Teleconverter: blurred at short focal length

Mamuang

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Hi! I am using the 1,7x Nikon teleconverter with a 70-200mm/f2.8 and paradoxically the photos are blurred at a short focal length (e.g. 119mm) at a big aperture until I stop down up to f10 or so - but they are not when at 340mm under the same settings - so it is not a matter of depth of field or motion blur. How come??
 

480sparky

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Have you checked for back-focus?
 
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Mamuang

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Well, there is no focus problem with the lens as such, and even with a long focal length at a wide aperture it is sharp. So it shouldn't be a back or front focus issue.
 

PaulWog

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Minimum focus distance?

Sample photos?
 

480sparky

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Which TC?

Don't assume that focusing without the TC means there won't be focus issue with it. Check for back-focus to either confirm or eliminate it as the problem.
 
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Mamuang

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I am using the Nikon AF-S Teleconverter TC-17E II with a Nikon 70-200mm/f2.8 VRII on a D810.
There are no AF problems when shooting wide open at 340mm under the same conditions - so I exclude this as the culprit.
The minimal focus distance is not undercut. It occurs with objects many meters away as well as with closer objects.
I don't have a photo handy right now, but it's just an unsharp object anyway - no motion blur and no signs of front or back focus. These should be way more obvious at 340mm with f4.8 anyway...

Could it be that the optimal aperture at 119mm with the teleconverter is just much smaller than with the other setting??
 
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Mamuang

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Hm. Shouldn't that be much more apparent with the long focal length? I don't have a problem there at all - even at f4.8 ...
 

480sparky

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The only way to know if it's back- or front-focusing is to test it.
 
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Mamuang

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Well..... After a lot of puzzling out I now discovered that there actually is a front focus - but only at shorter focal length - with AF fine tuning "+3" it is better, but this is not the case with longer focal length. As I said, at 340mm and wide open it is sharp - without fine tuning only. How can this be? Do I have to activate fine tuning every time I want to shoot at the lower focal length range? ;(
The lens itself, without the teleconverter doesn't have any front or back focus issues.
 
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Mamuang

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I guess I'll send it back then...
Thanks for your replies!
 

petrochemist

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Hm. Shouldn't that be much more apparent with the long focal length? I don't have a problem there at all - even at f4.8 ...
No!
Minor errors in registration are more significant with shorter focal lengths. Longer focal lengths need more extension to bring about the same change in focusing distance.
 
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Mamuang

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I see... Thanks!
So does it make sense to replace the teleconverter?
 

480sparky

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Does your camera allow you to adjust for backfocusing?
 

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