion)
Albinar 2x Macro Focusing TC
Note: The excellent Pentax XL convertors won't fit this
lens. Darn.
I heard good things about the Kiron 2x manual focus
teleconvertor, so when I had a chance to get one
recently, I did. It is even better than I expected. It
was the best TC of the lot, slightly better than the
Pentax 1.7x AF Adapter. I believe this Kiron 2x TC
rivals any K-mount manual focus TC out there, except
for the Pentax XL series.
The Pentax 1.7x AF Adaptor was good at f2.8, very good
at f4, and excellent at f5.6.
After being impressed with the Kiron 2x TC, I had high
hopes for the Kiron 1.5x TC. But I was disappointed.
The 1.5x is probably matched well to some lens, but
it's sure not this Tokina 300/2.8.
The el-cheapo Promaster 1.7x AF TC ran a close third in
the tests. But it loses so much light that it hunts in
AF mode on my F* 300/4.5, so I stick with the Kenko
1.5x for an AF teleconvertor.
As for the original poster's problem with out-of-focus
images, I would think that any TC that gives
exceedingly soft images should be tested further to
isolate the problem. Even a bad TC should not cause
completely out-of-focus images unless there is some
alignment or other mechanical problem somewhere in the
lens/TC/camera system.
--
Happy Trails,
Texdance
http://members.fortunecity.com/texdance
http://members1.clubphoto.com/john8202
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