Pekka Buttler 11/2025
Canon AC mount specifications and identification
Current: 1985–1986
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); 6 electronic contacts for communication and powering AF motor
Key characteristics of the mount of Canon AC lenses
[1] Female bayonet mount with three openings.
[2] Lens release button (pops out when lens locks on body/adapter).
[3] Alignment pin.
[4] Stop-down lever.
[5] Aperture indicator lever.
[6] Bayonet activation pins (bayonet is locked until body’s or adapter’s prongs or lens cap depresses these).
[7] Electronic contacts for AF an electronic communication
[8] Housing for AF electronics.
Introduction
This article is about the Canon AC mount. The Canon AC mount is a short-lived extension of the Canon FDn mount and Canon’s first attempt at creating an Autofocus system. While there are several similarities between the Canon AC and Canon FDn mounts, there are also significant differences and mount-specific quirks that merit giving each a separate treatment. Also, as the Canon AC mount and Canon AC mount lenses are rather rare, discussing them as part of the Canon FDn mount article would not serve that article well.
Before going further, it is worthwhile to note the position of the AC-mount in the 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 1985–86
Canon EF 1987–2020
Canon EF-S 2003–2020
Canon EF-M 2012–2020
Canon RF 2018–today1
Back story
The 1970s had seen several attempts at devising systems for autofocus cameras and some of them had been rather successful, but none of them had been for interchangeable lenses. The late 70s and early 80s were rife with plans by major SLR makers on how to surmount the technological (and economical) summit that was a working autofocus SLR system. Importantly, at this stage it was by no means a foregone conclusion how (using which technical means) workable autofocus should be facilitated (if you cannot think of other ways to do autofocus than the currently dominant approaches, feel free to browse the JAPB article on autofocus technology). These alternative avenues were not unknown to Canon which had experimented with a self-contained AF lens2 in 1981.
For Canon (as well as for most of its competitors) one key question was whether their existing system and lens mount could be morphed into an autofocus system, or whether the company would be best served by devising a totally new system (and mount) and thereby abandon the existing user base. This was always a discussion that focused partially on technology, but also on economics. One key question was whether there would be a customer segment who neither needed nor wanted autofocus3, and how big that segment would be.
I have no insider knowledge on the discussions between the ‘Sarariman’ and ‘Gijutsu-ya’ at Canon, but I could imagine a situation wherein a decision was made to split the difference and initially allow both avenues to be seriously explored, resulting in:
• one project to develop a wholly new autofocus-centred system that would become the Canon EOS system and EF mount (which would ultimately be launched in 1987)
• one project to develop an autofocus camera and lenses within the existing Canon FD/FDn system, that would become the Canon T80 and (to begin with) one nifty fifty prime and two zooms.
The Canon AC mount
In April 1985 Canon launched the Canon T80 camera and an initial trio of autofocus lenses:
• AC 50mm f/1.8 [data sheet]
• AC 35-70mm f/3.5-4.5
• AC 75-200mm f/4.5
The lenses are based on the Canon FDn mount, with the crucial addition of 6 electronic contacts (spring-loaded on the camera end) and with the key omission of not sporting an aperture ring. Given that the T80 system was centred on program auto exposure and did not have a manual mode, it is natural that the lenses would not have needed an aperture ring. On the other hand, this clearly indicated to all audiences trying to read tea-leaves that the AC system had no intention of serving a pro-oriented audience.
Besides entirely omitting an aperture ring, these lenses were quite clearly geared towards autofocus only, as manual focus (while possible) had been made extremely unwieldy.
Not a successful first product.
While certainly not a perfect product at launch, it is possible that the camera and its trio of lenses would have received a less unenthusiastic welcome, had not Minolta launched the Alpha/Maxxum/AF 7000 camera with its all-new mount two months before the Canon T80 was introduced. The pundits were unanimous in that the Minolta product was closer to what an Autofocus SLR was envisioned to be, and were unambiguously impressed by Minolta’s bet-the-farm commitment to its new system4.
For anyone even looking at the Canon T80/AC system one thing was clear: it was not as good as the Minolta system and that was enough for them to either i) immediately switch to Minolta or ii) hunker down and wait for Canon’s inevitable response. After only a year the T80 and Canon AC line was discontinued in the midst of Canon’s strongly voiced commitment to pick up the gauntlet Minolta had thrown.
Adapting Canon AC lenses
Adapting to SLRs: Besides mounting this lens on the Canon T80 it was intended to be used with (“Go for the genuine 1980s, bleeding edge AF experience!”), there are also some other options for using this lens (or any of the other Canon AC lenses).
Looking at the Canon AC lens’ mount (Essentially it looks like an new FD mount with some added electronic contacts) you might be thinking “So I should be able to mount this on any FD body, and as long as that body is designed so that I could just put the lens’ aperture ring in ‘auto’ and go shooting this lens should work also…” And you would be justified in making this assumption.
Indeed, if you can manage with a lens that looks like a weird 80s Sci-Fi product; that forces you to always leave aperture control to the camera body and that is so designed for AF that the usability of the manual focus ring is by far the worst I’ve seen (and I’ve seen many), then you can mount this lens on any Canon FD camera that is centred of shutter priority or program AE8. “Why would you bother?” is, however, a justified question.
Due to the inability to control the lens’ aperture through an aperture ring, you cannot mount the Canon AC lenses on any other SLRs or dSLRs9.
Adapting to mirrorless: This is – again – an entirely different ball-game. You can mount a Canon AC lens on any mirrorless camera using your average Canon FD lens adapter. You can even use that adapter’s control ring that you would normally use to engage the aperture ring’s manual mode to exert a modicum of control on the lens’ aperture, but it’s a clunky affair and does not give you precise control. If you have especially nimble fingers you can even manually focus the lens.
But unless you gain a lot of joy from circumventing limitations that might slow down a lesser lens hacker, the question is: why bother5.
Obviously I bothered (just to show that it can be done), and am offering some images below (Sony ⍺7R2, Canon AC 50/1.8, no-name FD->NEX adapter, ACR default conversion, Resize to 2K, save as JPG). While the AC 50/1.8 is decently sharp and my copy seems to have seen very little use, I am not going to change that. It simply is too unwieldy to use.
Footnotes:
- 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. ↩︎
- See the FDn 35–70mm f/4 AF at the Canon Camera museum ↩︎
- Historically there had been several occasions where the crucial pro segment had expressed serious disdain towards any added automation. Assuming this might happen again with autofocus is not far-fetched (and many sub-segments of the pro users were among the last to adopt autofocus). ↩︎
- Minolta had gone for a system with a longer flange focal distance than its well-loved Minolta SR-mount, hence making it very difficult to adapt existing Minolta SR lenses to the new system. At the same time Minolta launched one impressive body together with a total of 13 lenses (6 primes, 1 macro lens and 6 zooms) and was brandishing an ambitious transition roadmap. ↩︎
- Obviously I bothered (just to show that it can be done), but I’m definitely not going to make a habit out of it. The experience was simply so immensely clunky even though the 50/1.8 lens seems quite sharp (as sharp as any FDn 50/1.8). If someone wants to borrow or donate a working T80, I might take some film through it, just to know what I missed out on in 1985… ↩︎