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Old Sep 19, 2008, 6:52 PM   #1
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Hello!

I have a Nikon D40 with the kit 18-55mm lens, a 55-200mm VR lens and a 400mm lens that I have to focus manually (it is AF, but not "S")

I've purchased a Tamron 1.4x converter and it works pretty good with the kit lens (all the functions and AF) and with the 400mm tele (all functions but AF).

However, with the 55-200mm lens it does not work so good. AF works pretty good up to 105mm focal lenght; from there up to 200 it is erratic. Lens work very well without the converter.

Is the above a normal behavior with my equipment?

Thanks!
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Old Sep 19, 2008, 8:02 PM   #2
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The 55-200mm has a maximum aperture that varies from f/4 to f/5.6 over the focal length range. With a 1.4x TC you lose 1 stop so your maximum aperture becomes f/5.6 to f/8.

So, when you are using a longer focal length, the focus sensor in your camera body is trying to do its work using the light from a lens where the aperture is approaching f/8. This may be too dim for your camera to AF correctly.


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Old Sep 19, 2008, 8:47 PM   #3
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Thanks, FrankD!
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Old Sep 20, 2008, 10:56 AM   #4
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You're not going to have much luck with TC's and consumer grade, slow zoom lenses.
Using it with the 18-55 is useless as the 55-200 more than covers that range with much better quality and faster AF. As was mentioned, the 55-200 becomes too dim with a tc attached at anything approaching mid to full zoom.
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Old Sep 20, 2008, 12:56 PM   #5
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You are right; however I'm pretty satisfied using it with my 400mm f5.6 tele. I don't see the point using it with the 18-55mm, as you said.
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Old Sep 20, 2008, 1:46 PM   #6
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artmustel wrote:
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You are right; however I'm pretty satisfied using it with my 400mm f5.6 tele. I don't see the point using it with the 18-55mm, as you said.
Primes were what teleconverters were made for, and results are typically quite acceptable.
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