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Author Topic: AFMA & distance?  (Read 1885 times)

Marsu42

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AFMA & distance?
« on: November 07, 2013, 01:10:25 PM »
Since my good ol' 60d doesn't have afma (thanks, Canon!) I'm new to this with my 6d and would like to ask some people with more afma experience for advice.

I just adjusted my lenses with Magic Lantern's dot_dune module which is basically a free in-camera version of FoCal and found that neither of my f4 zooms need afma (well, the 17-40L +1) at all at either end, at least not at the distance I could test them with in my room. I wonder...

1. My 100L seems to need more afma the longer the lens-subject distance gets. Did I do something wrong, is this normal behavior so I actually arbitrarily need to pick my favorite distance I'd like the lens to af? Do I need to test my 70-300L at longer distances than the 2m right now for accurate results?

2. I don't understand the scale of the afma values - does for example make the +5 of my 100L @1m a difference at the dof of f2.8 in real life? Or is this overcompensated by the camera's and my human's focusing variance anyway unless I shoot on tripod (and in this case would use focus peaking or contrast af)?

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AFMA & distance?
« on: November 07, 2013, 01:10:25 PM »

neuroanatomist

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Re: AFMA & distance?
« Reply #1 on: November 07, 2013, 01:23:34 PM »
1. My 100L seems to need more afma the longer the lens-subject distance gets. Did I do something wrong, is this normal behavior so I actually arbitrarily need to pick my favorite distance I'd like the lens to af? Do I need to test my 70-300L at longer distances than the 2m right now for accurate results?

It's normal for AFMA to vary with distance; at longer subject distances, the delta becomes very small, which is why the usual recommendation is to test at 25x to 50x the focal length.  But if you almost always use a lens at a given distance, test at that distance.  FWIW, my 100L macro on my 1D X needs +4 at 25x, +2 at 50x, and +3 near the MFD (so I use +3). 

25-50x the FL means you should be testing a 300mm lens at 7.5-15 m, not 2 m...

2. I don't understand the scale of the afma values - does for example make the +5 of my 100L @1m a difference at the dof of f2.8 in real life? Or is this overcompensated by the camera's and my human's focusing variance anyway unless I shoot on tripod (and in this case would use focus peaking or contrast af)?

The scale for AFMA is one unit = 1/8th the depth of focus for a given lens at max aperture.  Depth of focus is similar to depth of field, but the former is measured at the image plane.  In practice, I'd expect to see a difference with a +5 adjustment for an f/2.8 lens (but it would be barely perceptable for an f/5.6 lens, if at all).  In part, that's due to the precision of the AF system itself - a high-precision f/2.8 AF point is specified to be precise within 1/3 the depth of focus, while a standard precision f/5.6 AF point is precise within one depth of focus. 
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Marsu42

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Re: AFMA & distance?
« Reply #2 on: November 07, 2013, 01:29:29 PM »
25-50x the FL means you should be testing a 300mm lens at 7.5-15 m, not 2 m...

Thanks for the advice, I'll re-test my tele zoom at longer distance to make sure even though if it's zero afma right now, but maybe I'm in for a surprise. With the macro lens I'll re-think the distances since I'm dual-using it a lot and if it needs to be use 2 afma values for very short and very long distance since the dof is so thin @f2.8.

I also have to mention that this was probably the most competent and conclusive answer I could have hoped for :-)

kaihp

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Re: AFMA & distance?
« Reply #3 on: November 08, 2013, 12:11:50 AM »
In practice, I'd expect to see a difference with a +5 adjustment for an f/2.8 lens (but it would be barely perceptable for an f/5.6 lens, if at all).

FWIW, I found that an adjustment of +4 on my 70-200/2.8IS II made a very visible and significant difference. At some point, I want to get around to testing the 70-200 for how much the AFMA varies with target distance because I feel that the portrait-type photos I take don't seem to be sharp where I expect them.

Mt Spokane Photography

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Re: AFMA & distance?
« Reply #4 on: November 08, 2013, 02:23:45 AM »
Generally, a difference in AFMA of +/- 4 makes only a tiny difference in sharpness.  This is easy to see with the curve plotted using FoCal software.  As long as the near / far variation is 5-8 points or less, its probably not significant.
 
Here is a typical curve showing sharpness versus AFMA for my 40mm stm  The curve is pretty flat near the top.
 

 
 
 
You see the same with my 100-400L at 400mm, the best setting is +2, but at 0 its just fine.
 

Marsu42

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Re: AFMA & distance?
« Reply #5 on: November 11, 2013, 06:09:06 AM »
Generally, a difference in AFMA of +/- 4 makes only a tiny difference in sharpness.

Thanks for the curves, now it's Marsu42 reporting back from afma adjustment outdoor test (if anyone ever reads this thread via search)...

... you're correct, the +5 of my 100L only makes a tiny difference @f2.8, but there it is, esp. at longer lens-object distance, so it was a good idea to adjust the lens. However, for my 60d w/o afma (thanks, Canon!) this +5 seems to be just outside the dof on crop as I never noticed any issues then, it's only on ff that the focus seems to be a little off wide open w/o adjustment.

P_R

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Re: AFMA & distance?
« Reply #6 on: November 11, 2013, 06:23:10 AM »
Slightly off topic but related, is it worth having ones camera and lenses calibrated? In doing so canon would presumably set all the correct amfa values for the camera and lenses.   
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Re: AFMA & distance?
« Reply #6 on: November 11, 2013, 06:23:10 AM »

neuroanatomist

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Re: AFMA & distance?
« Reply #7 on: November 11, 2013, 07:20:32 AM »
Slightly off topic but related, is it worth having ones camera and lenses calibrated? In doing so canon would presumably set all the correct amfa values for the camera and lenses.

Canon doesn't use AFMA. They electronically and/or mechanically adjust your lenses to a standard body of your type, and calibrate your body to match that standard.  They can perform more complex adjustments than AFMA.  It's free for items under warranty.  But IMO, if AFMA corrects the front/back focus, that's fine by me.
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Marsu42

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Re: AFMA & distance?
« Reply #8 on: November 11, 2013, 07:23:37 AM »
Slightly off topic but related, is it worth having ones camera and lenses calibrated? In doing so canon would presumably set all the correct amfa values for the camera and lenses.

I guess it's worth it if there's a large difference between afma needed for near and far object distance because unlike Sigma Canon's current afma approach doesn't account for that, but Canon service might be able to do something about it.

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Re: AFMA & distance?
« Reply #8 on: November 11, 2013, 07:23:37 AM »