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06-21-2011, 05:07 AM   #1
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Autofocus (Screwdrive) Gets Stuck at a Particular Close Focus Range?

Ok so this is a strange problem, Google has brought me very little insight.

So my F 70-210 is a great lens, and for far-away stuff it's been flawless. Recently I've been trying to use it for portrait work, and I've had some recurring problems with the autofocus (which sucks, because manual focus on this lens is not exactly pleasant).

It gets stuck around 1.5 meters (1.1 is closest focus). If I override the focus manually, I can nudge it past 1.5 and it will lock focus. If the subject is AROUND 1.5, I have to manually focus to get the image properly focused.

This is the thing.... the lens will run to each end of the focus range IF the zoom is extended, but will often get stuck if the lens is at 70mm. For this reason, I can't help but thing that this is some sort of electrical problem, rather than one with the gears... but I'm asking all the lens experts for hep!

Thanks!

06-21-2011, 05:10 AM   #2
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This definitely sounds like a mechanical problem, unfortunately. To test it, take the lens off when it gets stuck and try to rotate the focusing axle with your fingers. If there's significant friction, then my theory will be confirmed. Too bad though, because the 70-210 is otherwise a fantastic lens!
06-21-2011, 06:03 AM   #3
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QuoteOriginally posted by Adam Quote
This definitely sounds like a mechanical problem, unfortunately. To test it, take the lens off when it gets stuck and try to rotate the focusing axle with your fingers. If there's significant friction, then my theory will be confirmed. Too bad though, because the 70-210 is otherwise a fantastic lens!
There is a slight increase in friction past 1.5, but not substantial.

What are my options? The lens is optically magnificent and I don't really want to sell it at a dysfunctional price.

Any chance I could clean up the gears, or something?

---------------------

Update

---------------------

I just calibrated the lens for close focus.

I set up a tape measure for 4 feet, and autofocused the lens to a target at 4 feet. The distance scale actually reported something like 6 feet.

Then, I unscrewed the focusing barrel (or whatever it is called), and rotated it so that the distance indicator matched that distance (4 feet at 70mm). It's not perfect, but very close.

Now the lens focuses without interruption throughout infinity up to 1.1 meters.

The only *problem* is that the distance indicator says that *infinity* is only at 10 feet! It focuses properly at infinity, but I can't even SEE infinity on the distance scale!

Clearly there is some sort of problem with the lens, but seeing as it is now 100% functional, I'm happy. Any idea what would cause this?

Last edited by paperbag846; 06-21-2011 at 06:55 AM.
06-21-2011, 08:44 AM   #4
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I should add that the lens will not physically focus to infinity anymore - it stops just past 10 feet. You cannot even do it manually.

So... something happened which reduced the focus throw from infinity to close focus, but did truncate performance on either end of the scale... what could this be?

06-21-2011, 08:54 AM   #5
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Don't know the lens, but it does not have a focus-limiter?
06-21-2011, 09:07 AM   #6
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No focus limiter.

It's a normal, screwdrive, zoom.
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