Lens mounts: Canon EX

Pekka Buttler, August 2023 (Updated 12/2023)


This article is about the Canon EX ‘lens mount’.
Technically, the Canon EX is not a lens mount at all (as a big part of Canon EX lenses were permanently attached to the camera body), but is instead the attachment interface of the Canon EX family of set lenses.

If you’re unfamiliar with the concept of set lenses, we recommend reading the JAPB article on set lenses before going on.


As noted, Canon EX lenses are not actually lenses, simply the front-ends of lenses. The back-end (including 3 lens elements, the diaphragm etc.) of the Canon EX lenses is permanently attached to the camera body.

The attachment interface between the front-ends and back-ends of Canon EX lenses is a simple 39 mm thread. No communication between front-end and back end are possible.

Pictured: Canon EX 50mm f/1.8 front-end. See 39 mm thread at rear of lens.

Historical context of the Canon EX line

Canon is undoubtedly one of the great names of the camera business and Canon has been heavily involved in the development of high end cameras and lenses. Even though Canon also offered both compact and high-end rangefinders and viewfinders cameras, since the early 1960s, Canon’s main focus has been on developing interchangeable lens high-end cameras.

Such cameras by necessity need a lens mount, and the genealogy of Canon lens mounts can be summarised as:

Canon R 1959–1963
Canon FL 1964–1968
Canon FD 1971–1990
Canon EF 1987–today
Canon EF-S 2003–today
Canon EF-M 2012–today
Canon RF 2018–today

You might have spotted a discontinuity between 1968 (the year when the last Canon FL mount body was introduced) and 1971 (the year the first Canon FD mount body was introduced. In between – in 1969 – the Canon EXEE body and the first Canon EX lenses were brought to market.

I do not know whether the timing is a coincidence, whether the EX line was intended as a stopgap measure, or whether Canon was seriously toying with set lenses as a solution to its problems. Bear with me while I speculate and elaborate.

The Canon FL mount and how it did not satisfy.

(Read also: The JAPB article on the Canon FL, FD and FDn mounts)

Canon had since 1964 manufactured cameras and lenses using the Canon FL lens mount. The FL mount was a decent enough mount for the early 1960s, and Canon had produced a very nice lineup of camera and lenses, but the lens mount had a significant shortcoming. It did not facilitate exposure automation.

While the FD mount offered a lever by which the camera could close down a lens to whatever aperture the photographer had selected, there was no way the camera could know what that aperture was. Hence, aperture-priority auto-exposure could not be implemented.

Alternatively, to facilitate shutter priority, the camera would have needed to be able to set the lens’ aperture precisely to whatever value the exposure automation had predetermined, and the FL mount did not support that either. Here it must be said that this is not as easy as it sounds to implement precisely. Considering that the motion distance of most lens mounts’ aperture stop-down levers/pins is 4 mm +/- 2mm, it necessitates a lot of precision to be able to stop down a lens precisely to f/5.6 (especially as depending on the lens, that might be either stopping it down a lot or none at all).

I’m willing to speculate that Canon was torn whether they would
a) Develop an entirely novel approach.
b) Develop a new mount that would not support the significant installed base of Canon FL lenses.
c) Develop a new mount that would somehow be compatible with Canon FL lenses.

What Canon did was go for both A and C.

Alternative A: an entirely novel approach (the EX line)

In October 1969, Canon introduced the Canon EXEE camera body and the first Canon EX lenses. Or more precisely: lens fronts, as the Canon EX line was based on the concept of set lenses, where the rear of the lens was permanently attached to the camera body, and only the front-ends of the lenses were actually interchangeable. Initially three front-ends were offered (35, 50 and 95 mm) and another front-end (125 mm) was introduced half a year later.

Canon EX lenses
Canon EX 35mm f/3.5 (I), (1969–?), 6 elements in 5 groups, 122 grams
Canon EX 35mm f/3.5 (II), (?-1973), 6 elements in 5 groups, ? grams
Canon EX 50mm f/1.8, (1969–1973), 3 elements in 2 groups, 55 grams
Canon EX 95mm f/3.5, (1969–1973), 6 elements in 4 groups, 280 grams
Canon EX 125mm f/3.5, (I) (1970-1971), 7 elements in 5 groups, 445 grams
Canon EX 125mm f/3.5, (II) (1971-1973), 7 elements in 5 groups, ? grams

Importantly, by going for a set-lens based approach, Canon made the aperture mechanism (which was in the rear-end of the lenses) an integral part of the camera body, hence enabling more precise control of the aperture. Canon also made good use of this as the Canon EXEE was Canon’s first camera to enable shutter priority automatic exposure based on through-the-lens light metering. While Canon was by no means the first manufacturer to achieve this, one could argue that Canon’s approach made auto exposure more than a guessing game.

Alternative B: a better version of the FL mount (The FD mount)

In the spring of 1971 Canon introduced a new lens mount (the FD mount), two new cameras (the F-1 and FTb) and a swathe of new lenses (many of which were the same old Fl lenses but in a new housing and with the new mount). The FD mount was squarely targeted at addressing the previously discussed shortcomings of the FL mount. Not only did the lens now communicate the selected aperture, it also allowed for highly precise automation of aperture. And all this while still allowing photographers to mount and use Canon FL lenses (albeit only in stop-down metering).

Anyone who has ever taken apart a Canon FD lens knows that this came at a cost. The mount, especially with regards to the mechanical innards of the mount (that facilitate all that communication and control) is hideously complex and more than one commentator describes the FD mount as ‘over-engineered’.

Interestingly, even though Canon now had a lens mount that would satisfy its camera designers’ wildest dreams, it took until 1976 before Canon introduced a camera body (AE-1) that actually used any of those fancy features for shutter priority auto exposure and it took until 1978 (A-1) before Canon unveiled a camera that made use of the entire set of features (shutter priority and aperture priority).

The fate of the Canon EX line

In 1972 Canon introduced an upgraded version of the EXEE in the EX Auto, but no further developments to the EX line were made. While two bodies and four front-ends of lenses may not seem like much, it is worthwhile making a note of that for more than half a decade (1969 to 1976) the Canon EX line was the only auto exposure SLR in Canon’s lineup.

Why, then, did Canon not put more oomph into the development of the EX line? I cannot be sure, but I suspect that the inherent cons involved in set lenses was a major factor.

Adapting Canon EX lenses

Sorry to say, but the only meaningful way one can use a Canon EX lens (actually, only the front party of the lens) is when mounted on a Canon EX line body (EXEE or EX Auto). The main reason for this is that that lens front-end is designed to work together with the optical arrangement that was permanently attached to the body.

If you’ve always considered Gyro Gearloose to be your alter ego, you might disassemble a Canon EX body and try to mount the resulting contraption to your mirrorless camera, but even then you will need to somehow jury-rig the aperture mechanism (with the Canon EX line, aperture was controlled by the camera body – either automatically or by a dial on the top left of the camera).

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