DotTune: Video Tutorial for AF tuning without photographs

Started Feb 18, 2013 | Discussions thread
TOF guy
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Kaj's question is a good one IMO
In reply to Robin Casady, Feb 21, 2013

Robin Casady wrote:

Kaj E wrote:

Knowing that the sharp focus is reduced more rapidly from perfect focus in the direction toward the camera than away from it , why do you propose setting the focus at the middle of the acceptable range and not for instance at 1/3 from the closer edge of the range? I would suspect that the green dot goes to blinking more abruptly at the closer focus side. Have not tried your method yet, this is just how I would expect things to behave.

Have you done any test to confirm that the middle of the range corresponds to the best Live view focus?.

Kaj,

We look forward to your testing of your hypothesis.

The 1/3 -2/3 rule is far from general of course. But Kaj's objection certainly would apply IMO in any situation where the DOF does not extend the same in front and back of a target.

Example:

  • Testing a 200 mm lens at 10 feet (about 3 meters). Distance of focus is centered (give or take a very small difference) in the range of distances within the DOF  (rule of 1/2 1/2)
  • Testing a 28 mm lens at 10 feet. Distance of focus is 1/3 away from the nearest distance  within DOF and 2/3 away from the furthest (rule of 1/3 2/3)
  • Of course there are many other ratios possible, up to "infinity" (far distance extends to "infinity")

For the typical distances used to test a lens one would most likely be within the range of a 1/2 1/2 rule (you're more likely to test the 28 mm with a target closer to the camera), but there may be a correction to apply in this test as Kaj suggests i.e. consider how far the DOF extends behind and in front of the focus plane and apply the necessary correction. See for instance this calculator(which I did use just prior to posting just to make sure ).

Very valid point The O/P should add a comment in his video IMO.

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