]Jim Alexander wrote:
]> I'm running an experiment that employs 3 Canon EOS 20D bodies with
]> EF 35mm f/2 lenses. I'm running into a problem with the autofocus
]> accuracy of this combination suddenly dropping while lighting
]> conditions remain constant according to my light meter. I have 5
]> of these 35mm lenses, and I've had two degrade to the point I basically
]> can't get an accurate autofocus at all, and the remaining lenses are
]> edging toward unacceptable territory as well. Since I have more than one
]> camera, I can tell that the problem must be in the lens because if I move
]> the suspect lens to another camera, the focusing problem follows the lens.
]> The lenses have only been used in the lab, and are very clean.
]>
]> The only explanation I can think of for this problem is that the focusing
]> mechanism must be using plastic gears, and the lens is developing some play
]> or backlash. I realize that these are relatively cheap lenses, and they have
]> gotten a lot of use (the oldest ones have probably taken more than 30,000
]> images in the last 6 months). Does someone want to offer their opinion on
]> whether I should really expect to see wear problems of this nature so
]> quickly? Or is there another possible explanation for this problem?
]>
]
]Can't opine as to this, but my question is: in a lab setting, are you
]not able to satisfactorily use manual focus?

Right, I should have thrown in a paragraph about that in the original
post. Unfortunately, I am not able to use manual focus because I need
to be able to trigger the cameras simultaneously (to within a few
milliseconds). In order for this to work, I need the shutter lag time
to be predictable, and by shutter lag time, I mean the delay between
pressing the shutter release button and when the shutter opens, when
metering and focus lock are already done. In autofocus mode with the
20D, this lag is predictable (interestingly, the constant is slightly
different for each camera, but I built electronics to compensate for
that difference). In manual focus mode, however, the shutter lag is not
a constant - it seems to vary by as much as 20ms from shot to shot.
The cameras must take a different control path internally in auto- vs
manual focus, but why this random element exists only in manual
focus mode, and not in autofocus mode, remains a mystery to me.