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.

[diverhank]

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@Waddizzle wrote:

"Luckily the green dot (telling that you are in focus) remains working when the lens is switched to MF.  The green dot will also take into account the AFMA correction so that's why this method works." 

That's the part that I'm not convinced about.  You can see the AFMA data in EXIF when the lens is in AF, but not MF.  In fact, it makes sense for it not appear in MF mode, because no focus micro-adjustment should be made.

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I hear you and I'm not saying that adjustment is actually made to the picture while the lens is on MF...it's just the green dot is still working just fine with MF and this enables you to obtain the adjustment value.  You should try it and see for yourself :).

[Waddizzle]

"I hear you and I'm not saying that adjustment is actually made to the picture while the lens is on MF...it's just the green dot is still working just fine with MF and this enables you to obtain the adjustment value.  You should try it and see for yourself." 

Oh, I have done many times.  I think it is best to perform the calibration a few times, and use an average value of the repeated tests as the final number to enter into the camera. I use focus charts.  I've oriented the charts vertically, horizontally, and at an angle so that you can better see the DoF.  I think I'm pretty meticulous about it.

However, Dot-Tune just doesn't seem to yield highly repeatable results with zoom lenses, where you perform two calibrations at either end of the zoom range on some cameras, but just one calibration on some of the older cameras.  On the older cameras with just one AFMA point for both zooms and primes, there seems to be little purpose in doing a calibration with zoom lenses.

I think Dot-Tune may be adequate for primes, but I wonder the validity of performing the second calibration by manually focusing the lens.  It seems to me that you're creating two independent setpoints, when the cameras use them together.  There just feels like there is something inherently wrong with that picture. 

Pay me no mind, though.  I'm guilty of always thinking too much, anyway.

[diverhank]

I myself have used the traditional method...I've recently tried dottune on a prime and the result matched perfectly with my previous method that's why I've been recommending it for beginners because it takes the thinking out of AFMA.

AFMA is a double-edged sword though...there are just so much variables to have just one set of numbers for a lens...I was able to prove to myself that by just turning on an extra light, the AFMA number changed significantly from front to back focus...it's just nuts...the only useful AFMA that I have is on my 100mm f/2.8L IS Macro where the light I use is fairly constant...a big difference a correction of -1 makes for razor sharp IQ...On my 85mm 1.2...I was thinking about setting it back to 0...I shoot in too many lighting conditions...

[kvbarkley]

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@diverhank wrote:

AFMA is a double-edged sword though...there are just so much variables to have just one set of numbers for a lens...I was able to prove to myself that by just turning on an extra light, the AFMA number changed significantly from front to back focus...it's just nuts...

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I believe your problem could be infrared related

http://www.bobatkins.com/photography/technical/focus_vs_light_source.html

[diverhank]

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@kvbarkley wrote:

I believe your problem could be infrared related

http://www.bobatkins.com/photography/technical/focus_vs_light_source.html

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Thanks. Great article.  This was exactly what I was experiencing...until now, I thought nobody knew what I was talking about. 

[kvbarkley]

Obviously, Bob did  not know either. 8^)

Whiich actually makes sense when you think about it, the focus assist light on the strobes is mainly IR, so we know the phase detect is sensitive to IR. And us old geeks know that there were IR focusing marks on older lenses.

[Eaglewing]

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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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Thanks Bob, it is good to know. I don't think I have a problem but it is good to be able to confirm that the lens is focussing accurately.

[Waddizzle]

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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.

[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.