# Would you keep a lens that required +18 AFMA?



## switters (Jan 10, 2013)

Just bought the new Sigma 85/1.4. It requires a +18 AFMA on my 5D III. I'm concerned that if I ever get a new camera body, and it front-focuses even a little bit more, that I won't be able to adjust it. 

What do you think?


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## neuroanatomist (Jan 10, 2013)

I'd exchange it.


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## verysimplejason (Jan 10, 2013)

send it back.


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## AlanF (Jan 10, 2013)

What is an acceptable level for a high end L lens? What are Canon's official tolerances?


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## Mt Spokane Photography (Jan 10, 2013)

My 35mmL required a +2 on my 5D MK II and a +17 on my 1D MK III. Different bodies react differently to a lens. Canon adjusted the lens so it was on with both bodies, and it was also near perfect with my 1D MK IV and 5D MK III.
The issue with third party lenses is that Canon is not going to fix it for you, and you can have Canon and Sigma pointing fingers at each other. Exchange the lens for another. 

It will be interesting to see if Sigma's docking station solves more issues than it creates. On paper, its a step forward.


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## neuroanatomist (Jan 10, 2013)

AlanF said:


> What is an acceptable level for a high end L lens? What are Canon's official tolerances?



I've never heard anything 'official'. But personally, I'd prefer to stay out if double-digit values. 

What I can say is that the AF system spec is 'within one depth of focus' for standard-precision AF points, and 'within 1/3 the depth of focus' for the high-precision f/2.8 point(s). Also, each unit of AFMA is 1/8 the depth of focus. So, anything over |8| units of AFMA means the lens is outside the accuracy spec for the AF system. 

On my 1D X, the tele end of my 16-35L II needs +7, all other AFMA values are |4| or less, many are zero. On my 7D, they're all |4| or less. On my 5DII, values were also low - but at one point, I dropped the body to the pavement  after which the camera functioned fine, but all my AFMA values were negative shifted by ~10 units, so at that point, I had a couple of -9's and a -11.


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## switters (Jan 10, 2013)

neuroanatomist said:


> Also, each unit of AFMA is 1/8 the depth of focus. So, anything over |8| units of AFMA means the lens is outside the accuracy spec for the AF system.



That is very helpful, and intuitively makes sense to me as well.


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## Dylan777 (Jan 10, 2013)

Ask for another lens...+/-8 is my max

Exp. I tried 3 diff. Canon 50L lenses. Two were around -6 to 8. I returned two and kept @ -1.


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## AlanF (Jan 10, 2013)

I spoke to a techie at Canon this morning asking whether +8 for my favourite camera and lens was OK and what were the canon specs. There are no Canon specs for AFMA as depth of field varies with distance and aperture, and the setting varies with distance and zoom focal length. They would be happy to calibrate my lens and camera, but we left it that + 8 was quite tolerable.


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## neuroanatomist (Jan 10, 2013)

AlanF said:


> There are no Canon specs for AFMA as *depth of field varies with distance and aperture*, and the setting varies with distance and zoom focal length. They would be happy to calibrate my lens and camera, but we left it that + 8 was quite tolerable.



Note that above, I indicated depth _of focus_. There are Canon specifications for that (provided to me by Chuck Westfall), as related to the accuracy of the AF system and how AFMA affects that. Depth of focus is related to depth of field, but depth of focus applies at the AF sensor itself, and is measured in the µm range. That's the relevant spec, because as you point out, subject distance affects depth of field - but it doesn't affect depth of focus. The spec applies at the max aperture of the lens (whatever that aperture is), because AF is always performed with the lens wide open, regardless of the aperture selected for the shot.

I agree that +8 is fine. My preference to keep to single digits is an attempt at future-proofing - if a lens needs +8, and a future body purchased is 'off' by a similar amount in the same direction relative to my current body, the new combo will still fall within the 20-unit range.


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## MichaelHodges (Jan 10, 2013)

switters said:


> Just bought the new Sigma 85/1.4. It requires a +18 AFMA on my 5D III. I'm concerned that if I ever get a new camera body, and it front-focuses even a little bit more, that I won't be able to adjust it.
> 
> What do you think?



Way too much, IMHO. Plus, if you ever decide to sell the lens to someone with a non-MFA body....

I have a rule of no more than "10" either way for a new lens. Good luck.

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http://michaelhodgesfiction.com/


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## wickidwombat (Jan 10, 2013)

mine was +13 i sent it back to sigma to be recalibrated it came back a week later and has been perfect ever since
just get it recalibrated


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