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Lens jumps when focusing

lens focus

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#1 JeremyA

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Posted 07 September 2016 - 11:22 PM

Hey Guys,

 

Recently I picked up a used Tokina 28-70 f2.8 on eBay. For the most part it seems like a nice lens but this one seems to have some an issue:

 

 When I switch to manual focus it seems to hit some bumps and makes a grinding noise when I focus it. I  would normally be okay with it if it didn't affect the image but it does. I placed the lens on a sandbagged locked down tripod and attempted a rack focus. When it hits a certain point from focusing the entire image jumps like the tripod shakes like someone hit it.

 

Furthermore the auto focus is really really noisy (not a big deal since I don't really take stills but might be indicative of the issue).

 

I tried several different attempts and verified the tripod was locked down.

 

Has anyone ever experienced this? Is there a fix? Should I just return the lens?

 

Thanks! 


Edited by JeremyA, 07 September 2016 - 11:24 PM.

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#2 Macks Fiiod

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Posted 08 September 2016 - 04:56 AM

I looked at the construction of the lens. I assume it was designed for stills.

 

This was a problem I ran in to when I first got a DSLR with the 18-55 kit lens. Is the grip to pull focus directly glued to the front element? If so, every time your fingers make the slightest amount of contact, your image will wobble, no matter how stable the tripod is. Like literally ANY play at all on that front element and it will happen.

 

As for the auto focus feature, it might just be a "get what you pay for" deal.

Cheaper lenses made for stills won't be silent for video.


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#3 JeremyA

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Posted 08 September 2016 - 09:03 AM

The grip is glued to the focus element . I've seen the "jump" (I'm not sure a better way to describe it/I'll upload a video later in the evening) before on canon 18-135 kit lens but just assumed he dropped or damaged it as the lens was in horrible shape, I wasn't aware it wasn't a super rare occurrence.

 

Do you think the element just needs to be cleaned?


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#4 Dom Jaeger

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Posted 08 September 2016 - 08:09 PM

A lot of stills lenses have what's called image shift when you rack focus or zoom. It's very common, and not just limited to lenses where the grip is attached to the front element. It's the result of any play whatsoever in the focus or zoom mechanics, where an element might shift in centration or become tilted. It tends to be more noticeable in wider angle lenses.

 

But your issue seems to be a bump at one point in the focus travel, which is a different problem. It could be that there is a fault with the auto-focus clutch that allows you to override the motor and manually focus, or there might be an issue with the focus mechanism itself. I'm not familiar with that lens, so it's possible that's just how it is, but even consumer grade lenses shouldn't have big bumps in the focus travel.

 

It might be an easy fix, like a loose focus cam screw, but maybe returning it is the best bet.


Edited by Dom Jaeger, 08 September 2016 - 08:09 PM.

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