I was having a discussion with Hamish from the (excellent) compact camera blog 35mmc.com on Twitter recently when he was trying to figure out exactly how a Canon A-1 (which is an FD system camera) behaved when using an older Canon FL mount 55mm 1:1.2 lens. The simple answer is that you can use the stop-down metering lever and the camera behaves more or less like it’s in Aperture Priority (Av) mode – the microprocessor calculates the “best” shutter speed based on the light coming through the lens with the aperture closed down to whatever the value set on the lens. This is basically the only method of using older lenses recommended by Canon in the A-1 manual and is common to most FD mount cameras, with differences based on the exact controls available.
Now, I’ve used FL mount lenses quite a lot with my A-1 and so I can say although that’s pretty much how I use them a lot of the time it’s not the only way, so I thought I’d expand a little on what I said to Hamish. Firstly a bit of background on Canon lens mounts.
All Canon SLRs since the EOS 650 in 1987 have used the EF mount, which has no mechanical connections between the camera body and lens, but instead uses eight electrical contacts to control aperture and autofocus. This is a pretty good thing since electrical contacts aren’t subject to mechanical wear and tear, and reduce the opportunities for foreign objects making their way inside the lens.
Earlier lens mounts for SLRs all use mechanical connections to allow the body to communicate with the lens, starting with the R-mount, and then followed by the FL, and then FD. These three mounts are all compatible, but the later mounts allow the camera to do more because the lenses have additional features. The R mount (as I understand it – I ‘ve handled an R lens myself) is a manual preset lens where the aperture is set by the user, and is stopped down when the shutter fires, then re-opened when the film is advanced. The FL-mount is a redesign of the R-mount which allows for instant return to fully open aperture as soon as the shutter has closed. The FD-mount added the capability for the camera body to control the aperture automatically.
I’ve written before about incompatibility of the EF- and FD-mounts (and earlier), and what you can and can’t do about it, so I won’t talk about that again here. What I want to cover this time is some of the details of using FL lenses on FD cameras. This may not be applicable to all the FD range, but it’s certainly true for the A-1 and the AE-1.
First up let’s have a look at those lenses. I have both FD and FL 50mm 1:1.8 lenses which make for a good comparison. Aside from the heavier metal construction in the FL version these lenses are pretty similar despite being a couple of decades apart, but you can see the FL lens has only a single lever, at the bottom which stops down the aperture when the shutter fires. The FD on the other hand has a second lever on the left of the lens mount – this is used by the camera body to communicate the aperture setting. When the lens aperture ring is set to “A” then whatever mode the camera is in the aperture setting is selected by this lever and then stopped down when the shutter fires by the bottom lever.
Of the two pins only one – the one on the left, between the two levers – actually does anything, the other is “reserved for future use”. As far as I know that future use never arrived, so on FD lens/bodies that pin on the right has no function at all. So what of the one on the left? This is the “full aperture signal pin” and tells the camera what the f-number of the lens is when wide open.
If you look at the lens mount you can see how this works – the aperture control lever is visible below the mirror and the aperture signal lever is the vertical lever on the right of the mirror. In the corner between the two is a small cam. When the lens is mounted the full aperture pin on the lens pushes on this cam and the amount it is pushed in tells the camera the maximum aperture of the lens. The further it’s pushed in the smaller the f-number, from f/5.6 when not pushed in at all, to a minimum of f/1.2. If it’s pushed in beyond the f/1.2 position it will read 4.0 then “OP” in the viewfinder.
This pin is the key to how the metering works in any of the automatic modes on FD cameras. During normal operation the lens is wide open and with an FD lens the camera knows from the position of this pin what aperture that is, so it’s a simple calculation to measure the actual light level at that aperture and set either the shutter speed (in Av), taking aperture (in Tv), or both (in P), given the constraints of ISO setting and exposure compensation. Or if the lens is set to manual it will tell you in the viewfinder what the settings would be without actually being able to set the aperture.
This also explains why this doesn’t work with non-FD lenses. Obviously anything without an aperture signal lever isn’t going to let the camera set the aperture, but also without the full aperture pin the camera doesn’t really know how to calculate the values anyway since what it thinks is the full aperture is probably wrong. If you mount a 1:1.8 lens but the camera thinks it’s 1:5.6, you’re going to end up nearly 4 whole stops out. Trying manual mode doesn’t really help in situations where the light is too dim for your chosen shutter speed at f/5.6, since the camera just blinks 5.6 at you since as far as it knows that’s the best the lens can do.
There is a complication too depending on the lens. If you look at this profile view of three lenses you’ll see that the FD lens is completely flat at the mount except for the levers and pins. The FL lenses on the other hand show a significant bulge at the lower edge of the mount, presumably to accommodate the aperture linkage. Not all FL lenses have this bulge – I have an FL 35mm 1:3.5 which is totally flat – but if they do have it, you’ll find that it presses in on the full aperture pin. For example my FL 58mm 1:1.2 shows the max aperture as f/2.5 – around 2 stops out.
Canon’s recommended method of using stop-down metering for these situations avoids all these issues – the aperture is already fixed, so given the film ISO all the camera need do is calculate the one remaining variable – shutter speed – based on the light coming through.
However if you really want a proper manual mode – and given the fixed semi-spot metering method in a lot of these cameras there are situations where you might well want that – there is one thing you could try. If you can fake the full aperture signal pin, then you can tell the camera the correct f-number of your lens when wide open and use it properly in full manual mode.
A word of warning – I take no responsibility for any problems caused to cameras or lenses by trying this idea out. There is plenty of potential for this to go wrong and then have something jam the mirror, shutter or some other mechanism, so don’t do it unless you’re sure you can cope with the consequences.
The idea is to fix something of a suitable thickness on to the lens in the correct position to push in the signal cam the correct amount. To prove to myself that it works, I built up the small area on the lens where the pin would be with layers of electrical tape. By trial and error I got to the right thickness for f/1.2 and sure enough manual metering worked just fine. I haven’t yet come up with a permanent solution, since I don’t particularly want to permanently modify such a nice old lens with my clumsy fingers – the chances of getting some sort of non-removable blemish on the rear lens element are a bit too high for my liking. With a bit of work you could easily figure out the correct thickness to specify any aperture from f/1.2 to f/5.6 so that almost any lens could be modded this way.