20 Sep 2014

Mamiya C33 TLR Repair - Part 2

In Part 1, I showed you the body's mechanics. In this part, I will show you the focusing screen and what I did to calibrate it.

The camera came with a waist level finder and an eye-level porrofinder. Both were good except that the flaps on the WLF needed to be pulled by hand every time. That was alright.

The problem though was that the focusing screen itself seemed dirty. I know that in a DSLR you should not try to clean a focusing screen, in fact, you can't, but I wanted to give this a shot anyway.

I do not recommend you attempt to clean your focusing screen, and you will see that I could not do anything to it anyway, but the pictures below may come in handy for anyone wanting to know how this thing is constructed, in case you want to replace your parts or something.

In this picture, you can see the smudges that made me want to clean the screen. They looked like moisture formulations and I was worried about possible fungus growth.

To remove the focusing screen first you need to remove the finder. You will find that there are 5 aluminium screws holding it in place. Remove these very carefully and try to pull the metal mount vertically upwards. The screws hold the shims that you need so much to keep the screen at the precise right distance from the lens. DO NOT LOSE THOSE, and do not drop them when you remove the metal plate. Especially if you have the rubbish bin right between your legs while working.

This is what would happen if you do. Can you find the shim?

It's in the lowest end of the picture near the abc sticker.

There are three long black paper shims, four small round shims (two yellow, and two thicker white ones), and two very thick (about 1mm thick?) shims.

And this is how the ground glass is mounted on the aluminium plate. Notice the lack of any light seals in this model. I heard the C330 has light seals that function as cushions which push the glass up. In this model you only have the two retainers holding the glass in place, and they are held in place with two screws each. The glass is also glued to the aluminium so I did not bother disassemble any further.

But mainly because I found out that the focusing screen is in fact two layers stuck to each other. The smooth side of each layer is facing outwards so it is safe to wipe clean, but as the formulations are in the middle between the two, separating them for cleaning meant certain irreversible destruction.

In case you want to clean or replace your mirror (if you can source and cut a similar mirror, that is), there is one screw holding the mirror in place and you can access it from the front through the opening of the viewing lens.

Loosen this screw and you can pull the mirror out.

Here you can see the screw again without the mirror in place.

And this is the top of the camera without the mirror. There are some metallic plates that will push the mirror against the edges to hold it firm in place.

If you do not need to mess around with the focusing screen, keep it in place. If you had to clean or change the focusing screen, put everything back in place and pay special attention to the shims. When I took mine off, the shims fell off and I had to figure out where to put each par. In the end, I put the three paper shims in the front, and each screw in the back got a single one of each of the three types of shims. When I forgot one thick shim, the focusing calibration was visibly wrong.

So now that everything is back in place, you need to make sure your focus is right, and to do that, you need to be able to look at both the focus plane, and the film plane.

On this model, you can remove the back cover completely by undoing the locks you can see in the following picture.

Then in order to be able to see what the film will actually record, you need to put another 'focusing screen' where the film goes. For that, semi-transparent paper lamination will do. It's quite thin but only hard enough to maintain its flatness. I used double sided tape to fit this cross-shaped piece of lamination (which is a leftover from this post, if you remember) right where the film should go.

Calibrating the focusing screen is simply a matter of putting the right shims in the right place. You need to have the focusing glass at the same distance from the viewing lens, as the film plane is from the taking lens.

It is very interesting the way you can see the image forming on this plane when you open the shutter. See the video in the previous blog post. However, I decided to a cut a new square piece to cover the entire area because I needed it to be completely flat. Flatness is of absolute importance.

Fix the camera on a firm tripod (Manfrotto Magic Arm in my case) and focus it at any given object. I normally calibrate either for infinity or at the minimum focusing distance of a lens depending on the requirements of the job, but due to the nature of the camera and other limitations, I simply focused at the clearest object I could see which did not cause the parallax correction arm to appear, which was the chrome ring on a lens on my window sill. The shutter's bulb mode will be essential at this stage.

To verify you're doing it right, you now need to look very closely at both images. A large-format magnifying loupe can be your friend, but if you're like me (on a budget) and don't have one, you can, like me, use the front element of an old, broken, Sigma 70-210mm lens along with its own hood. Just insert the element in the hood until it is firm in place. Now you have a magnifying loupe which you can use on both the focusing screen, and the film-plane's lamination screen.

May take a while to get it right, and will take removing the upper focusing screen and putting it back several times, but once you do it right you'll be happy.

Test for the focus at the centre and also at all corners of the frame. Also test focus match at all distances: from infinity down to close-enough for parralax not to interfere. If I had one of those mounts that correct the placement of TLR lenses (y'know, they shift the entire body so that the taking lens is exactly where the viewing lens was), then I would also test it for macro focusing.

Finally, when I was happy that the focusing screen is calibrated right, and before putting the back cover back, it was the right time to replace the light seals. As you can see, the seals were completely deteriorated on my camera. In some cases it may not have an effect as some cameras still prevent light even without foam seals, but I did not want to take my chances. 

First off, the old deteriorating seals need to be removed. Cotton pods, q-tips, wooden sticks (broken matchsticks?) and lots of proper solvents should be used until the thing is pretty clean. I used %99.8 Isopropyl Alcohol.

Then I simply cut the right size and thickness of foam from a big piece that I have. You can probably buy a seal kit pre-cut for your camera model, but it works out a lot cheaper if you do many cameras to buy bigger pieces and cut to your desire.

This camera requires one wide 1cm piece and three long 2mm pieces of 2mm thick open-cell foam. I used self-adhesive ones but you can use latex adhesive to stick a non-sticky foam.


  1. Hi!! I'm very interested on your article. I have a Mamiya c33 and I opened the focus screen, well, the shims have fallen (surprise!!!)) and now I have to figure how to put them in place again. I read your article but I need to see the pics (they arent here). Can you help me? Mi email is hello@miguelpagano.com . Sure I can donate if you can help me! ;-) Tnx

  2. How do I see the pictures for the mamiya c-33 focus screen? Thank you, Ron