This guide will give advice on how to repair some of the common faults with Canon EF lenses.
If you're an avid photographer, but feel the cost of good lenses hold you back, then this can be overcome if you are good at detailed work, and have a lot of patience. A quick search on eBay will show various faulty Canon lenses available. Although some may be uneconomical to repair, there are also a few which can be repaired at very little expense to you.
One of the most common faults with Canon EF and even the more recent EF-S series of lenses, is the result of a torn ribbon cable. These are flat, flexible cables which carry the electronic signals from the lens connectors, to the focus motor, the aperture, zoom sensor and Auto/Manual focus switch.
As some of these parts have to move back and forth to follow the zooming action, the cables are being flexed and will eventually develop a 'weak point'. This will soon develop into a tear across the cable, which results in the camera body being unable to fully communicate with the lens
With luck, the tear may only affect one of the wires within the ribbon cable, but sometimes the tear can be across all of the wires.
The first step is to begin disassembling the lens. It is highly recommended you do this in a closed room free from dust. Using latex gloves also helps prevent fluids from your fingertips from getting onto the lens elements.
It also makes good sense to document each step of the dis-assembly by photographing the lens before removing each part, and keeping the relevant screws with the part they came from.
If you do not do this, you WILL regret it!
You should treat the ribbon cables delicately, they can rip quite easily. It is also important that you don't try to forcibly remove any parts of the lens. If a part is not moving where you want it to go, find out what's holding it back. Your patience will be rewarded.
As you progress, closely inspect all of the ribbon cables as they are revealed. It may only be a hairline tear, a couple of millimeters long. Once you locate the torn cable, you have to remove it from the lens assembly. Some cables may be soldered to a part, others may be stuck down with double-sided tape.
If the cable is torn in two, you must align them on a flat surface, using tape to hold them both firmly in place.
You should leave 1 centimeter of the cable exposed at both ends of the tear to allow you to scrape the insulation off from the wires. Once they are exposed, you need to bridge them by soldering a short strand of wire across each broken wire. This is extremely tricky, and requires both patience and a steady hand. You should then insulate the join by wrapping some tape around that part of the cable. Don't go overboard, just enough tape to cover the join and loop back on itself.
Now, having ensured that you've checked all the ribbon cables for tears, it's time to re-assemble the lens.
This is where the photographs you took (you did take them, didn't you?) will become indispensible.
Some parts of the lens came apart easier than the go back together. It can get quite frustrating, so if you feel your temperature rising towards enragement levels, take a break. A cup of tea and some fresh air later, you'll feel ready to continue without the urge to pulverise the lens with your bare knuckles.
During the re-assembly, closely inspect the glass lens elements to ensure that they are clean. Usually, a few specks of dust will attach themselves to the optics, so it's quite handy to have a lens blower on standby. They can be bought quite cheap on eBay, and are an essential part of a photographer's kit.
It should be noted that blowing dust off using your mouth will probably result in tiny blobs of saliva and condensation getting on the the lens. This should be avoided, as it may cause mould to develop at a later time.
If all goes well, the lens will be back together and you'll have no screws left over. (I always end up with one left over and have to backtrack. It's not amusing I can tell you...)
Hopefully, you'll now have a fully working lens, at a fraction of the cost.
It is quite a difficult procedure and it's easy to make mistakes, I practiced on an unrepairable lens which I bought for buttons. This is a good way of building up confidence and experience which will be of great importance when repairing some of the more expensive lenses.