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Just how bad is this Tamron 28-75 2.8 ?

E

Edward Bussa

Member
I recently received a copy of the Tamron 28-75 2.8 XR Di. Then I started to notice others talking about the possibility of a "centering defect".

I had already noticed a dramatic effect on sharpness when going from 2.8 to 3.5, etc. but I don't know what is considered normal.

So, in the following image, is the increase of sharpness from left to right within what might be called a normal range or not?

tamron2875.jpg
 
Asher Kelman

Asher Kelman

OPF Owner/Editor-in-Chief
Hi Ed,

With hesitation I'm jumping in! My understanding is that there should be greater resolution at wider aperture but there also maybe poor contrast wide open. Of course, wide open the focus plane is thin. If you are off, the image looks soft.

So here the wider images might be soft because they are out of focus or because the contrast is poor.

I'd take an image of a ruler at 45 degrees to see whether or not the lens is back or front focusing. That could explain things as when you close the aperture you are getting a wider depth of field and the mis-focus is now disguised.

Asher
 
E

Edward Bussa

Member
Yup, you're right. It's front -focusing. The focus point ends up at the very rear of the DOF.

Okay, so now what?! Is this just a function of this lens design on this camera or is it a misaligned camera or misaligned lens?

Ed.
 
Asher Kelman

Asher Kelman

OPF Owner/Editor-in-Chief
So how old is thelens? There must be service centers to recalibrate them. You don't want to fiffle with your Camera unless other lenses you have show the same problem. Then it might be that one calibration would improve all your lenses. It can be adjusted. I wonder what tells the Canon the identity of the lenses. It might be possible to trick the 1DIII to correct the focus of the Tamron. The Leica M8 has a code of spots that has been decoded so now all non-Leica lenses can be matched at will to the closest Leica lens for the built in corrections.

Asher

AQsher
 
E

Edward Bussa

Member
CA dramatically spikes wide open on the wide end. However, I'm more inclined to think I'm experiencing a front-focus issue.

I don't know what causes front-focus, so if I send this lens back, is it possible a new lens will focus more accurately?
 
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