Canon EOS EF Lens Repair: 20-25mm f/3.5-4.5 USM autofocus / manual focus
failure
This is my trusty wide angle lens: It has gone in numerous small dusty
caves, damp caves, muddy caves, and I take it hiking in the sun and in
the rain alike.
The Problem
The problem was failure to autofocus, followed shortly by failure of manual
focus. Within 1/2 hour, it was shot. It was stuck at a focus distance of
about 1.0ft. High ISO and small apertures got a few more shots out of it
that day at Natural Bridge Caverns, TX, but that was pretty much the end
of it.
This lens was about $400 new, had served me well, and I was confident it
could be fixed. With an upcoming climbing trip to Crestone Needle (Ellingwood
Arete ascent), I didn't want to spend the time or money sending it off
to be repaired. I was sure it could be fixed in my kitchen, using my Swiss
Army knife and a set of small Wiha screwdrivers.
While this report is specific to the 20-35mm f/3.5-4.5 USM wide angle lens,
the nature of the failure is such that it is likely the cause of other
"my lens cannot focus even in manual mode" problems, and thus
generally useful.
Many will be familiar with the operation of the full time manual mode,
which allows both the USM drive and the manual focus to occur at the same
time, by virtue of the focus ring rolling on three small wheels sandwiched
between the manual focus ring and the USM armature ring. Without sufficient
compression force between the drive rings, neither can turn the focus ring's
three small wheels, and thus the lens loses ability to focus in both auto
and manual modes alike.
There is an excellent description of the workings of the USM lens here
(which I discovered after the repair):
A guided tour through
the inner workings of the EF-28-105 USM
Procedure
To disassemble the USM workings of the lens, remove four screws of the
mounting plate, two screws for the electrical interconnect, and one screw
for the bayonet stop. To remove this black plastic part, you first need
to pop four tabs from below. Use Swiss Army screwdriver blade. Extreme
care must be taken here so as not to damage or rip off the flexible PCB,
which is attached at this time. Then use the Swiss Army knife blade to
ease around the now-visible gap on the top-side, and pop out this part
as pictured below:
From here, the control PCB and electrical interconnect are visible:
Carefully disconnect the four thin ribbon cables (two are friction fit,
two are tabbed low-insertion-force type), and remove the PCB, which is
held in place by one screw only:
Now, remove the four black screws holding in place the large plastic collar,
taking care not to damage the ribbons -- two of the ribbon cables are secured
in place by each fitting onto a plastic pin.
Life the collar off:
Now, we can remove the components of the USM drive:
Numbered in the order they were removed, we can see the various rings:
1. Retaining ring,
2. Wavy spring,
3. Felt-like spacer,
4. USM motor stator,
5. USM motor armature.
Wear component
The problem component is the felt-like spacer, indicated with the red arrow.
Clearly a wear-out-and-send-lens-to-service component, and probably suceptable
to moisture, it is very disappointing. It was visibly compressed in some
places where in contact with the wavy spring, and no longer provided sufficient
spacing and consequently sufficient compression forces to allow the focus
ring wheels to operate properly.
I took some measurements:
Above, on the left, we see the nominal thickness of the felt-like spaer.
On the right, we see the thickness in one of several "thin spots".
I found my wife's card stock to be of use, and simply cut three different
rings of cardstock, whose total thickness was a little thicker than the
felt-like ring was at is thickest spot. My paper rings were not perfect,
because I couldn't find the blade to my wife's circle cutter, so I used
the Swiss Army Knife scissors...
This collection of three cardboard spacers now replaces the felt-like spacer
in the USM motor focusing unit.
Reassemble the USM motor components in the correct order, using the card
rings as appropriate, and replace the lens components in the reverse order
to which they were removed.
Verify manual focus operation, and attach the lens to a camera and verify
auto-focus operation.
Notes
Note that no optical glass components were touched, altered, misadjusted,
or otherwise compromised in this procedure. Note also no special service
tools were required.
Conclusion
This whole process took under an hour, and I bet many lenses that suffer the same failure mode have the same
felt-like spacer, which seems to be a wear component and needs replacing. You may want to order the correct part
from Canon, if you can find it -- the Parts Catalog I found here only lists the "Focusing Unit" as a single part
number.
If you are competent at tinkering, have suitable small tools, a trusty
penknife, and especially if you are the "engineer type", do not
be disuaded by the thousands of comments on other forums that claim "there
is nothing that you can fix in a Canon USM lens", "DO NOT take
it apart", "Only Canon Service can work on these lenses",
etc. There is fear of the unknown, and hopefully these pictures will remove
some of the mystery of at least some parts of this lens.
Disclaimer
The author makes no guarantee that the reader can repair his or her lens,
and no warranty (expressed or implied) is provided. Perform this procedure
at you own risk. If your lens is under warranty, an Athorized Canon Service
Center, through your local camera shop, should be your first choice.
, Jul 25, 2008; 12:16 a.m.
