Mount Conversion of a Canon New FD Mount supertele lens Canon 500 mm f/4.5 L to the current Canon EOS EF mount for DSLR

An owner of this lens asked me to convert his lens to Canon EF mount - as I did that for my own Canon FD 300 mm f/2.8 L lens before.
I tried to to that much better than my own lens, and did some test configurations - more will follow.

This supetele had a defective mount, it was partly borken, because it once hit the ground hard. For me it seems, that the lens was opened before. A screw was missing, and one screw was loose. The backwards format change part had more tolerances than my Fd 300/2.8L.

First I deconstruct the lens into front part with all the lenses, and the backpart with only the filter and iris inside. To do this, I screwed three screws out of that ring. Opend that ring. For that I have to remove the filter holder.

I opend the screws on the outer barrel of the FD mount ring / breech look. After that the coupling ring part could taken out. From this side one gets no deeper inside the lens!

To get the lens apart, one has to work from the iris side. Best way ist to remove the iris (first make a mark on the iris and the tube housing).
Three screws have to bee opened, then the iris assembly can be removed.
The screws on the upper assembly should not been touched, as long on can work without it. The screws inthe longer holes are for the adjustment of the maximum opening of the iris. The screws in the round holes are for complete disassembly of the iris. Do not touch the iris blades, To not grease them!

As you can see on the right picture, the iris coupling crank is bend (not normaly)

With a long Phillips screwdriver one can open the screws in the tube inside. Remove the tube.

Here you could see the lever for the iris actuation for the bright viewfinder and fast closing to working aperture - what is the English expression for that?
That hole must beo closed afterwards.

With the outer screws one can now detach the Canon FD mount assembly with the iris ring and the inner tubular.
On the left image one can see the bend iris leverage.

Futher disassembly work

Outer iris maipulating ring, ant the inner tubing. .

Now you could see the defective parts! A part of the inner tube was broken at the screw holes.

There are different ways to repair that. I made a metal "L" as repair part.
With this way the I have not to alter other Original FD mount parts. The lens conversion is theoreticly reversibel!

For the iris coupling / connection I made a new arm.
I tested a way with a round metal part with was exactly fittet into the FD mount outer tube. But it was not moving soft enough, I suppose because of the long arm which is needed for the FD 500/4.5L.

I tought about making a ball bearing fot that, but decided not to test that - because of the time needed.
I prepared a part which fits into the Canon FD housing back, directly under the comming EF adapter. This part was partly lathed, and afterwards filed to fit. A metal blank was bend to fit. And glued for the boring.
Afterwards I could thread in this narrow spaces.
The iris work with this adapting part from 4.5 to 11 or 16.

The iris assembly could be opend. This iris is relativ easy to reassemble.

That iris cleaning is not needed for the normal conversion!

I have the bore some new holes for the adjustment of the iris.

For the main adaption to Canon EOS cameras I used a Mf42 to Canon EF adapter. I lathed this adapter to the diameter of the part it has to fit. With this step I get smaller tolerances for decentring.

The best is to glue some velvet inside the lens. Here you could see the difference between some the velvet like velcro, ant the black paint. Black paint reflects much better low angeld light / sided light.

As one can see on the picture below the orientation is not correct.
My first adapter was correct orientated - but the holes are to big. The second adapter was with much better holes - but not correct orientated - what is for this lens very bad because of the tripod mount. So I mande a third adapter, with correct orientation, and furthermore correct holes.

Parad of the Canon supertele lenses:
It seems, that I has assembled the plate upside down on my DIY converted Canon 2.8er 300mm. On the right image one can see the parts that are not deedet for the conversion. But I needed many screws.

This kind of conversion is full reversible. You do not need to alter Canon FD parts in a way that they are no longer usabel for the lens as FD mount lens. I only drilled some holes in the original FD parts.

Now some example pictures of that lens with a Canon EOS 5D.
You can click on the images to get it in full resolution. I think a very sharp and contrasty lens.

Please be aware of all the dangers of a lens mount conversion! You do all on your very own risk!
I am pretty sure that this instruction manual has some faults! At least in this translated version :-)
The conversion was not that easy as it might look.

With more exact parts I have converted the Canon NewFD 400/2.8L to EF Mount now (German site)!

Please visit my own work site!