Cleaning a Canon LTM 50mm f1.8 Lens

Sometime ago I bought a Canon 50mm f1.8 LTM lens, also known as Serenar. It had some haze on the internal elements, but between one thing and another it took me a while to get to it and clean it. I managed to remove the haze from all the optical surfaces but one.

One of the internal elements has some haze that doesn’t come off with anything I’ve thrown at it. I tried H2O2, H2O2 with windex (for the ammonia), windex, alcohol, acetone, lighter fluid. None of these worked. My guess is that it is either a coating damage, in which case there is nothing to do, or perhaps it is the doublet that is separating (though it looks to me the haze is on the outside surface, not in between the elements).

I took some pictures of the steps to take the lens apart, since I couldn’t find much online. To take this lens apart you will need a rubber tool. Also, I didn’t lubricate the aperture blades at all after cleaning them. I’m not sure if that’s the way to go, but even the smallest amount of oil was migrating on the blades so I decided not to do it.

 

Use a rubber tool to unscrew the front ring

Use the same rubber tool to unscrew the front element

As you can see, there was haze and what appears to be fungus on the inside of the front element. I used Hydrogen peroxide mixed with ammonia to clear the lens. After that, I used lighter fluid and a cotton swab to remove any residues. I also used a clean microfiber cloth to wipe the lens clean. After that, I used an air blower to get rid of the lint left behind. The front element came out perfect, nothing could be seen with the flashlight test.

With the spanner wrench, remove the outmost ring that keeps the mount attached

The mount part of the lens can be slid off

The mount, with the helicoid, was working fine, so I didn’t take it apart any further

Remove the spacing ring and don’t loose it

You can now unscrew the rear element

Remove the three screws just like the one in the red square in this picture

Remove the pin that controls the aperture

Now the front element mount can come out of the aperture ring

If you want to clean the aperture blades (They were very oily in my lens), remove the brass retaining ring

Now you can remove the slotted ring that controls the aperture

With the aperture controlling ring off (on the left) we can now remove the blades

It might be easier to remove the blades when they are in this configuration. Don’t worry, it looks scary but it is not that hard to remove the blades and put them back in. Just use a good set of tweezers and take your time

One of the blades

You can also, if you haven’t done it already, remove the last doublet. this is the one that has marks on the coating

The blades cleaned and ready to be reassembled

The blades reassembled. Compare this picture with the one before cleaning, and you’ll see how much oil was on them

The aperture ring (the one you move to change aperture once the lens is fully assembled) was stiff, so i decided to take it apart. Remove the screw with the red arrow.

Remove these two screws. The brass spring comes off and be careful not to loose the metal ball that the spring keeps in place. Be sure to scribe the screw position before removing them.

Remove these 4 screws. As before, scribe the position of the screws on the brackets they are holding, so you will be able to put them back in the right position when reassembling.

Remove the last two screws and remove the slotted piece

Remove the brass retaining ring

Now the two pieces come off. Clean them and lubricate them slightly with some grease.