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.

[RobertTheFat]

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@Waddizzle wrote:

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@RobertTheFat wrote:

There's no reason you shouldn't give it a try. But I've yet to see an "L" lens that needed AFMA.

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I don't know what to think of AFMA, anymore.  If it is not needed, then what about manufacturing tolerances?  Why would a company like Sigma come up with a specialized tool like Sigma Dock?  I don't know what to think of AFMA, anymore.

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Believe me, there are times when you do need it. My EF-S 17-55mm f/2.8 IS would not be usable without it. OTOH, my wife's 17-55 doesn't need it, which is lucky because she used it for several years on her T2i before I got her a 7D Mark II. I could not have used my 17-55 on a Rebel.

[Waddizzle]

"Believe me, there are times when you do need it. My EF-S 17-55mm f/2.8 IS would not be usable without it. OTOH, my wife's 17-55 doesn't need it, which is lucky because she used it for several years on her T2i before I got her a 7D Mark II. I could not have used my 17-55 on a Rebel." 

Yes, some correction is most definitely needed in some cases.  I'm just not certain how to determine what might be needed, and then how to best determine and implement it. 

The posted link the article about light explains a lot about the discrepancies and lack of repeatabiity that I have observed.  I've repeated the data gathering phase of Dot-Tune under a variety of lighting conditions as it turns out, both indoors and out.  Light sensitivity may also explain why the manuals suggest performing AFMA at the venue where you will be shooting.

I'm pretty sure that no AFMA is applied when the camera is in MF mode, even though the AF system may beep and report that the camera is focused.  The only way for Dot-Tune to succeed would be for AFMA values to be ignored when the camera is in MF mode.  If not, then both AFMA values should be at zero when the focus range measurements are taken.  In fact, I seem to get better results when I don't enter AFMA values until I've completed focus range measurements at both wide and tele ends.

[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.