AF degradation of Canon EF 35mm f/2 lens

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?
--
________ Jim Alexander __________________ jalex@cis.upenn.edu ________________
I have yet to see a problem, however complicated, which, when you looked at it
in the right way, did not become still more complicated. -- Poul Anderson

Jalex@cis.upenn.edu (Jim Alexander)

Re: AF degradation of Canon EF 35mm f/2 lens

...
The AF/MF switch fell off my EF 35 f2, and I was able to see inside.
Unfortunately the photos I took don't make it clear whether the gears are
metal or plastic. I can see 3 in my photos. I glued the switch plate back
in place and it's been fine since. (Could email you a snap if you're
interested.)
--
M Stewart
Milton Keynes, UK
--
Posted via a free Usenet account from http://www.teranews.com


Malcolm Stewart

Re: AF degradation of Canon EF 35mm f/2 lens

In article <C5mdnaPiP5KqJ9XYnZ2dnUVZ_v2dnZ2d@comcast.com>,
]
]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.
--
________ Jim Alexander __________________ jalex@cis.upenn.edu ________________
I have yet to see a problem, however complicated, which, when you looked at it
in the right way, did not become still more complicated. -- Poul Anderson


Jalex@cis.upenn.edu (Jim Alexander)

Re: AF degradation of Canon EF 35mm f/2 lens

Can't opine as to this, but my question is: in a lab setting, are you
not able to satisfactorily use manual focus?
--
john mcwilliams


John McWilliams

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