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Dot-Tune: Autofocus Fine Tuning in under 5 minutes
  
 
snapsy
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p.9 #1 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


WrightImage wrote:
Thanks for this snapsy, been very interesting. I tried an half hearted attempt last weekend with my 7D & 400 L 5.6 but the blizzard cut it short. I must say that doing the 50 times the focal length (around 21 yards) was a pain to manual focus for critical focus! I shoot mostly small song birds and am 90% of the time inside 15-20 feet. Do you really have to do the 50X for this to be correct?
Dave


Thanks Dave. You can actually use any focus distance you'd like. I only suggested 50x because that allows for a workable distance when a good AF target like a focus chart is available. You can use other distances and/or other AF targets, provided there is good detail and lighting.



Mar 01, 2013 at 01:10 AM
Zenon Char
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p.9 #2 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


Not really. 50X is suggested by Canon when you search for some info on this which I find puzzling - that you have to actually search for it. Here are a few things out of a typical manual.

The first paragraph is interesting as well.








Mar 01, 2013 at 01:21 PM
WrightImage
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p.9 #3 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


Thats some good info. I feel like i might have a slight back focus issue but after the quick MA at 20 ft i ended up with a -8 setting! Then i took several shots of birds at my normal shooting distance. Needless to say i then had a front focus issue, lol. I'll give it another shot Sunday. Thanks guys.
Dave



Mar 01, 2013 at 04:28 PM
Davis B.
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p.9 #4 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


Puzzling info
I ran this on my 150-500, and it showed +14 to start to focus and > +20. when I ran this on focal, I get +2, could there really be that big of a difference focusing thru the mirrors vs sensor?
Maybe I am not doing this right, did the 50X caculations and ended up with approx 52 ft.
I dont know whether to send the lens to Sigma or not.



Mar 01, 2013 at 06:17 PM
WrightImage
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p.9 #5 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


My 400 mm at 50 x the focal length = 65.61 ft. A 500 mm x 50 is 82.02 feet unless I'm doing the calculations wrong!



Mar 01, 2013 at 08:44 PM
robinlee
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p.9 #6 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


snapsy, still waiting for your auto AFMA tutorial using ML


Mar 01, 2013 at 09:06 PM
snapsy
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p.9 #7 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


Davis B. wrote:
Puzzling info
I ran this on my 150-500, and it showed +14 to start to focus and > +20. when I ran this on focal, I get +2, could there really be that big of a difference focusing thru the mirrors vs sensor?
Maybe I am not doing this right, did the 50X caculations and ended up with approx 52 ft.
I dont know whether to send the lens to Sigma or not.


50x was just a guideline. Can you try a shorter focus distance and see if that gets you within the +/- 20 range?



Mar 01, 2013 at 09:15 PM
snapsy
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p.9 #8 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


robinlee wrote:
snapsy, still waiting for your auto AFMA tutorial using ML


, I'll get it going when DotTune becomes available in the next non-beta release of ML.



Mar 01, 2013 at 09:17 PM
Wilba
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p.9 #9 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


WrightImage wrote:
Do you really have to do the 50X for this to be correct?


For the absolute best results, do it at the distance you're shooting. 50f is an arbitrary compromise that may or may not have a sound basis - no-one seems to know. Canon's Service Centres don't stick to that rule so you shouldn't feel compelled to (see You Can't Test Autofocus with a Slanted Target in Busted! Digital Photography Myths).



Mar 02, 2013 at 06:18 AM
WrightImage
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p.9 #10 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


Thanks wilba, good read and info.


Mar 02, 2013 at 03:09 PM
 

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Zenon Char
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p.9 #11 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


This has been my argument from day one. Dozens of methods and all types of suggested distances. For me it is hard to comprehend. Expensive precision equipment being judged with household measuring tapes. I sure hope they don't apply this approach when they service any planes I fly on . Forget the technical service manual, we found a short cut that works better. Sorry for the rant. 25 years technical training in manufacturing. I have seen it all. Wide eyed, stunned looks on how well equipment runs after we got the crews to put their tool boxes away that contained snakes and rattles and set it to manufacturer specifications. Problem here is Canon does suggest a method but few follow it. Why?

I like DotTune because it is quick, simple and much like FoCal the system tells me what is out. I don't want to make that decision. If any lens I own is way out it will go to Canon regardless. Minor tweaks - I'll use DotTune.

Edited on Mar 02, 2013 at 04:11 PM · View previous versions



Mar 02, 2013 at 03:10 PM
kevindar
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p.9 #12 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


How is this method any better than the computer patter interference method? for one, knowing you have achieved critical focus with the computer screen method is a lot more easier. second, why move all around when you can just repeatedly and reliably see the focus is not moving/changing, once you have achieved perfect phase detect focus?


Mar 02, 2013 at 03:36 PM
Zenon Char
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p.9 #13 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


Forgot to mention. Chuck Westfall suggested 50X in an article/interview I read while the manual states at location. Chuck also stated that if you could do better go for it. I hope he does not service planes


Mar 02, 2013 at 03:38 PM
clarence3
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p.9 #14 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


Wilba wrote:
(see You Can't Test Autofocus with a Slanted Target in Busted! Digital Photography Myths)


I like the photo of Canon's AFMA test rig that they left on the memory card of a camera/lens combo sent in for calibration...
http://2.static.img-dpreview.com/files/articles/2978485979/2407739.jpg

The moire pattern on the left really helps find the center of the plane of focus.

But that doesn't seem like 50x focal length, unless it was a wide lens. (ah... EXIF shows 18mm... x50 = 900mm = 3 feet)



Mar 02, 2013 at 03:43 PM
Zenon Char
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p.9 #15 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


I did some calculations at 50X a few years ago and got these numbers.

50 = 8.5 ft

100 = 17 ft

105 = 18 ft

200 = 33 ft

300 = 50 ft




Mar 02, 2013 at 04:45 PM
Mike K
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p.9 #16 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


kevindar wrote:
How is this method any better than the computer patter interference method? for one, knowing you have achieved critical focus with the computer screen method is a lot more easier. second, why move all around when you can just repeatedly and reliably see the focus is not moving/changing, once you have achieved perfect phase detect focus?


Kevindar, see my slightly different AF calibration method at the beginning of this thread.
Both approaches use magnified Live View as the "standard" of accurate focus. I suggest an interference pattern only because it can be used on screen, without having to print it out. I like it as the pattern appears to change as the the focus changes, so its easier to assess AF comparisons. However, as the focal length gets much longer, the recommended 50X focal length will place you away from your computer, perhaps outside.

The main difference is that the dot tune uses the focus confirm "dots" as a measure of establishing the boundaries of PD AF. I simply press AF to see if magnified LV MF matches the PDAF setting. If not, change the micro adjust setting until Magnified LV matches PD AF. I repeat this comparison several times until I am satisfied I am within the normal imprecision of the PD AF method.
I believe Snapsy developed this AF calibration method on Nikon bodies, where the focus confirm lights provide more directional information than with the Canon set up.

As for the 50X focal length, this was suggested by Canon (yes Chuck Westfall) to minimize the impact of focus shift, a common phenomenon with wide aperture lenses (ie f1.2) at shorter (portrait or closer) distances. Obviously calibrating using the distance you are going to shoot at is the most accurate answer (but only for that specific distance).
Mike K



Mar 02, 2013 at 09:43 PM
BrianO
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p.9 #17 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


Zenon Char wrote:
Forgot to mention. Chuck Westfall suggested 50X in an article/interview I read while the manual states at location. Chuck also stated that if you could do better go for it. I hope he does not service planes


Of course adjusting it to the exact distance from which you'll be shooting at a particular location will be the most precise; but not everyone has time to do that at every shoot.

50X is a rule of thumb that gives good -- if not perfect -- results for most uses.

Using your airplane analogy, I can calibrate my magnetic compass while facing north in the hangar at my home airport and it'll be spot on...when facing north in the hangar at my home airport.

Alternatively, I can check it at the compass rose on the field, while facing in several directions, and apply a correction that will average variation and deviation and use that. Since I'm not usually sitting in the hanger when flying I'll choose the latter as being the better method for my use, even though it may not be exact in any one circumstance.



Mar 04, 2013 at 05:13 AM
Zenon Char
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p.9 #18 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


Interesting. I have never had come back with this type of explanation.


Mar 04, 2013 at 01:31 PM
EL_PIC
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p.9 #19 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


Do any camera or lens manufacturer use this method at final QC ??
or any other method ??



Mar 04, 2013 at 08:09 PM
Zenon Char
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p.9 #20 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes


I doubt it. They use projection equipment and have pretty sophisticated measuring methods. I also doubt there is much measuring or anything else at final CQ. Maybe a sampling from time to time but not all. Final QC is a waste of time, resources, a bottle neck and an added expense in the world of Japanese or also known as the lean manufacturing process. The process itself is supposed to be fool proof so a final product that meets all specs is produced all the time. Not just the Japanese. They were first the first adopters. Any serious big player in manufacturing these days has to apply these methods to compete.

Also it is been proven that people who do QC make mistakes. Not their fault. They are human and it is very boring. If there was QC at the end of the line then my new 70-200 2.8 II should not be in Purolator's hands today on it's way to Canon.

Edited on Mar 04, 2013 at 10:21 PM · View previous versions



Mar 04, 2013 at 08:28 PM
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