Autofocus Microadjustment Epihany

Posted by on Jun 5, 2012 in Blog | 5 Comments

I just had an Autofocus Microadjustment Epiphany!

I recently got the Tamron 24-70 f2.8 VC. I also have a really neat lens cal system called the Reikan FoCal (a autofocus microadjustment system ). So as soon as I got the lens I set it up with my Canon 5D Mk III and ran a MA series on it.

Now Canon says you should run this kind of check a a distance of focal length X 50. I also have the LensAlignPro ( another autofocus microadjustment system) and it says that you can cheat on that a bit and run X 25. FoCal says you can can also run the test at a distance where you take most of your shots.

I use the full range of 24-70 (I also have the Canon) but I also like to take close up shots near the minimum focal distance. Vegetables at the farmers Market.

So I did the calibration at around 28″ from the target and got an autofocus microadjustment of 20. That’s pretty high but I checked it and it repeated.

I go to market and get some good close up (<2′ away) shots. I also take some shots of friends from about 8′ away. The later shots are out of focus. They are all the same focal length.

So I have been doing both lens align and Focal calibrations on both the Canon and the Focal for most of today.

What I am finding is that when you do a calibration for autofocus microadjustment using any system the autofocus microadjustment value is only good for the distance that you test at. It changes for distance.

What I though I was seeing was for a fixed autofocus microadjustment point, your DOF focus moves back the further from the target and it moves toward you the closer to the target.

So…… Does this make sense?

Assuming it does I think that means that if you are using AF don’t get close to the minimum focus distance. Just because you can get a focus lock does not mean it will be in focus. And if you set up your autofocus microadjustment for distances that are close to the minimum distance then you will be compromising your other shots from further away.

I think that means I need to calibrate my lens in the 25-50X range and go get a prime between 50-85mm (calibrated for closeup distances) for my vegetables.

Then I thought…. lest spend some more time on this and look at the numbers for several different cases.

So lets check this all out with a series of tests  I will used my “rocket scientist” background……

Hypothesis:

Autofocus microadjustment is affected by distance.  This is particularly true near the minimum focus distance.

Test Equipment:

1.  Canon 5D Mark II

2. Canon 5D Mark III

3. Tamron 24-70mm f2.8

4. Canon 20-70mm f2.8 I

5. Reikan Focal Calibration system ver 1.5 beta

6. LensAlign Pro

7. Tripod

8. Two 6500K CLF lights

Methodology:

I will get the best autofocus microadjustment at three distances for both lenses on both cameras at 70mm at f2.8 in Av mode at 100 iso.    The distances I will use are

  1. My favorite distance for close up shots 28″  ( minimum focus distance is 1.25′)
  2. 5 feet ( close to 25X as recommended by LensAlign)
  3. 9 feet ( close to 50X as recommended by Canon-   50x is really about 11′ but my room has limitations)

So I will do both cameras and lenses at each distance on the Focal system.   Then I will repeat that on the lensAlignpro system.   Sounds like fun…… well its raining outside and my wife has a bunch a women over for an exciting domino’s tournament…..so this is what I can do.

Data

Reikan FoCal Results

Hang on ….. there are 12 of these things.  The summary and conclusions are at the end.

5DMk II Canon Lens 28″

Comment:  Decent curve fit with most points together at each autofocus microadjustment

5DMkII Canon Lens 5′

Comment: fairly wide spread of points for the near autofocus microadjustment’s…. not sure why

5DMkII Canon Lens 8′

Comment:  Really nice tight curve fit.

5DMkII Tamron Lens 28″

Comment: Except for -20 a pretty good tight fit

5DMKII Tamron Lens 5′

Comment:  This was rated, by the software, as a poor cal.   The points are all over the place.  But there were a predominance of points around 6 0r 7.

5DMKII Tamron Lens 8′

Comment: Fairly good fit with some stray points

5DMKIII Canon Lens 28″

Comments:  Again fairly good fit with some stray points

5DMKIII Canon Lens 5′

Comments:  Again fairly good fit with some stray points

5DMkIII Canon Lens 8′

Comment:  Better fit of points

5DMkIII Tamron Lens 28″

Comment:  Lots of spread at the high negative range but then tight after that

5DMkIII Tamron Lens 5′

Comment: Decent fit except for a few places

5DMkIII Tamron Lens 8′

Comment: Really nice fit

LensAlign Pro Results

I did not include images of the results of these tests.  There is a fair amount of subjective judgement with this system in determining the middle of the depth of field.  Its pretty straight forward at close distances and is easy to see.   So I have I lot of confidence in those results.   The more distant readings at 8′ were subjective in that the DOF was almost as wide as the ruler.  I will have to get the longer ruler that they sell.   The autofocus microadjustment’s that I did come up with are shown below with the FoCal numbers from above.

Here is a summary of all of the AF

Autofocus microadjustment values:

Focal Method
Distance 5DMkII 5DMkIII
Canon Tamron Canon Tamron
28″

-5

7

-4

20

5′

-3

4

-3

13

8′

-3

2

-4

13

LensAlign Pro
28″

-5

6

-3

12

5′

-2

4

-3

9

8′

-3

1

-4

11

Observations:

  1. If you compare each camera and lens combination between the FoCal and LensAlignPro, they are all pretty close except for the combination of the Tamron and the 5DMkIII.  If you look at the calibration curve it looks like it almost requires an adjustment of greater than 20, which is not possible.  But it does not behave that way on the 5DMkII.
  2. There is a small tendency to require Higher autofocus microadjustment adjustment, in the FoCal sytstem,  near the minimum focal distance.   This is most noticeable in the Tamron lens near the closer 28″ test.
  3. It was satisfying to see that the LensAlignPro test showed that the Tamron/ 5DMkIII combination a MA adjustment of only 12 was required and not the 20+ that the FoCal system showed.
  4. Most of the tests with the FoCal system at 8′,  closest to the recommended 50X, seemed to have the best curve fit
  5. DSLR AF systems are NOT 100% repeatable.   Stray bad focusing happens

Conclusion:

If you are going to be taking photos of images close to the minimum focus distance you DOF may not be where you think it should be.  I am thinking that several shots, with the focus point a slightly different distances,  may help to insure you get the focus where you want.    At least that is true with my lenses on these cameras.

Its probably best to set your AF MA at the 25-50 X distance.

For the most part Focal agrees with the Lens Align systems.   Note that I was using a FoCal beta version.

There is something odd going on with the Tamron Lens that you can see in both cameras.     I may try to get my hands on another Tamron and see what results I get.

 

5 Comments

  1. Zoom Lenses Make You Lazy
    August 12, 2012

    […] faced with changing the MA, in the field, for close ups or going to manual.  I also covered that Here in […]

    Reply
  2. Skip
    September 19, 2012

    Thanks for this work. I’ve been struggling with focus calibration for a while, and your exploration of what works and doesn’t has given me a lot of insight.

    blue skies!

    Reply
  3. yh
    October 31, 2012

    You need to do a little of your own adjustment to read the figure. The curve just give an estimate (+-2). You need own jugement based on the curve.
    From the figure, I suggest the best adjustment for canon 24-70 is -4 across all distance and both mrk ii and mrk iii.

    Also the canon lens are way better than tamron only based on AF accuracy!

    Reply
    • bobbyk
      October 31, 2012

      I agree that you have to use your own judgment from the curve. In my camera, Canon 5DMK III you can set one MA for wide and another for tele. The camera interpolated between the two points.

      Reply
  4. paco
    November 2, 2012

    Thanks for this empiric document, very good scientific method, very helpful!

    Reply

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