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Author Topic: +18 AFMA out of the box....return?  (Read 3981 times)

cervantes

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Re: +18 AFMA out of the box....return?
« Reply #15 on: May 17, 2013, 01:52:28 AM »
Sorry jdramirez, that doesn't really make sense to me.

AFMA is used to correct for manufacturing tolerances. Tolerance means that nothing can be made to an exact value but it can be made for sure within certain boundaries. Canon knows these boundaries for their lenses and cameras. I believe when they allow AFMA to be set from -20 to +20 it should be enough to correct worst case scenario. I don't know that for sure but it would be logical from my point of view.

E.g: The maximum amount a camera can be "off" would be +/-10 and the maximum amount a lens can be "off" would be +/-10 so a correction of +/-20 would be sufficient.

In this case the lens is probably off -9 and the body -9 (so they are both around their extremes) but you can correct it by using +18. Due to the known tolerances your next body will not be off more than -10 so correction should always be possible.

You - Ewinter - could test this by looking at the AFMA values of your other lenses. If you have a big enough amount of lenses you should see a tendency of AFMA values around +9 or at least they should be quite consistently >0. If you can't see this and the values are for example around -15 then there is probably something wrong with your lens.

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Re: +18 AFMA out of the box....return?
« Reply #15 on: May 17, 2013, 01:52:28 AM »

R1-7D

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Re: +18 AFMA out of the box....return?
« Reply #16 on: May 17, 2013, 05:09:36 AM »
Well I just ran my 50mm f/1.4 through Focal Pro about eight times tonight. EV was 11. When Focal worked (it kept erroring out saying it could no longer find the target and then it prevented me from closing any of the app's windows) I got results from +10 to +15. When I got the +15 it said the fitment quality was poor. I got +10 with a "good" fitment quality, +12 with excellent, and twice +13 with excellent fitment quality and confidence rating. I set the lens to +13.

I don't know whether it is the new version of Focal or just this particular lens, but every other lens I have was much easier to calibrate...with no errors when using the program. Of course, f/2.8 is the fastest aperture of all my other lens, so perhaps having a two-stop aperture difference makes a difference to focus reliability on the target?

Anyways, I have tomorrow to return the lens before the long weekend here. I'm wondering if I should and get a different copy of it, or just keep playing with this current one. When I first brought it home it front-focused something fierce before I calibrated it. Now it is pretty good with focusing. I'm just getting a little anxious with all this camera and lens micro adjustment talk.

Advice? :-\
Cameras: 1DX, Canon M. Lenses: Canon EF 24-70mm f/2.8L II, Canon EF 70-200 f/2.8L IS II,  Canon EF 100mm f/2.8L Macro, Canon EF 2X III, Tokina SD 16-28mm f/2.8 FX, Canon M 18-55mm IS, Canon M 22mm. Flashes MT-24 580EXII

neuroanatomist

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Re: +18 AFMA out of the box....return?
« Reply #17 on: May 17, 2013, 08:00:11 AM »
Sorry jdramirez, that doesn't really make sense to me.

AFMA is used to correct for manufacturing tolerances. Tolerance means that nothing can be made to an exact value but it can be made for sure within certain boundaries. Canon knows these boundaries for their lenses and cameras. I believe when they allow AFMA to be set from -20 to +20 it should be enough to correct worst case scenario. I don't know that for sure but it would be logical from my point of view.

E.g: The maximum amount a camera can be "off" would be +/-10 and the maximum amount a lens can be "off" would be +/-10 so a correction of +/-20 would be sufficient.

In this case the lens is probably off -9 and the body -9 (so they are both around their extremes) but you can correct it by using +18. Due to the known tolerances your next body will not be off more than -10 so correction should always be possible.

That would be true if manufacturing process was perfect. It's not. Tolerances aren't absolute, to make them so would be prohibitively expensive.  A common manufacturing practice is 3σ, which basically means ~3 out of every 1000 products that leave the factory will not fall within the tolerance range.  Of course, if you get one of those you may not even notice it, but if you've got a lens on the other extreme, you might.  If you do notice it (in time), Canon will fix it under warranty - doing so is cheaper than tighter adherence to tolerances in manufacturing.
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Re: +18 AFMA out of the box....return?
« Reply #17 on: May 17, 2013, 08:00:11 AM »