Camera & Lens Focus Calibration

[surfnron]

Over on FM, a member has come up with a dead simple way to calibrate lenses quickly. Basically you focus on your target and turn off autofucus. Then in the MA menu, you check the focus by half pressing the shutter, (or back button),and adjusting the ma setting until the focus light blinks. Do this for + and -. Your sharpest focus will be in the middle. Here is a link:

http://www.fredmiranda.com/forum/topic/1187247

This is my first post here - hope the link works...

Ron

[Plainsman]

No lens can maintain optimum sharpness right through from close range say 10m to infinity - in practice anything beyond a few hundred metres.

So ideally you need to do your AFMA for your normal approx favourite photographing distance.

Long lenses are birders lenses designed for optimum performance at relatively close range rather than infinity.

Interesting to read Thom Hogan's reviews on long Nikons and his comment that they loose some sharpness at long distance. Same probably applies to Canon.

[neuroanatomist]

Do I understand you correctly...zooms...short end tested at 25x and 50x, long end tested at 25x and 50x, and intermediates tested at 25x and 50x...

So a 70-200 would involve 6-10 tests with distances ranging from 1.75m to 10m?

And the results (5diii) would be an average (mean or median?) for the low end and an average for the high end?

And if you'll indulge me one more question...you run a complete, separate series of tests when using a TC?

Correct.  The number of intermediate focal lengths I test depends on the zoom range, e.g. for a 2-3x zoom like the 16-35mm or 70-200mm, I test just one intermediate focal length; for a ~4x zoom like the 24-105mm or 100-400mm, I test two, etc.  So for the 70-200, that's a total of 6 tests - 70mm, 135mm, and 200mm each at 25x and 50x the focal length.

The newer AFMA bodies (including the 5DIII) can store separate values for the wide and tele ends of zoom lenses.  I use the two distances and the intermediate focal lengths to help guide the choice - the values I enter are a compromise between the distance(s) at which I usually shoot with that lens, the effect of focal length on DoF, etc.  For example, with my 100-400L at 50x focal length, the FoCal-reported AFMAs at 100,200,300,400mm were 0,0,0,1 and at 25x focal length they were -3,0,-1,2.  The values I selected were W=-1 and T=1. 

The camera does a simple linear regression between the W and T values to apply AFMA at intermediate focal lengths.

Yes, with a teleconverter I do a complete separate set of measurements (6 for the 70-200/2.8 with the 1.4x and 6 more with the 2x).