Re: Dot-Tune: Autofocus Fine Tuning in under 5 minutes
jcolwell wrote:
I\'ve read information that says, one step in the EOS Utility LiveView focus control interface ( < or > ) corresponds to a single step in the AF system. IOW, it\'s the smallest AF increment that you can use. The proportion of DOF that this corresponds to will vary with the parameters that affect DOF; namely, focal length, aperture, and distance. Further, this AF step size is the same as a single step in the AFMA value that you register in the camera.
I use this approach with a Lens Align II target, where the number of LiveView steps required to get the centre of focus at \"zero\" on the target is the same value that I enter into the AFMA registry. This consistently works for the six EOS DSLR, twenty+ EF lenses, and two extenders that I have performed AFMA tests with.
I\'ll look for the original reference about the relationship between EOS Utility LiveView focus steps and AFMA steps, and I\'ll post it here if I find it. OTOH, I\'m not going to spend a lot of time looking for it, because I\'m confident that it does work.
Also, some lenses (probably most lenses) are designed to put the centre of focus more or less in the middle of the DOF, while others are designed for different behaviour. For example, the SMC Pentax-A* 85/1.4 \"portrait lens\" has the centre of focus near the rear of the DOF, which provides increased separation between the subject and the background.
That would be helpful. Until a few days ago I always assumed that an MFA shift was one value across the board. Someone found a reference to Chuck Westfall who said the same thing but stated ⅛ of the DO Focus in another article which I don\'t have a link to. So now there is him and Rudy Winston\'s comment so I have to wonder what they mean. One is DO Field and the other Focus but they are proportional. One is just easier to visualize. Actually if he had of left it at \"fine increments\" even that is easier to understand. If he had of said DO Focus I\'d be wondering what that meant even more than DO Field.
I\'m not so concerned about the back and front as much as total DOF, distance and the effect on MFA.
Also when I was goofing around with this I was still wondering about the MFA shift being one constant value no matter what the input is. I thought perhaps Canon uses the standard 50mm 1.2 for that number but he did no say that.
So both used ⅛ of either DO field and Focus so that has to mean something. If I got it wrong it was an interesting journey.
Jun 04, 2015 at 09:11 AM
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