John Mahan
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 RE-POLL: MkIII have AF issues? | |
This was posted to a different site, but I thought folks here might find my experience useful. This is the first time I have posted here--it's the first time I have had anything worthwhile to add to this excellent forum. Are there bad units in the wild? Yes. My 1D Mark III just shipped out to Canon CPS per their request following my description of my Mark III's AF problems.
I normally assume I am at fault when my results are sub par. That assumption is what made me keep this body beyond the time limit for returning to the dealer. First, I noticed many unsharp results amidst some stunningly detailed frames. Although not limited to telephoto shots, the soft results dominate my telephoto frames (70-200 F4 L IS & 300 F4 L IS).
Trying to sort this out, I was unable to get any sharp frames of oncoming traffic--but speeding trucks really slow down fast when you point a camera at them. Needing a more repeatable test, I have done a ridiculous # of test series of my wife riding toward me at 10-15 mph on her bike (in hot and cool, sunny and cloudy weather). The results? Nine gigabytes of distressingly out of focus boring pictures.
Had I suddenly lost all my photographic ability? Did I not know the magic CF combination?
OK, I needed a more controllable & objective test. Out came the metal yardstick and focussing chart. Crude, but very enlightening.
With the MKIII on a huge old Gitzo 'pod and Arca Swiss monoball head & focused on the 8 in the #18 on the angled yardstick at 45 degrees, I ripped off a burst at 10 FPS. Viewed on the monitor I watched focus shift, frame to frame, from back focus to front focus in the burst series. The series was shot in AI Servo, Center point with no focusing point expansion on. Focus shifted from almost #16 to #19(camera approx. 10-15 feet from ruler). These results were repeatable with two different 70-200 L lenses(yes, IS was off) and the 300 lens. The same lenses on my wife's 20D in the same test yielded (repeatedly) virtually no variation at all.
Next a focussing chart was used, again tilted 45 degrees to the camera's focal plane. The results with each of the three lenses showed focus shifting from front focussing to back focussing by as much as 2-3 inches. This occurred in single shot, non-Servo mode as well as 10 FPS bursts in AI Servo mode. In some series, focus would shift from pronounced front focus to pronounced back focus in just two frames. The focus shifting was very repeatable except the degree and direction varied from series to series. The new focus microadjust feature can't cope with variable focus shifts. Neither can I. Next week I should know if Canon CPS can resolve this problem.
For me, this proves that some MK IIIs are indeed "bad". I don't question for a moment that many others have perfectly wonderful MK IIIs. But for those of you who are unable to come even close to results with other bodies, you too may have a MK III that keeps guessing where to focus.
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