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.