Introduction: Fixing an Expensive Photo Lens ( Canon 85mm F1.2 L II )
Fixing things is a great hobby, especially if you are a photo nerd with a limited budget and still want to have great gear. This is how I got into fixing photo lenses and by now I have fixed so many lenses I stopped counting.
Most lenses are constructed similarly and there are shared construction techniques between all lenses I have disassembled so far (all Canon EF). So while I am writing this article for a very specific lens, I will try to provide as many information as possible on the similarities and differences of each part.
The Problem of the Canon EF 85 f1.2 L USM
I got this lens from ebay for a spectacular price of 200€, and the only description it had was that the focus didn't work. This is a very common problem and can be caused by things like faulty electronics, loss of friction in a specific coupling, loose screws, broken plastics, and many many other causes.
Spoiler: this one problem was caused by falling and the lens being so heavy that it bent the 3mm metal barrel.
If you want to do something similar
If you want to get a lens for repairing there are a few things you want to look out for. Not everything is worth repairing and some lenses are just beyond repair for a hobbyist without an optical bench for re-aligning. Here is a list of show stoppers that are not worth your time:
- Don't buy water damaged lenses. Cleaning every piece of glass is doable, but a real nightmare.
- The actual glass lenses should be free of scratches
- Don't pay too much, you will need some room for spare parts
- Zoom issues in any zoom lens are bad. Often caused by tiny broken pieces of plastic. The lens will need a lot of work for optical re-aligning
- Issues with IS are generally not good (IS assemblies are expensive to replace or hard to obtain)
- Don't buy super-old L lenses. You won't be able to get any spare parts.
Oh, and if you've never taken apart a photo lens before: Start with something super-cheap that you can just destroy without being frustrated. It's a lot of fun, but sadly the fun stops if you have to worry about destroying a piece of professional grade high-tech worth more than 1000 $/€.
Step 1: First Look and Assessment
So you know that the focus doesn't work, what now?
You check what exactly doesn't work. This doesn't require any disassembly, you just take a look at the lens and mount it to your camera. Does the AF work? Does the MF still work? Is the MF coupled mechanically or is it just a sensor that will trigger the AF motor to move? If something moves, does it move over the whole focus range or is there a problem in a specific area?
The 85mm f1.2 has an electronically coupled MF, so the whole focus assembly is just one large motor that turns a barrel with grooves which will move the inside of the lens back and forth. Mechanically it is rather simple, which is good news.
The focus did move, but only a small amount between 0.95m and 1.2m distance. This means electronics are fine and we have a mechanical issue.
Step 2: Tools
First picture shows the tools you need, which are:
- good PTFE grease
- soldering iron for SMT
- good tweezers (sold for SMT soldring, but useful for everything)
- lens pen for removing dust
- small piece of plastic foil (what??)
(Sorry for the wrong lens in the picture, I didn't want to get the tools out to take a new photo)
Also if you have to store your disassembled lens until spare parts arrive some boxes are very useful. I use those stackable boxes for everything except large barrels that don't fit in them.
Pro-Tip: Use a sheet of paper with little patches of sticky tape for the screws. Label each patch, so you know where everything goes.
Step 3: Disassembly (finally...)
This is where the fun begins.
You always start at the 'bottom' of the lens.
Never at the front. It is a very common mistake to think you have a problem somewhere near the front, so you start there. The front glas element is usually the only exception.
The first disassembly step is a bit different with this lens. Usually you would unscrew the silvery bayonet, remove the plastic ring inside and remove the flex cable for the contacts. Here you remove only the plastic around the bayonet (so far this is the only lens where I have seen this).
Pro-Tip: Use a piece of thick, flexible plastic foil like from a zip-lock bag and put it between screw and screw driver. This way you don't damage the delicate paint on the screws.
After all screws are removed the large plastic piece can be wiggled free and slides off easily. The window for the focus scale is glued with some nasty double sided sticky tape. It can be a bit difficult to get off, be careful not to break it.
Step 4: Disassembly (removing the Electronics)
After the outer plastic is removed you can see a lot of details. All relevant parts are made from very thick metal. It is the most heavy-duty construction I have seen in a photo lens and it makes a lot of sense since the glas itself is just huge, heavy and needs a very rigid construction to stay optically aligned during focus movement.
The electronics consist entirely of flex PCBs wound around the back of the lens with some solder connections to other parts. They'll have to be removed so we can reach the USM motor.
To do this, carefully peel of the black tape that holds everything in place. Then remove all connected cables. Some of the flex cables can get damaged very easily and bending them a few times can already break them.
The solder connections require a bit more work, but anyone with good soldering skills can easily pull them apart. If you don't know how to solder this is a show-stopper for you. However, usually there is no need to solder anything, this lens is a bit special.
Step 5: Disassembly (USM Motor)
This is the first step where an optical element is removed. That means you have to be careful to note adjustable elements and if there are any, note their exact position.
In this case there, however, there is nothing to be adjusted and after removing the screws along the outer ring of the top element the whole thing with bayonet and lens comes off. We have successfully opened up the so called 'focus assembly'.
By now we also know that the failure lies inside the focus assembly. If we were Canon service we would just replace that assembly (and we can do that too). Problem is: the assembly is about half of the lens and costs 695£ at my usual supplier.
Step 6: Detour in USM Motor Technology
Most people who don't take photo lenses apart probably never heard of ultrasonic motors.
Stator & Rotor
The main components are two large diameter rings, which are pushed against each other tightly by springs. These are the stator and the rotor. The rotor is the passive element that is moved (rotated) by the 'flexible' stator. This stator will be deformed ever so slightly by ceramic elements on the other side of it.
The 'deformation' of the stator basically lifts and moves those feet like notches. With this movement the 'feet' are pushing the rotor and thus rotating it. This 'walking' motion is about 1µm with each step, so it happens with very high frequencies to generate fast and strong rotations.
This of course requires some friction between rotor and stator, so they have to be pressed together tightly. The most typical failure mode of these motors is the spring that does this. If it loosens the motors torque is drastically reduced and it may not be able to move the heavy lenses around.
Step 7: Found the Problem
The springs for the USM motor did not loosen in this lens. The unusual construction makes this impossible, so the problem had to be somewhere else.
I noticed that the focus scale doesn't have any clearance in one particular place. It basically scraped against the metal where it shouldn't. When the inner ring (with the focus scale) was rotated there was a little bit of clearance, and when it got stuck it scraped against the outer ring at this position! We finally found the real problem!
Taking a close look at the shape of the outer ring it was obvious that it wasn't a perfect round curve as it should have been. There was a dent in exactly the place where it got stuck. In the last picture you can even see a tiny crack in the metal.
Step 8: Fix It!
So to fix the blockage there were two options:
- remove the metal that pushes inside and against the inner ring
- bend it back
Removing e.g. with a lathe is very messy, but bending has the danger of making everything worse by introducing even more error. Weighing the pros and cons in my head I decided to carefully bend the outer ring back in place.
To match the original curve as exactly as possible I made a custom jig. The outside was supported by a 3d printed part with high infill to make it as strong as possible. The first picture shows the part held against the undamaged side and it fits perfectly. On the other side it can be seen how much the barrel has deformed.
A steel bar was used as a lever and also had a printed plastic piece at the end that matched the curve inside the barrel.
After carefully working the lever the shape was much closer to how it should be and the inner ring turned freely!
The last picture shows the part of the focus scale where it got stuck around the 2.5m mark. It is a bit damaged, but has no influence on the functionality of the lens.
Step 9: Reassembly and Testing
Reverse all the steps and you have to put everything back together again. It helps if you take a lot of pictures and sort all parts into boxes and make notes for every screw as described in the tools step.
Dust is everywhere, so one important tool is a lens pen to remove every spec of dust on the lenses inside. (Unless you are fortunate enough to have a clean room in your basement)
Parts that were greased may need some new grease as you usually remove some during disassembly.
If you've done everything right you now have an amazing lens for a fraction of the normal used-condition price!
Now get your favourite subject and take some sample pictures!
After disassembling a photo lens you should take some measurements for sharpness/optical resolution, so you know if everything is good or optical adjustments are needed. This excellent blog post from lens rentals explains the process very well: https://www.lensrentals.com/blog/2012/07/optically...
For taking your own measurements you can use the tool "MTF Mapper". When you got it working it can be very useful. http://mtfmapper.blogspot.com/
First Prize in the
Fix It Contest