Auto Focus Microadjustment - 5D III

[Eaglewing]

I am about to experiemnt with AFMA on my 24-70mm L f/2.8. I read from the manual that I need to adjust the wide end as well as the telescopic end separately.

Can anyone who has made similar adjustments please advise at what distance should this be carried out.

I read from various sites that the recommended distance is to multiply focal length with 50. So for the wide end, it will be 1.2m (50 x 24mm) and 3.5m (50 x 70mm) for the telescopic end.

And for the 70-200mm zoom, it will be 10m for the telescopic end (50 x 200mm). I plan to check out focus accuracy for this lens too.

I am also making a DIY GhettoCAL which I printed from the internet as against purchasing the DataColor SpyderLENSCAL.

This post is all about the question of using the correct distance when doing a focus test. 

Thank you in advance.

[kvbarkley]

If you know feedback theory, you know that residual errors like this are a camera *and* lens issue, not just a lens issue.

Which is why pre AFMA you sometimes had to send the lens and the camera back to Canon for adjustment.

[diverhank]

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@kvbarkley wrote:

If you know feedback theory, you know that residual errors like this are a camera *and* lens issue, not just a lens issue.

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Yes...definitely a combination of camera and lens.  I have different AFMA values for different cameras for the same lens.

[Waddizzle]

"If you know feedback theory, you know that residual errors like this are a camera *and* lens issue, not just a lens issue." 

I am well aware of that.  That's half the source of my doubts.  The same is true for the adjustments.  Adjustments at one end of the focal length on a zoom have an impact across the entire focal length, not just at the endpoint.

[Eaglewing]

Thank you all for various input, I will very likely take a basic approach as follows:

1. Adjust in 5DIII using AF Microadjustment at recommended 50x FL for both 24-70 and 70-200 at f/2.8.
2. Use USB Dock for Sigma 35mm 1.4 at recommended zones 0.2, 0.3, 0.6m and for the last zone I shall set at 3m. And turn off AF Microadjustment.

I shall see how this will turn out to be. Hopefully I do not have serious issues with front and back focus. Will report with another post once I have a chance to work on the above. I have also completed making a DIY GhettoCAL target which I will use to ascertian front/back focus error.

[ebiggs1]

"But I've yet to see an "L" lens that needed AFMA."

This is my findings also.  If I had one or two "L" lenses I would not make such a statement. But I have actually had the opportunity to use many.

It is possible to really screw things up but it is easy to set it back to O so why not go for it?  Most of the time people that think the lens needs focus adjustment are using the gear wrong.  Or simply have poor technique.  Make sure that isn't you!

[Eaglewing]

Thanks ebigg1,

I am with you on the same page with regards to mis-focus which might have been the result of poor technique on my part. Hence the need to establish first of all the the lens is focussing accurately as it shoul be.