Modifying Canon FD lenses for use on EOS cameras
This is the way to do things right.
Thee are a number of people (discussed elsewhere)
offering kits (for $120 to $140) to modify various FD lenses
for use on EOS cameras.
This page discusses some of the issues if you want to attempt it
yourself.
Some people do this with hand tools, JB weld, and a dremel tool.
There are things to learn from their process, but since I have
access to a machine shop, this is not the approach I intend to take.
Machine shop work
The game is to remove the FD locking ring, and then in some way attach
a flange with the proper EF mount bayonet lugs.
Some means of controlling the aperture must also be considered.
Some cases will require some lathe work, but since I am seeing kits
offered to do the conversion, I am convinced that modifying parts of
the original FD lens is not essential in many cases.
If you plan to put a modified lens on a full frame camera,
you need to worry about the FD control levers protruding far enough
back into the body that the mirror crashes into them
when you take a photo, causing catastrophic camera damage.
I have heard that lenses 28mm and shorter cause this sort of trouble.
Infinity focus fine adjustment
Most FD lenses (perhaps most lenses period) are designed to allow some
fine adjustment of infinity focus in the first place. Typically there
are setscrews holding a lens body within a focus helix which can be
loosened and the lens body repositioned.
FD mount details
The FD mount was designed in 1971.
The FD mount is a "breech mount" system.
The main advantage is that the lens is not rotated against the camera
when it is mounted, avoiding wear on critical surfaces.
There are two "flavors" of the FD mount, the old and the new.
The new system allows faster mounting of the lenses.
FD lens linkages
If you have ever looked at the back of an FD lens, you may have been
puzzled by the various pins and levers:
- There is a lever, usually with a bevelled section that the camera uses
to stop the lens down to the chosen aperture when the picture is taken.
In a conversion, some means must be provided to manipulate this lever.
- There is a lever that tells the camera what the chosen aperture is.
This would have communicated with the metering system.
It would serve no purpose in a conversion.
- Maximum aperture indicator. This is a fixed pin on the lens.
As near as I can tell at a different angular position for lenses with different
maximum apertures. This was linked to the meter for full aperture metering.
One guys job on a FDn lens
The above link describes how this fellow modified a FDn mount 135mm f/2 lens.
I may need to buy a lens spanner.
He used epoxy to customize some internal parts, and JB weld to bond his
thinned EF mount to the thinned piece that screwed to the lens.
I can't say for sure until I try it myself, but I would have been tempted to
drill holes in the EF mount or his thinned piece so I could use screws to screw
both to the lens. This way the thinned piece would just be a spacer or washer.
He also says that all the ball bearings that fell out of the lens did not need
to be replaced. Someone elsewhere said that these were used because these lenses
had to function with motor drives that would be working them at high speeds.
Once they are customized for manual use they can probably be eliminated.
Also it seems that he has not provided a ring or lever to work the automatic
lens mechanism, so this modification works with a totally manual diaphram.
Another effort on a 50/1.8 FD lens
He says that these sorts of lenses are very desirable for video work because the focus
helix on manual lenss has a long taper. Lenses designed for autofocus have a short helix.
He does not show all the details and it is not clear if this is an old or new FD lens.
He uses epoxy to customize the internal aperture linkage, joining a U-shaped part to
the aperture ring. He also uses epoxy to join his flange to the lens.
He uses a chipped M42 adapter for a mounting flange.
He says he ground 1.0 to 1.5 mm off of the back end of the lens, but did not
show or describe how he did the grinding. He then uses trial and error and shims
to achieve infinity focus. His modification yields a totally manual diaphram.
He says to take care about the "clocking" of the lens when you mount the flange
since you are free to choose any angular position.
Great video: Jeff Johnson converting a 50 f/1.8 lens (old FD)
He does things right, using a lathe and doing quality work. He says that there are many variants of the 50 f/1.8
and he got lucky picking one that would be easy. He does not show all the details, but what he does show is
very instructive.
- Remove the silver ring.
- Remove 3 screws and lift the back piece, the levers stay with it.
- Determine the required registration distance by trial and error.
- Use a lathe to machine an aperture control adapter (ring with flange).
- Use a lathe to reduce the aperture control section to get proper registration distance.
- Mate the two pieces and drill holes for the mounting screws.
- Use and end mill to countersink the drill holes.
- Use the lathe to modify the silver ring (plated brass) to become a trim ring.
- Reassemble everything.
It is not clear what EF mounting flange he is using, or what the details of the aperture control adapter are.
This is an exercise left for the reader.
Feedback? Questions?
Drop me a line!
Tom's Digital Photography Info / tom@mmto.org