Lens mounts: Canon FDn

Pekka Buttler (11/2025)

Canon FDn mount specifications and identification

Current: 1979–1990
Mount type: Bayonet/breech-lock hybrid
Flange focal distance: 42 mm
Film format: 36mm x 24mm (‘Full frame’)
Mount communication: one lever for actuating aperture stop down at time of taking the shot (camera-to-lens); another lever to report aperture selected on lens (lens-to-camera)

Introduction

This article is about the Canon FDn mount. The Canon FDn mount is the direct successor of the Canon FD mount and the last manual-era Canon mount before the introduction of the Canon EF mount . While there are several similarities between the Canon FDn and FD mounts (and JAPB has previously treated them together), there are also significant differences and mount-specific quirks that merit giving each a separate treatment. Also, these three mounts and their details and significance cannot be adequately discussed in one readable-length article.

Before going further, it is worthwhile to note that the FDn-mount is one in a long lineup of Canon interchangeable lens mounts for SLR’s. A short genealogy of the Canon SLR mounts is:

Canon R 1959–1963
Canon FL 1964–1968
(Canon EX 1969–1972)
Canon FD 1971–1979
Canon FDn 1979–1990
Canon AC 1985–1986
Canon EF 1987–2020
Canon EF-S 2003–2020
Canon EF-M 2012–2020
Canon RF 2018–today¹

Back story

Since 1971 Canon had used the Canon FD mount as the lens mount as the basis for their immensely successful SLR system. The period 1971–79 saw Canon moving up the ranks of premium Camera manufacturers. This 71–79 period not only saw the relegation of the West German camera industry to niche player, but it also evidenced several Japanese competitors throwing in the towel. Miranda camera went bankrupt in 1976 and Petri camera followed suit a few years later. At about the same time Topcon – a previous contender for the top spot left the camera business. Having been a top-10 contender in the early 1970s, by the end of the 1970s Canon was making serious claims to be considered to be part of the top-3.

At the same time, there was one aspect that seriously chafed: While the entire camera industry had coalesced around an understanding that bayonet mounts were THE way to mount interchangeable lenses onto high-end cameras, Canon stubbornly (and since 1959) used the (fundamentally) same breech-loch mount.

In theory, breech lock mounts have several advantages, most notably that with breech-lock mounts the act of attaching and detaching lenses creates less friction on crucial parts than bayonets and bayonets therefore are liable to wear out. At the same time, breech-lock mounts do have a key disadvantage in that with breech-locks secure mounting is usually based on tightening and facilitating a breech-lock that would have a locking mechanism easily becomes over-engineered. Hence, very few breech-lock mounts have any snap-locking mechanism.

As a result of combining a mounting type that necessitates a bit more care with the human tendency to become distracted, breech-lock mounted lenses have a de-facto higher risk of high-speed encounters with hard surfaces. Granted, the likelihood of a lens coming disconnected from a breech-lock is not that much higher than with bayonets, but Canon did feel the squeeze…

The Canon FDn lenses

In June 1979 Canon introduced the “New FD” series of lenses. These lenses were designed to be compatible with the entire range of Canon FD cameras, but so that the way you attached lenses to the cameras no longer worked based on a breech-lock (insert lens in correct, final position and twist locking ring) but worked according to bayonet principles (insert lens in an off-angle position and twist the lens into the correct position to lock in place).

Canon’s marketing material did not want to paint this as Canon having given in to consumer/market pressure. Instead, Canon describe the FDn lens mount as a bayonet-breech-lock hybrid.

Anyone who has ever been forced to try to repair an FDn mount lens can attest to that the resulting mechanism is one of the most complicated contraptions out there (as the excerpt above says: Mounting does not turn the flange but turns the entire lens barrel, meaning that as you mount/unmount a Canon FDn lens, there are two parts of the lens that change relative axial rotations).

But the upside was undeniable: Canon managed to change from a breech-lock mount to (for all intents and purposes) a bayonet mount, without incurring any penalties in terms of compatibility issues. That – as they say – was no mean feat.

This is also why the Canon FDn lense did not, technically, change anything at the camera end. Canon FD system cameras used FD and FDn lenses without knowing which was which and with perfect compatibility. When the FDn lens line was launched this made next to zero impact on Canon’s lineup of cameras.Technically, it is even questionable whether one should call the FDn mount a lens mount (as the mounting is identical to the FD mount and the main difference is how the lens behaves when mounted).

The introduction of the New FD mount gave Canon the opportunity to remake their entire lens lineup. In doing so Canon not only switched the mounting mechanism but also utilised the opportunity to take further steps in directions that were part-and-parcel of the photography trend throughout the 1970s:
1) aiming for lighter, smaller lenses, in part by
2) increasing the use of plastics

Smaller, lighter lenses…

Within 12 months (6/1979–5/1980) of the launch of launch of the Canon New FD system, Canon had launched 36 lenses for the new system. Of these 36 lenses
• 8 were entirely new designs (three consumer zooms, three wide-aperture primes and two lenses to replaced aged designs (the 135/2.8 to replace the 135/2.5 and the 35/2.8 to replace the 35/3.5).
• Four fundamental redesigns where the existing lens was kept (same mainline specifications), but the optical design was significantly changed (24/2.8;28/2; 35/2 and 200/4)
• The remaining 24 lenses were rehoused to match the Canon FDn mount and looks. Because of the internal changes in Canon FDn lenses, this was never a small feat, but as Canon decided to shift away from the 55 mm filter thread, the result was a lens lineup that was quite fundamentally changed.

Lens (in order of introduction)	weight	opt.des.	filter mm	old weight	old filter mm	Notes
FDn Fisheye 7.5mm f/5.6	365 g	11e 8g	Built-in	380g	Built-in
FDn 24mm f/2	285 g	11e 9g	52			New lens
FDn 24mm f/2.8	240 g	10e 9g	52	330g	55	Opt.redesigned
FDn 28mm f/2	265 g	10e 9g	52	343g	55	Opt.redesigned
FDn 28mm f/2.8	170 g	7e 7g	52	230 g	55
FDn 35mm f/2.8	165 g	6e 5g	52			New lens
FDn 50mm f/1.4	235 g	7e 6g	52	305 g	55
FDn 50mm f/1.8	170 g	6e 4g	52	200 g	55
FDn 50mm f/3.5 Macro	235 g	6e 4g	52	310 g	55
FDn 85mm f/1.8	345 g	6e 4g	52	425 g	55
FDn 100mm f/2.8	270 g	5e 5g	52	360 g	55
FDn 135mm f/2.8	395 g	6e 5g	52			New lens
FDn 135mm f/3.5	325 g	4e 4g	52	385 g	55
FDn 200mm f/2.8	700 g	5e 5g	72	700 g	72	Old FD introd. 1978
FDn 200mm f/4	440 g	7e 6g	52	675 g	55	Opt.redesigned
FDn 300mm f/4	945 g	6e 6g	drop-in	965 g	drop-in	Old FD introd. 1978
FDn 300mm f/5.6	635 g	6e 5g	58	685 g	58	Old FD introd. 1977
FDn 35–70mm f/4	315 g	8e 8g	52			New lens
FDn 70–150mm f/4.5	530 g	12e 9g	52			New lens
FDn 80–200mm f/4	765 g	15e 11g	58	750 g	55
FDn 100–200mm f/5.6	610 g	8e 5g	52	765 g	55
FDn 100mm f/4 Macro	455 g	5e 3g	52	530 g	55
FDn 28–50mm f/3.5	470 g	10e 9g	58	470 g	58	Old FD introd. 1976
FDn 35–70mm f/2.8–3.5	545 g	10e 10g	58	575 g	58
FDn 17mm f/4	360 g	11e 9g	72	450 g	72
FDn 20mm f/2.8	305 g	10e 9g	72	345 g	72
FDn 24mm f/1.4 L	430 g	10e 8g	72	500 g	72
FDn 35mm f/2	245 g	10e 8g	52	345 g	55	Opt.redesigned
FDn 24–35mm f/3.5 L	495 g	12e 9g	72	495 g	72	Old FD introd. 1978
FDn Fisheye 15mm f/2.8	460 g	10e 9g	Built-in	485 g	Built-in
FDn 100mm f/2	445 g	6e 4g	52			New lens
FDn 85mm f/1.2 L	680 g	8e 6g	72	756 g	72
FDn Reflex 500mm f/8	705 g	6e 3g	drop-in	740 g	drop-in
FDn 135mm f/2	670 g	6e 5g	72			New lens
FDn 300mm f/4 L	1060 g	7e 7g	drop-in	1235 g	drop-in
FDn 100–300mm f/5.6	835 g	14e 9g	59			New-lens

As is evident from the table above, many Canon lenses recorded a marked weigh loss in going from the Canon FD to the FDn mount. At best the weight-savings were around 35% and –even without an optical redesign contributing – many lenses lost more than 20% of their weight. While there were a number of lenses that lost very little weight in going to FDn, these were almost invariably newer additions to the Canon FD lineup (introduced after 1975) that had already been designed according to the “svelte and light” -rule. Even so, comparing the FD and FDn line-ups (inasmuch as they overlapped) the average weight-savings were around 14%.

While this is at least partially due to manufacturing more compact and smaller lenses, one key factor in the transition was the …

Increased use of lighter alloys and plastics

As the dictionary says, an ‘alloy’ is “a metal made by combining two or more metals, especially to give greater strength or resistance to corrosion”. Alloys are at the core of metallurgy and we have them to thank for many of the key material improvements around ourselves. Yet another (except strength or resistance to corrosion) purpose of alloys is to achieve lighter materials, and in the late 70s and early 80s, these alloys started becoming a key element (pun intended) in the camera industry’s striving for lightness.

What alloys would be used depended rather clearly on whether the metal would be used in places where it would have to be able to deal with bumps and knocks (bottom and top plates of cameras, lens’ front ends) or whether the component was internal or otherwise well-shielded. In the first-mentioned places alloys like Zamak (Zinc-Aluminium-Magnesium-Copper) were used extensively, while in most other places Aluminium-based alloys like 6061 (Aluminium-Magnesium-Silicon) would replace heavier, more traditional metals.

At the same time, plastics also moved in and started finding increasingly wider uses. This was especially true for cameras, but also on lenses where typically the first metal parts to be replaced by plastics would be aperture rings and name rings (aka. vanity rings), but most manufacturers stuck with metal alloys (instead of plastics) for external parts of lenses – until the advent of autofocus.

Another typical use-case for plastics were the focusing rings of lenses that had originally been rubber (work-intensive, liable to harden and crack) where replaced with TPE-S plastics (styrenic block copolymers such as SBS (styrene–butadiene–styrene) and SEBS (styrene–ethylene–butylene–styrene)

Hence, while plastics certainly played a part already in 1979 (and would become a crucial factor in the late 80s), the weight-savings achieved by the camera and lens industry in during the 1970s were due to a systematic search for lighter, high quality metal alloys.

A storm on the horizon

While the Canon FD system and the FDn lenses clearly were a competitive technology, it was also clear that a major change was coming. Already two years before the launch of the FDn lenses Konica had launched a popular autofocus point-and-shoot camera, and – as part of a long series of user-friendly innovations – it was only a question of time before autofocus would upend the SLR industry.

Several key players were openly experimenting with the technology:
• Pentax (1981) launched the ME F camera and autofocus lenses;
• Canon (1981) launched an autofocus standard zoom that could be used on any old FD camera;
• Chinon (1983) released the CE-5 camera and autofocus lenses
• Nikon (1983) released the F3AF – a pro body with limited AF ability with specialist lenses.

It was clear that manual focus lenses days were numbered, and that the future of the photographic industry would be decided by the companies’ ability to manage the transition to autofocus.

While the most crucial element of managing that transition was the development of workable, dependable autofocus technology, one unanswered question was whether transitioning to autofocus would necessitate an entirely new mount, or whether the transition could somehow be achieved without abandoning the entire installed base.

While Canon in the end went for the EF mount – a new, fully electronic and in no way compatible lens mount (and was at the time strongly criticised by its loyal customers for doing so) – Canon also made an experiment to continue within the limitations set by the FD mount. That story is discussed in the article on the Canon AC mount.

Adapting Canon FDn lenses

If you are in the possession of some nice Canon FDn lenses, and want to use them to shoot some film, you have a wide range of opportunities for doing so as the lineup of Canon FD cameras is both wide and many models were manufactured in their millions. Moreover, if you want to you can also use your Canon FDn lenses on a Canon FL era body, but you will obviously be limited by that body’s metering abilities. However, due to the short flange focal distance of the Canon FDn mount, using Canon FDn lenses on any other film camera becomes a rather unpractical (while not entirely impossible) concept.

The issue of the FDn mount’s short flange focal distance also is the key impediment should you want to use your Canon FDn lenses on any digital SLR. Due to the negative flange focal distance between the Canon FDn mount and any of the main dSLR mounts (Canon EF, Minolta A, Nikon F, Pentax K), adapting Canon FDn lenses on dSLRs certainly would necessitate an adapter with optics to allow for infinity focusing. Again: Not the optimal solution, but certainly a possibility.

Should your interest be to adapt your Canon FDn lens to a mirrorless camera, nothing could be easier as the difference in flange focal distances between the Canon FDn mount and any digital mirrorless format is in the double digits. However, due to the design of the Canon FDn mount, you will need an adapter that has a control ring that allows you to engage the lens’ aperture stop-down function. Otherwise you’re stuck in using the lens at it’s maximum aperture.

If you’re relatively new to adapting legacy lenses, you might want to have a look at this article on the practicalities of adapting legacy lenses.

Footnotes:

1. Technically the Canon RF mount is not an SLR mount because Mirrorless interchangeable lens do not fully qualify as SLR cameras (as there is no mirror), but I’ve added the mount for the sake of anchoring the genealogy in current events ↩︎