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Canon 70-200 F2.8 IS II: Does yours maintain focus while zooming?

Canon 70-200 F2.8 IS II parfocal backfocus

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#1 Ian Carleton

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Posted 21 February 2013 - 05:04 PM

Hello all!

I'm looking for insight on the Canon 70-200 F2.8 IS II.

 

Canon says that this lens is not parfocal and more often than not this is true where the image looses focus while zooming.

 

For example:
- at 70mm you focus on your subject

- you then zoom to 200mm

- your subject is now out of focus

 

From time to time however I have come across version that magically hold focus when you crash zoom in.  Has anyone discovered why this happens, or if there is a way to identify versions that work better than others?

 

Any insight would be much appreciated, even if it's just letting me know that yours holds focus while zooming.

 

Thanks in advance!


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#2 Gregory Irwin

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Posted 01 March 2013 - 11:14 AM

Hello all!
I'm looking for insight on the Canon 70-200 F2.8 IS II.
 
Canon says that this lens is not parfocal and more often than not this is true where the image looses focus while zooming.
 
For example:
- at 70mm you focus on your subject
- you then zoom to 200mm
- your subject is now out of focus
 
From time to time however I have come across version that magically hold focus when you crash zoom in.  Has anyone discovered why this happens, or if there is a way to identify versions that work better than others?
 
Any insight would be much appreciated, even if it's just letting me know that yours holds focus while zooming.
 
Thanks in advance!


It sounds like an easily correctalble problem by a certified lens technician. Basically with zoom lenses, the front element group controls focus, etc. on the long end of the zoom while the rear elements control the wide end. The middle zoom range is controlled by what's called a field lens. Somewhere inside of the barrel, at least one of these groups is out of collimation. it's an easy fix.

G
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#3 Mark Dunn

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Posted 01 March 2013 - 12:00 PM

The 70-200 is designed for stills photography. It doesn't need to be parfocal and probably can't be. It's not out of adjustment, it's not meant to be used in that way.

That said, if you zoom in and focus, then zoom out, you might have a better chance of being in or near focus when you zoom back in as the DOF is more restricted at the long end.

Only works for a rehearsed shot, of course.


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#4 Dom Jaeger

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Posted 02 March 2013 - 08:39 AM

With any zoom the front focus (controlled by the focus ring) is more critical at the long end, while back focus (the distance behind the lens where an image forms) is more critical at the wide end. The back focus is always set at the wide end, so if it is correct (and the corresponding flange depth of the camera is also correct), when you focus at the long end and zoom all the way out to the wide end, the image should be sharp. But with a non-parfocal stills zoom the focus will drift during the transition from zoomed all the way in to all the way out. And/or the focus may shift a little depending on which direction the zoom ring is turned.  

 

Stills lenses have larger mechanical tolerances than cine lenses, less fine adjustability and less stringent quality controls, so it could be that the back focus is a bit out, as well as having the focus drift or shift through the zoom range. 

 

But as Mark mentioned, focussing at the wide end then zooming in is not a good test. Even with a parfocal cine zoom, it can be hard to focus by eye at the wide end and zoom in to a perfectly sharp image. 


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