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Back focus caused by lens?

AlanDunne

Ottawa, CA
148 posts

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AlanDunne Registered since 09th Nov 2005
Tue 19-Dec-06 06:42 PM

I am not sure if this is the best forum to post this question. Regardless, hopefully some of you technical types can provide some insight.

I have a D200 and the autofocus, all modes, works extremely well with my Nikon lens, i.e. 12-24DX, 18-200VR, 70-200VR, 50F1.8, and a Sigma 105 macro. I recently purchased a Tamron 28-75 F2.8 as it had recevied some very positive reviews, with some cautions about unit to unit performance variation. Alas, I think I did not get a good unit.

It looks like I am getting back focus with this lens, that I don't get with my opther lenses. I will be doing some tests shortly with the infamous 45 degree test to see if I can identify this more precisely.

My question is how can a lens cause back focus? I can see how a body incorporating an out of calibration autofocus mechansim would cause back or front focus problems, but that would be seen on all the lenses, would it not? I cannot figure out how a lens would cause back focus given that the mechanism is based on searching for a maximum contrast along the focus point. If the lens was not sharp, you would still end up focusing at the point of maximum contrast, at least to my understanding.

So, can a lens cause back focus and how? Any insight is much appreciated as it will help me try to diagnose the problem further.

My early impressions are that it is a focus problem and not a sharpness problem as there are parts of the image in focus, just not where I was expecting them to be.

Regards, Alan

Regards, Alan
www.dunnephoto.ca

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