Canon lens mounts - from the R to the FDn

This page is about manual focus Canon SLR lens types and lens mounts.   Canon's lens mounts are somewhat unique, especially the latest FD mount, called the FD new (or just FDn).   This lens mount makes one wonder; "Why can't I open the aperture when the lens is off the camera? What are all those pins for?" With this work I want to answer these questions and some others.

The evolution of Canon lens mounts in manual focus SLRs

Many camera users who have always worked with 35mm SLRs don't realise that the origin of these SLRs can often be traced back to range-finder predecessors.   When you take a good look at an early Nikon F, a Contaflex, or a Zenit C camera for that matter, you can detect the legacy of the range-finder ancestors.   It is not a surprise then, that some of the early Canon SLR lenses were not much more than a range-finder lens with a new lens mount tacked to the end of it.   The 135mm lens on the next picture (on the far left) is an example of this.   It has the new breech-lock mount, but otherwise looks very much like the range-finder model.

Here I am showing an example of each of the manual focus SLR lens types that were developed over the years.   From left to right they are the following:

  • R lens with a manual aperture.   This is a short telefocus lens; 135mm f/3.5.

  • SUPER-CANOMATIC R standard lens with an automatic aperture.   This lens also has a manual stop-down ring which enables the photographer to check the depth of focus.   This ring is independent of the aperture setting ring.

    The R lenses were used on the Canonflex cameras, such as the R2000 and RM.

  • CANON FL standard lens.   This lens has an A(uto) - M(anual) switch instead of an aperture stop-down ring.   The 'Canon Camera Co., Inc.' inscription was superseded by 'CANON    LENS MADE IN JAPAN' on most FL lenses. Noteworthy cameras of the period were the FX, the FP, the FT, and the Pellix.

  • FD lens.   The initial release of the FD lenses still had the breech-lock mount.   They have a bayonet fitting on the front rim for lens hoods.   The aperture setting ring also has an A(uto) click stop for aperture-priority automatic exposure control.   Several minor variations exist among the FD lenses.

  • FDn standard lens.   Here Canon abandoned the classic breech-lock mount and created something that resembles the bayonet mount of Nikon, Minolta, and other lenses.   The FD new lenses are much lighter than their predecessors, in part due to the use of plastic for some of the components.   (However, it would be wrong to assume that they are not well built.)

Next, I am going to examine the rear of these lenses - the interface between camera and lens.

  • Pic. 1: The R lens is very simple.   There is nothing else just the locating pin and the red dot on the locking ring to help align the lens when mounting on the camera.

  • Pic. 2: The CANOMATIC lens adds a measure of complexity by introducing the automatic diaphragm.   This is actually an advanced version of those semi-automatic lenses, where the user has to open the iris and tension the stop-down spring by rotating a ring or a lever on the lens before each exposure.   In the case of the CANOMATIC the camera does this.   When the shutter is cocked, the pin on the right hand side gets pushed to the left by a lever on the camera and that readies the lens for the exposure.   When the shutter release button is pressed, another lever in the camera hits the other pin and that causes the aperture blades to close down momentarily.   (While all this is happening, it is important to have the manual stop-down ring set to the largest opening.)

  • Pic. 3: The FL lens is very simple.   It has a fully automatic diaphragm which is operated by a single pin.   By default, the aperture is fully open.   For the duration of the exposure, an arm in the camera pushes the pin to the right causing the aperture blades to close down to the preselected f-stop.   Conveniently, on all FL lenses there is some way of stopping down the aperture manually.   Most often this is with an A(uto) - M(anual) switch.   Other implementations include a push button or a stop-down ring.

    Light measurement with an FL lens is, by necessity, of the stop-down type.   The lens does not relay any information back to the camera.

  • Pic 4: With the FD lenses the lens-camera interface got a lot more complex.   Here, Canon made not only one, but two improvements; introduced full aperture exposure metering, and made aperture priority automatic exposure control feasible.

    A Signal Lever (on the left hand side) communicates the aperture value - set by the user on the aperture selector ring - back to the camera.   The Full Aperture Signal Pin (at the 7 o'clock position) transmits the largest opening f-stop of the lens.   Both of these data are necessary for full aperture exposure metering.   The second establishes the reference point in relation to which the first one is considered.

    When the aperture selector ring is set to the A mark (or o on earlier lenses), a tiny pin pops out and it makes mounting the FD lens on cameras without automatic exposure control impossible.   (This pin is hidden on the first picture, but you can see it on the second picture at the 1 o'clock position in the ditch.) So, it is a good idea to check the aperture ring before mounting an FD lens on an older camera.

    The lever at the 6 o'clock position is the one which is responsible for closing down the aperture when an exposure is made.   Its shape is different, but its function is the same as that of the single pin on the FL lens.

    There is a pin at the 3 o'clock position about which Canon said it was 'reserved for possible future additions to the SLR system'.   Since the 'future' is now history, we know that this pin was never used for anything.   One can only wonder what kind of 'additions' the designers envisaged back then.

    When the lens is not on the camera, the iris is fully open and it cannot be closed down by rotating the aperture selector ring.   The Aperture Signal Lever is firmly locked in position, and nothing happens when one flicks the Aperture Lever.   This might frustrate people, who are used to other manufacturer's lenses.   Fortunately, there is a solution.   Find the positioning pin at the 12 o'clock position.   Slightly to the left of this, there is another pin hidden under the mounting ring.   If you push this pin in, you can turn the mounting ring to the left (counter clockwise).   Once this is done, everything works as expected.

    In their documentation, Canon stated that FD lenses can be mounted on cameras designed for FL and Canomatic R lenses.   The reverse is also true; FL and Canomatic R lenses will mount on contemporary Canon SLRs, such as the F1 and FTb.   I tried this and, although non-matching lens pins and camera levers are dangerously entangled, these setups work - within their obvious limits.   What I mean by this is that an FD lens will work like an FL lens on, say, the Pellix or FT and will work only in manual mode on the Canonflex.   How do you turn an FD lens into a manual lens? By pushing the Aperture Lever (the one at the 6 o'clock position) fully to the right and locking it in that position.   On the lens pictured below, the locking is accomplished with the little lever near the L mark.   (L for 'lock', obviously.) On other lenses there is no locking lever, but if you push the Aperture Lever with some force fully to the right, you find that it stays there. And now you can mount the lens and it will work as a manual lens.

  • Pic. 5 and 6: Now we come to the most recent manual focus lens, the FDn (n for 'new').   This is even more mysterious than the first version.   I will not repeat the pins that are the same as before, only discuss the new features.   The most obvious difference is that the breech-lock mounting ring disappeared and the lens now mounts similar to other manufacturers' lenses; you press the back of the lens against the camera's lens mount while aligning the red marks and then twist the lens until a click is heard.   However, this is not a true bayonet mount even though it might feel like one.   (As a matter of fact, it is this 'feeling' that Canon had to reproduce, as most people ended up preferring the bayonet mount over the breech-lock.   Personally, I like the breech-lock mount and am not in need of a 'third hand' when changing lenses.   Nikon lenses confuse me more.)

    The same lens is depicted on the last two pictures above with some difference.   See, if you can tell the difference.   The first of the two pictures shows the back of the lens as it was taken off the camera.   The second, as if the lens was on the camera.   Part of the lens stays put while the rest of it rotates around it, when the lens is mounted.   And, unlike with other lenses, you press a button on the lens (the release button), when you want to remove it from the camera.

    The FDn lens also seems like a puzzle when you want to operate the controls.   (You can't even open the aperture blades to examine the glass for fungus, etc.) Again, for anything to work, one has to put the lens into an 'on camera' state while it is not on the camera.   This is harder to do on the FDn than on the FD.   With reference to the second last picture, find the two small pins in the ditch at the 10 o'clock and 2 o'clock position.   These two small pins have to be pushed in simultaneously while you also turn the inside of the lens clockwise.   (Now you do need a third hand.)

    In some specialised applications, the lens is mounted back-to-front.   Canon made a ring that attaches to the back of the lens, so that the aperture can be manually operated.   Unfortunately, this accessory is very rare, but if you had one, you could achieve the same result as with the difficult exercise described previously.

    Another rare accessory is the manual diaphragm adapter.   This is a small item that keeps the Aperture Lever in the same position as the lock lever on the FD lens above.   Without this, the Aperture Lever cannot be locked in the rightmost position on the FDn lens.   (I guess Canon made this simplification to the FDn lens, because on modern cameras manual aperture control was seldom required.)

Lens caps

With every new type of lens (FL, FD, etc.) came a new release of lens caps.   With the front lens caps the change was, for the most part, cosmetic.   In the case of the rear lens caps the underlying reason was different, as we will see below.

Each lens cap above typically goes with one type of lens, although there are overlaps.   The silver cap is for the non-automatic lenses used on the early Canonflex cameras, as well as, their rangefinder counterparts.   The standard CANOMATIC R lens was also fitted with the silver metal cap, but the others had slip-on plastic caps.   These plastic caps later found their way onto some FL lenses, too, but most FL lenses had black metal slip-on caps, such as the one in the middle, bottom row.   The clip-on lens cap on the top right is for FD lenses and the one below it is for the FDn lenses.

There are subtle but important differences among the rear lens caps.   In some cases a new lens mount necessitated a change to the rear lens cap. For example, when shutter priority automatic exposure was introduced (in the Canon EF), a new pin was added to the rear of the lenses.   To accommodate this pin, an extra recess was cut into the perimeter (or more precisely the 'lip') of the rear lens cap.

This also means that not all lens caps are interchangeable.   The FL rear lens cap will not mount onto the FD lens, if the aperture selector ring is set to the o (auto) mark.   The FDn lenses will only take the lens caps specifically designed for them (i.e. the latest version).   However, lens caps are backwards compatible (you just need to know their chronological order).   On the picture above, the lens cap on the left will fit the Canomatic and FL lenses.   The middle one is for the FD lenses and the rightmost one is for the FDn.


  1. Fantastic information for Canon SLR fans! Thanks.

  2. I'm really enjoying you pages. Mine are at

    1. Thanks Kevin. Your site is great, too. I surely will be visiting it again and again for the excellent info it contains.

  3. This comment has been removed by a blog administrator.