Canon FD lenses

Canon FD lenses - background information

There was a similar FL mount that preceded the FD mount. The FD mount was revised and modified at some point, so there are actually two variants of the FD mount.

The original FD mount (sometimes called the "old" FD mount) has a silver ring that is used to attach the lens. (The ring was silver in lenses manufactured by Canon, lenses made by other manufacturers can be any color). When mounting the lens, the lens never rotates, it is mated with and held against the camera and the ring is rotated to tighten the attachment.

The new FD mount (sometimes denoted "FDn") has a black ring and attaches in a different way that does involve rotating the lens.

Notes on the old style mount

It can be tricky to mate an FD to EF adapter to a FD lens if you have no experience with FD lenses. Some FD lenses have a little latch that prevents the silver ring from rotating until it is on the camera. The intent here is to keep the ring in the proper place until it actually is engaged with the camera lugs. Some cheap compatible lenses lack this latch and the ring can spin to any location. The ring needs to be fully clockwise (viewed from the back of the lens) to mate with the camera or adapter). Also remember that when mating a lens to an adapter, it is the ring on the lens that needs to rotate, any rotating ring on the adapter is for aperture control.

It is important also to ensure that any mechanism the adapter has for controlling the aperture is properly engaged with the lever that controls the aperture. This will usually "just happen" if the adapter is brought up to the lens at the proper position. If the adapter is rotated on the back of the lens while attempting to find the right position, it is possible for this lever to be improperly engaged.

Notes on the new style mount

Here, in contrast to the "old" FD mount, the lens rotates. You match up red dots, jiggle things a bit to get a little tab on the lens to drop into a slot in the adapter and rotate the lens counterclockwise (as viewed from the back of the lens). There is no loose silver ring. To dismount the lens you push a silver button and turn the lens the other direction.

This is much more like other manufacturers "bayonet mount" in contrast to the "breech mount" that the old FD lenses provided.

My 100mm and 200mm FD Canon macro lenses have this style of mount. The 50mm macro is old style.

FD lens levers and pins

This really calls for a diagram, but here goes without one (but visit the following link).

Early FD lenses

Hold an FD lens so you can look at the rear of it and so that the levers are down with one toward the right and one toward the left.

The lever on the left is the aperture indicator. It moves with the aperture ring and served to tell the Canon camera body what the aperture setting is. It could be done away with in an FD to EF conversion.
The lever on the right is the aperture control lever. When moved to the right it closes the lens down to the selected aperture. The camera would have controlled this lever at the time of an exposure. It is vital to provide some way to control this lever as part of an FD to EF conversion.
The big pin between the two levers indicated the full aperture of the lens to the camera to allow for wide open metering. As near as I can tell it is the height and not the position of this pin that is relevant. This is unimportant in an FD to EF conversion.
The smaller pin towards the top of the lens is fixed and apparently part of the camera to lens coupling, its purpose is unclear. There is a similar fixed pin "down in the groove" toward the upper left whose function is unclear.
There is a tiny pin down in the groove towards the right that moves up when the aperture ring is set to "A". From what I have learned the purpose of this is to lock out the "A" setting when the lens is mounted on certain cameras. In other words this pin needs to move up for the ring to be set to "A". A camera that allowed this setting would have a hole drilled for the pin to move into. A camera that did not would have a flat surface here and the pin would be unable to move. This is unimportant to a FD to EF conversion.

Distances and registration surfaces

My measurements and calculations using a glassless adapter on an old FD lens show that one of my adapters adds about 10.74 mm of extension to the registration surface of an FD lens (add 2mm to this to get the overall extension when mounted on an EF camera). Adapters can and do vary and typically extend the FD registration surface by 10 to 12 mm.

The FD registration surface is the surface "down in the groove between the lens body and the rotating ring, at least on an "old" FD lens.

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Tom's Digital Photography Info / tom@mmto.org