Pages 26 and 27 of the booklet “Canon Lenses Directions and Tables” (Pub. No. 202-A) shows the Serenar 800mm f/8 lens introduced in 1953 with its attached Mirror Box. The lens did not connect to the cameras rangefinder system but instead relied on viewing straight down through an eyepiece to view and focus through the lens.

Mirror Box 1

In 1956 Canon issued a 400mm f/4.5 lens with a Mirror Box but this one was a separate unit. Collectors have come to call this the Mirror Box 1 but Canon never used the numeral. It was just “the Mirror Box”. This is what is shown in the first image above and in the one immediately below.

The Mirror Box screwed into the camera body using the M39 mounting threads. On the lens side was a two lug bayonet mount with an M39 threaded mount in the inside. Notice that the lens mount has, in the top bayonet lug, the Positioning Pin Receptacle which is incorporated into later lens mounts R, FL, FD and FDn. As similar as this lens mount seems, none of these later lenses will mount on it. The lugs are wrong.

The Mirror Box 1 with a
Canon 400mm lens attached to it.
The Mirror Box screws into the rangefinder body
using the M39 screw mount. The lens uses the two
lug bayonet mount to attach to the Mirror Box. Notice
the chrome colored mounting ring on the lens. This rotates to
lock onto the bayonet lugs just as an FD lens would do. But the FD  lens mount
has  three lugs and so will not mount on the Mirror Box.

The Canonflex (“R”) Lens Mount

In May of 1959 Canon released their first true Single Lens Reflex. The lenses for this camera were the “R” Series and they had a new lens mount: the “R” or Canonflex lens mount.

The rangefinder lenses had to communicate focus distance to the rangefinder but that problem was solved with Through the Lens (TTL) viewing. But this introduced a new difficulty. For accurate focus and clearest view one wanted to focus at the widest aperture possible. However, proper exposure and depth of field usually required that the lens be stopped down. To accommodate both requirements the new mount and the lenses were designed so that the lens remained open as wide as possible until the moment the shutter was to fire. Just before the shutter fired the lens was stopped down, the shutter opened and closed and then the lens was opened again.

To hold the lens securely on the camera Canon used the Bayonet Lug Ring from the Mirror Box 1 but they created three bayonet lugs instead of the two on the Mirror Box and so lenses for one will not fit on the other. A locking

The mount end of the Super-Canomatic 50mm lens.

This Brochure (Publication No. 5088) describes the “R” Series lenses and demonstrates the confused state of “R” Lens design.

Importantly Canon chose a flange distance of 42mm which became the standard for SLR’s until the advent of the EOS cameras. The Bayonet Lug Ring inside diameter was a hair under 48mm and this remained a constant as well until the end of the “T” Series.

The Mirror Box 2 Lens Mount

When Canon released the Model 7 rangefinder camera in September of 1961 it had an M39 lens mount so it could use the wide range of Canon lenses available. However, around the M39 opening there was a three lug Bayonet Lug Ring. This was included to accept the new Mirror Box 2 which was released about the same time. Mirror Box 2, the Canon name for it, was a considerable improvement over the eariler effort. The prism could be removed allowing the user to view the image looking down or, with the prism, he could view the scene from the back of the camera like the earlier Canonflex and later SLR cameras.

For the lens mount Canon retained the Bayonet Lug Ring of the
Canonflex camera so most lenses of the R, FD, and FDn series
lenses will mount on it but they won’t function properly. A
special series of lenses was created for this Mirror Box, the
M Series lenses, which have the longer flange distance
necessary for the Mirror Box 2. they are completely
manual in operation so the auto and light meter functions
of the later FL and FD lenses will not work.  And the flange
distance is larger so infinity focus is impossible with any but
the M lenses.

The long and short of it is that, although the other lenses may
mount on the Mirror Box 2, only the M Series lenses will
work properly with it.

The FL Lens Mount

The FL lens mount was an interim design that basically corrected weaknesses in the previous R Mount. Only six “F” Series cameras used it: the FX, FP, Pellix, Pellix QL, FT and TL. In that short time over thirty lenses were released for it from 19mm to 1200mm. Because the same locking arrangement was used on later FD lenses most of the FL lenses were useable on the later cameras. We shall look at the compatibility issue in more detail below.

In terms of functionality the R and FL mounts were similar. One set the aperture on the lens but the lens stayed open and when the shutter was fired the lens automatically stopped down for the exposure.

However, Through the Lens (TTL) metering was beginning to appear. The FX and FP did not measure light through the lens. They were essentially manual cameras. However, the other four cameras, the two Pellix, the FT and the TL, did measure light through the lens and light readings had to be measured with the lens stopped down. FL lenses did not communicate the f/stop  of the lens to the light meter in the body. Provision was made to momentarily stop the lens down for this purpose but it had nothing to do with the lens mount other than it was the Aperture Lever Actuator that moved to the side moving the Aperture Lever on the lens.

Pushing the Aperture Lever to the right, in the picture, the lens stops down to the f/stop set on theAperture Ring on the lens.

The FD Lens Mount

It was the advent of ‘Through the Lens’ (TTL) light readings drove the next development in Canon lens mounts. There were two problems. For focusing the lens should be at maximum aperture. As a result, light readings had to be as well which meant that the lens had to communicate to the camera body what its maximum aperture was so that this could be taken into account.

Secondly, the metering system had to know at what aperture the image would actually be captured so that the light reading taken at full aperture could be properly adjusted for the stopped down value.

And on top of that, Canon did not want to leave users with older lenses high and dry so the lens mount had be be backward compatible as much as possible. And you must remember that this mount was developed before electronics began to control cameras so all of the information flow between camera body and FD lens had to be analogue and mechanical. The result was a complex  mechanical lens mount with no electronics.

The result was the FD lens mount which was to serve Canon for the next two decades until it was retired by the Canon EOS lens mount.

For fully automatic exposure operation the lens must communicate many pieces of information with the camera body. This communication is totally mechanical as the design was created before the coming of the electrically operated cameras. These began their appearance with the Canon “A” Series but the lens mount design was already in use and remained so until the advent of the EOS 650 in 1987. The advantages of the EOS lens system marked the end of the FD lens mount.

The FD lens mount had the same Bayonet Lug Ring as the R and FL, and even Mirror Box 2, mounts. As a result backwards compatibility was, for the most part, achieved. There were some differences in operation and a few lenses were not compatible but most were not a problem.

So let’s go around the mount and look at how the magic was accomplished. The Aperture Signal Lever tells the camera the preset f/stop at which the picture will be taken. This is communicated throught the Aperture Signal Sensor in the camera body.

The Full Aperture Signal Pin tells the camera the maximum aperture of the lens. This is the full aperture at which the light reading will be taken. The height of the pin corresponds to the aperture. For an f/3.5 lens it is 1.0mm high, for f/1.8 it is 2mm high, flr f/1.4 it is 2.5mm high and so on. This height is sensed by the Full Aperture Signal Pin Sensor in the camera body.

The Automatic/Manual Aperture Lever stops down the lens to the preset f/stop for the actual exposure. This is accomplished by the Aperture Lever Actuator in the camera body moving sideways.

The EE Switch Pin on the lens extends outward when the lens is set to Automatic, the Green “A” or “o” on the aperture ring. The pin fits into a corresponding hole in the Bayonet Lug Ring, the EE Switch Pin Receptical. However, if the camera is not capable of automatic exposure, such as the TL, TX or TLb models, the  EE Switch Pin Receptical is not present and the EE Switch pin has no where to go. This prevents the lens from being set to automatic exposure. Also, if the lens is already set to automatic you cannot then mount it on these cameras.

This is all rather obscure which testifies to the sophistication of this lens mount. There is a lot going on as the lens is telling the camera body what it needs to know about itself. And it is all done through mechanical linkages. To really grasp this mount one should sit with a few different lenses and camera bodies and work your way through this explanation.

The FDn Lens Mount

The FDn lens mount is easy to discuss because it does not exist! In June of 1979 Canon introduced a redesign of the FD lenses to make them more like a true bayonet mount. Collectors have come to call these the FDn lenses. The locking ring was done away with and the lens mounted on the camera by simply inserting it into the mount and turning it. But, no changes were made to the lens mount on the camera body. The FD lens mount remained unchanged. So, there is no FDn lens mount, only FDn lenses.

Lens Mount Compatibility

Because the Bayonet Lug Ring has not changed through the R, Mirror Box 2, FL and FD lens mounts pretty well all of the R, FL, FD and FDn lenses will fit on any of the mounts but there are problems. So let’s discuss now what lens will work on what mount and what problems might we encounter. This information is hard to come by because it is not explained by Canon in their manuals and guides and the answers you find on line are confusing and inconsistent. I have come by it by sprreading out on a table all of these bodies and lenses and trying them on each other. And what follows is what I have learned.

The Mirror Box 2 is easy to dispose of: only M series lenses will focus correctly to infinity on it. Although the FL, FD and FDn may fit on its Bayonet Lug Ring they will not focus correctly. The flange distance is much too great. M Series lenses will fit on FL and FD bodies but they are unsable. The flange distance is far too short.

R Series Lens Compatibility   The lenses designed for the FL and FD lens mounts will work on the Canonflex body, sort of. All will certainly mount correctly however the aperture will have to be set manually. Exposure will have to be read with the lens stopped down and focus will be wide open. It is awkward but it does work and good pictures can be taken.

FL Series Lens Compatibility   FL lenses will fit on the other lens mounts with two exceptions. The first is the FL 19mm f/3.5 lens which must have the mirror locked up on any SLR. If you do not the lens extends so far into the mirror box that the mirror will strike the lens. One or both may be damaged.  The Canonflexes and not all FL and FD bodies have this feature. The second is the FLP 38mm f/2.8 lens made for the Pellix camera. It will not fit on any other bodies without modification which is not recommended. That lens too sits too far back into the mirror box.

Except for these the FL lenses will fit on the Canonflexes but the aperture will have to be handled manually. They will also fit on the FD lens mount bodies and the aperture will stop down automatically for the exposure. However, the lens cannot communicate the maximum aperture of the lens to the camera so the internal metering will not work correctly. To use the camera’s light meter the lens will have to be stopped down for the exposure reading and then reopened for focusing and composition. 

FD Series Lens Compatibility   FD lenses will mount on the Canonflex R lens mount but the mechanism in the camera body to stop the lens down does not connect with the lens so it is necessary to stop the lens down manually. And this is where it gets a bit complicated.There are two types of FD lenses which are placed into manual mode differently.

On the second version manual mode is is set by pushing the Automatic/Manual Aperture Lever up and locking it into position with the Manual Aperture Lock Lever.

The two types on FD lenses are shown above. Look at them carefully. Both are set to manual mode. Read the captions for how to lock them into manual mode. Once there the aperture will be set at the setting on the Aperture Ring and will not remain open at full aperture unless that is the setting. Notice that in Style 1, on the left, the Automatic/Manual Aperture Lever locks almost up to the EE Switch Pin while on the second style, on the right, the Aperture Lever locks at a much lower position because there must be room for the Manual Aperture Lock Lever to fit between them. And that is a problem.

Style 1 can be mounted on any of the Canonflex cameras and operated in manual mode. However if you mount Style 2 on a Canonflex and then wind the camera you will find that the Automatic/Manual Aperture Lever jams against the Aperture Lever Actuator because it has not rotated high enough to clear it. These lenses cannot function on the R mount.

On the FL lens mount the FD lenses stop down automatically as designed but metering must be done while stopped down using the Stop Down Lever on the front of the cameras. However, on the FX and FP there is no such lever and so the lens must be set to manual mode. Both style will function in manual mode just fine.

FDn Series Lens Compatibility   The FDn lenses present a different problem in that there is no mechanism in the lens to set it into a manual mode. The exception are the macro FDn lenses which can be set to manual mode just like the Style 1 FD lenses. So what do we do?

On the FL lens mount the FDn lenses operate just as the FD lenses do. The aperture stops down automatically for the exposure and for metering the lens can be stopped down with the stop down lever on the front of the camera. On the FX and FP cameras without the stop down lever the lens must be stopped down manually. Which we cannot do except for the Macro Lenses. On the Canonflex lens mount we have the same problem. The lens must be used manually and so only FDn Macro Lenses can be used.

We can summarize what fits on what in a chart like the one on the left. However, let me emphasize again that when using a lens on a mount that it was not designed for you must use great caution.

Some lenses that reach back into the mirror box must be watched because they may interfer with the moving mirror.

Some lenses may not engage with the sensors and levers in the camera body. And just because the lens fits in the lens mount, that does not mean it will operate correctly.

Read the manual for the camera body very closely. They often warn about which lenses will and will not work on that body. But if the manual says nothing about the correct lenses do not assume that means things are OK. It probably means that they did not comment assuming you would only use the correct lenses.

There are two big rules. The first is never never never force anything to get it to fit. Parts that are meant to be together fit easily. Secondly, never let WD-40 or any similar spray lubricant anywhere near a lens of any make or model. This is bad stuff for cameras.

What about other Lenses?

We have looked at the Canon lenses for the SLR’s but there are other lenses to consider, both from Canon and from other manufacturers. But first we have to look at Flange Distance. That is the distance from the face of the lens mount that the lens sits against to the surface of the film or sensor. The R, FL, and FD lens mount all have a value of 42mm.

his is important because any lens with a longer flange distance can be made to work on one of the R, FL or FD lens mounts by the addition of a short spacer to bring the lens out to the correct flange distance for that lens. If this is done the lens will focus to infinity correctly without additional lenses.

However, lenses with a smaller flange distance cannot focus to infinity without the addition of an extra lens in the mount. And it may not be possible to create an adapter for such a geometry.

Canon actually made an adapter that allowed R, FL and FD lenses to mount on their M39 rangefinder cameras. This was the Lens Mount Converter B. It did not connect to the rangefinder so it was necessary to “zone focus”. Canon’s various lens converters are discussed under the heading “Accessories”.

The EF lenses have another problem: their apertures are controlled electronically only and so no matter how you mount them on an SLR you will not be able to control the aperture. And using full aperture, focus will be manual as the SLR camera bodies have no autofocus controls.And a final word. Look at the Flange distance of the RF lens mount: 20mm. That is substantially less than the S, R, FL and FD lenses. This means that adapters are available to mount these lenses giving them new life in our electronic age.

Many third party makers of lenses also made optics for the R, FL and FD lens mounts.  An example is Tamron and their Adaptall 2 adapter allowing their line of SP lenses to mount and function perfectly on the Canon SLR  cameras. And there were many others.