# Amount of front focusing shifts between f2.8 and f4.0 70-200 II



## langdonb (Aug 6, 2016)

I am doing some manual AFMA on a new 70-200 mark II on a 6D body. I set a value of -7 for 200mm, 2.8 which makes it perfect focus. When I do test shots at different f stops, there is a very noticeable change to front focus at f4.0. Same problem setting up for 70mm. I have never experienced this with past AFMA's on different bodies/lenses. Can anyone tell me what the problem might be?


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## Mt Spokane Photography (Aug 7, 2016)

Focus shift is caused by uncorrected spherical aberration, the 70-200mm f/2.8 L II has very little. If your measurements are correct, its possible that the lens has a internal problem, some of the elements may have issues.

Exchange the lens if you can, otherwise, send it to Canon for testing and repair.


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## Hjalmarg1 (Aug 7, 2016)

Mt Spokane Photography said:


> Focus shift is caused by uncorrected spherical aberration, the 70-200mm f/2.8 L II has very little. If your measurements are correct, its possible that the lens has a internal problem, some of the elements may have issues.
> 
> Exchange the lens if you can, otherwise, send it to Canon for testing and repair.


+1,
Canon 70-200mm f/2.8 L II normally is the best 70-200mm lens in market with regards to focus shift, yours may need a visit to Canon workshop


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## AlanF (Aug 7, 2016)

I don't have direct experience of uncorrected spherical aberration so please correct me if I am wrong. Uncorrected spherical aberration usually means that at wide aperture the lens front focuses. As the aperture, decreases then the the focus moves further away. But, if I understand the OP correctly, his focus, his focus is moving to front focus when stopping down. It's like his lens has over-corrected spherical aberration. Right or wrong, his lens needs to go back to Canon.


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## langdonb (Aug 8, 2016)

Thanks to all who responded. I will send it back for repair as it is beyoond the time to exchange (it is a refurbished direct from Canon).


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## AlanF (Aug 8, 2016)

As a matter of interest, what focusing mode were you using? I would guess not just centre point.


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## langdonb (Aug 8, 2016)

AlanF said:


> As a matter of interest, what focusing mode were you using? I would guess not just centre point.



I always use center point focus point, particularly on the 6D for the obvious reasons (that being the 6D is weak otherwise) .


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## GMCPhotographics (Aug 19, 2016)

langdonb said:


> I am doing some manual AFMA on a new 70-200 mark II on a 6D body. I set a value of -7 for 200mm, 2.8 which makes it perfect focus. When I do test shots at different f stops, there is a very noticeable change to front focus at f4.0. Same problem setting up for 70mm. I have never experienced this with past AFMA's on different bodies/lenses. Can anyone tell me what the problem might be?



The focus point will shift also depending on the distance to the chart. Less so at infinity focus. It'll also change according to the focal zoom and aperture. So AFMA is a best endeavors solution than a perfect solution scenario. Also the colour of light can affect it too. Do the adjustment at home, at night under tungsten light and it'll be completely different compared to daylight.


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