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Tom Cambridge May 16th, 2010 01:30 AM

canon 7d focus issue
 
I have a 7d with the 18-135mm kit lens. I am experiencing a strange issue when doing a focus pull. As I go from a shallow depth of field to focusing on a further away object, at the end of the focus the image jumps to the right slightly. I thought it may be an effect of the image stabilizer on the lens so I turned it off and had the same problem. Any similar problems anybody or any ideas much appreciated, have done some unsuccessful searches of the forums already.

Noah Yuan-Vogel May 17th, 2010 09:59 PM

It is not unusual for still photo lenses to do weird things like have shifting elements (and breathe and lose focus while zooming etc), especially the cheaper lenses. These issues are generally acceptable for still photography but pretty ugly for video. Its definitely the 18-135 not 28-135? I believe the 28-135 is the standard kit lens and it is an incredibly cheap lens. If it is the 28-135 I would not be surprised as for the short time I owned one I had many issues with it including breathing, internal dust, a loose telescoping section, and a malfunctioning/loud image stabilizer. It's great, however, if you want to spend <$200 on a lens with a lot of range for a full frame sensor camera (the range isnt great for APS-C like 7D) and dont care about much besides range and price.

How much does the image shift? it only happens right as the lens hits the internal stop at the end of its range? Truth is stuff like this may be well within the normal performance of the lens. The lens probably focuses past infinity, right (most AF lenses do this I think)? does the shift happen if you stop right at infinity or even at hyperfocal distance without hitting the end of the focus range?

Leonard Levy May 17th, 2010 10:52 PM

I'm borrowing one of those lenses right now and can test out mine but tell the truth I can't really understand what your describing. ". As I go from a shallow depth of field to focusing on a further away object, " doesn't mean anything to me.

Be more specific about what mm the lens is at and what focus change you are making. Are you in manual focus mode? Are you zooming also?

I don't find any problems with mine just changing focus from close to far at any zoom setting. I do see some weird tracking problems including pulling to the left if I zoom in rapidly, though not if I do it slowly. This is probably just the nature of the lens though.

Lenny

Bill Pryor May 18th, 2010 09:29 AM

Most still camera zoom lenses "breathe" when changing focus. Even some expensive video camera lenses do it as well. This could be what you're seeing. Generally, though, the image gets bigger or smaller, not necessarily a shift to the right.

As far as zooming while you're shooting video, still camera lenses aren't designed for that. The zoom should be used to change focal length--more convenient than switching lenses--not for zooming while shooting.

Bob Hart May 18th, 2010 10:22 AM

If your zoom lens "kicks" at the long end of its travel, it might just be one of the followers in its slot hitting end just before the other and tilting one of the travelling groups a mere fraction or a dedicated end-stop inside the lens doing the same. It does not take much tilt to cause a kick.

I can't suggest much except to train yourself to stop the zoom just before it hits the end of its movement or glue a couple of tactile marks on the barrel and zoom ring to warn you when it is getting close. You could mod the lens by adding an end-stop and overhang on the zoom ring to physically halt the movement before the internals of the lens reach their limit.

That would involve cutting, drilling and finishing two little pieces of metal, four screws and adding threaded screwholes in your lens or glue inplace of the screws and threaded holes. Alternatively you could have two clamping rings made up to go on the zoom ring and lens barrel to the same purpose. Either way it is a workshop job and no guarantees against damage.

An alternative is to buy a bridgeplate, rods and follow-focus kit and gear the zoom ring. There are clamp style focus gears available for stills lenses in several sizes from several ventors. Cinevate (Dennis Woods) in Canada does a 7D kit. Cinecity in Chandrigagh India does one as well which integrates with their Jumbo mattbox and rods kit I think.

Tom Cambridge May 24th, 2010 03:15 PM

Just uploaded a quick clip to vimeo, It is not a zooming issue, the image jumps to the right at the end of the focus. The lens is the uk kit lens; 18 not 28mm at the wide end. Not a very good clip though sorry.


Burk Webb May 24th, 2010 04:34 PM

That really looks like a mechanical issue with the lens to me.


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