28-105mm lens problem

I have a 28-105mm zoom that is sharp across the range above F5.6 when auto
focusing. At max apperture of F2.8-4.0 autofocusing it focuses forward of
the focus point, if I manual focus it is much sharper. The 105mm end is
better wide open than the 28mm end.

What can I do about this?

 

I have a 28-105mm zoom that is sharp across the range above F5.6 when auto

It sounds like the AF isn't hitting the spot you really intend on hitting.
Download one of the various test charts, and find out just where it's
focusing.

steve

 

Yes I have done that, that is how it is focusing in front wide open but is
fine at everything above F4.0. I would have expected that if it was out it
would be across the range.

 

Yes I have done that, that is how I know it is focusing in front wide open
but is
fine at everything above F4.0. I would have expected that if it was out it
would be across the range.

 

Return it for another or for repairs? You may have to send the camera AND
the lens in to get this resolved.

 

6 other AF lenses are perfect so the problem is not with the camera.

 

Pete,
This is a problem on some canon lenses. The question is: is it a
lens or a camera issue? I have a link somewhere to a canon document
that acknowledges the problem (a pdf). If you have the same issue
on multiple bodies, it is due to a lens problem. You can apparently
send the lens (or camera) to canon and they can recalibrate it.
Not sure what it will cost. Try contacting canon service.

Roger

 

Thanks Roger, not a Canon and I am thinking that this lens and all others
like it are just crap, it was cheap, too cheap really and as I said 6 other
lenses on my Ds are perfectly fine.

 

There might be a firmware update for the camera out, check the manufacturers
website and see.

Might need warranty repair.

MontyX

 

Its the lens, not your camera.
The internal gearing mechanisms of many autofocus lenses is not what it
should be, but if the lens will lock on focus at all the mechanism is not
likely the problem.
Autofocus is really a search for maximum contrast, which is why autofocus
works best at high contrast edges,particularly at low light levels (neurons
in the visual cortex fire at edge boundaries also, but that is another
subject).
A possible explanation is that by some fluke that in the aperture range you
describe that flare or some other optical effect is such that maximal
contrast does not coincide with accurate focus under those particular
conditions.
If you want another demonstration of the problems that dubious optical
design can wreak on an autofocus lens go to a camera store and check out the
Nikon 55-200. I have never experienced an autofocus lens at any price that
has to hunt and peck beyond and below the focus point for so long: it is not
an exaggeration to say that you can time the focus with your wristwatch.
However if you look at a light source through the lens you see a general
smear which suggests why the camera autofocus sensor cannot find what it is
looking for.