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Jim <jen....not....home..remvdots...@....yahoo
>> I recently purchased a Nikon Zoom Telephoto AF Zoom Nikkor 80-200
>> f/2.8D ED. I would like to purchase a converter for it. I was
[quoted text clipped - 4 lines]
>> it. Is there truly a difference between the different brand of
>> converters? Thanks Joe
There are mainly three areas in which teleconverters differ (with
specific reference to use with a Nikon DSLR):
1) multiplying factor. This is usually 1.4x, 1.5x, 1.7x and 2x. This
controls by how much the focal length of the lens is multiplied. In
practice, the higher this factor, the higher the degradation of the
images shor with the TC. This factor also affects how many stops you
lose (1 with a 1.4x TC, 2 with a 2x TC). With a Nikon DSLR, you are
going to lose autofocus if the effective speed of the lens + TC is
slower than f/5.6 (although this may vary somewhat). Usually, 1.4-1.5x
produce good results, while 2x gives a noticeable degradation of the
image (but this is also affected by the quality of the lense).
2) optical quality and design. Some TCs (e.g., Kenko) are designed to
fit any lense, while others (e.g., Sigma) have a protruding front
element that prevent them from mounting on some (usually shorter FL)
lenses. TCs may also be designed to work best with short-medium teles
(Kenko) or long teles (Sigma, Nikon). While all popular TCs nowadays
have a rather good optical quality, they will perform best with the
specific range of FLs (and sometimes lense brands) for which they are
designed.
3) AF couplings. Nikon uses two: the mechanical "screwdriver"
connection and the electronic one. Kenko (and Tamron?) TCs provide
both, but Nikon and Sigma only the electronic one. Both types of AF
usually work slower with a TC.
With very few exceptions (e.g., the Sigma 300-800 and the Nikkor 300
f/4 seem to be so sharp to start with that the following argument does
not apply), any TC will introduce enough image degradation to produce
visibly worse results than using a longer FL lense without TC. So a TC
is a way to save money and/or carrying weight, but it is always a
compromise with image quality. In the worse cases, cropping an image
taken without a TC may give better results than using a TC.
In your situation, I would choose a Nikon 1.4-1.7x TC if your Nikkor
lense has electronic AF.
JPS@no.komm - 10 Feb 2006 22:27 GMT
>In the worse cases, cropping an image
>taken without a TC may give better results than using a TC.
If the TC is poorly made, or incorrectly used, this may be true, but in
general, if you meet the increased shutter speed requirement, and focus
properly, then the subject itself shouldn't suffer much at all, and
should be better sampled. It is the pixel-to-pixel contrast that
suffers most, not the subject_detail-to-subject_detail contrast. We
don't take sharp images to have great pixels; we do it to have great
subject details.
I used to believe than any TC-image that was showing decreased
pixel-to-pixel contrast was not worth taking with a TC, but I have come
to believe that a little bit of softness can still allow a net gain in
many cases. I can certainly see a difference between a black/white
border taken with a 400mm lens and a 2x TC and without, side-by side, at
100% pixel zoom, but with the real-world subject printed or scaled to
the same size for both, the TC image has a bit more of a natural look to
it, while the non-TC tends to show digitization and de-mosaicing
artifacts more, as they are closer in frequency to the maximum subject
detail.
Of course, some lenses are a bit soft already without the TC, and go
overboard with over-sampling if used with a TC, as I found out with my
Canon 75-300 IS when I bought a 2x TC for it. For the longest time I
thought that TCs were junk because of the results I got, but when I
bought my Canon 300mm f/4L IS I bought a Canon 1.4x TC for it, and it
worked great, and then I tried the old 2x, and it worked great, too.
(got some great shots with them stacked, actually). Of coure, you need
sufficient light for a good exposure at a fast shutterspeed (or flash)
to get sharp images like that.

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John P Sheehy <JPS@no.komm>
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