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This post follows on somewhat from yesterday’s and was prompted by a mail I got overnight asking a question I’ve had a number of times. It goes something like “I have an EOS camera and I want to use an FD lens on it – will it work?” This is not an unreasonable question especially for people with a collection of lenses from old film cameras which, after all, were either made by Canon or at least to fit Canon cameras. And that’s pretty much the situation I was in when I asked the same question a few years ago.

I have a Canon A-1 with a variety of lenses, and was wondering if I could use the fast primes on my shiny new EOS 20D to supplement the kit lens. Now Nikon users can feel free to chuckle away at this point, secure in the knowledge that the F-mount is more or less compatible with any lens made in its 50 year history, and that’s quite a lot of lenses. When moving into the brave new world of autofocus in 1987 Canon decided that abruptly changing the mount was better than gradually introducing changes and compromises into the existing design. The new EF mount had no mechanical connections between body and lens, a longer register, and a completely different mounting mechanism – bayonet instead of breechlock – and so left the entirely mechanical, breechlock FD user a bit stranded.

A quick bit of searching suggested that an adapter could be obtained, so I promptly found one, reasoning that it might not be great but could potentially be good value while I saved up for some decent EF lenses. I eagerly awaited delivery, and when it arrived stuck it on the 20D.

EF/FD adaptor on 20D

I mounted the FD 50mm f/1.8 from my A-1 and was delighted with the incredibly bright viewfinder. Stopping down manually worked and TTL metering seemed more or less accurate. I was pleased. However, on close examination of some shots I realised they were pretty fuzzy. They weren’t out of focus just suffering from really bad flare. I’d used the lens for years, a nifty fifty if ever there was one, so clearly the problem was the adapter. As you can see, the adapter has a lens in the centre and any time you add more glass to a system, you’re going to affect the quality of the image, so why is it there?

I mentioned the mount register earlier and this is the reason. The register, or flange-to-focal plane distance is a crucial quantity in a camera/lens system. When the lens is set to a certain distance, objects at that distance will only be in focus on the film or sensor if the lens is at the correct register. If the lens is closer to the sensor, then it will focus a little further away. That throws the distance scale off but it’s not a huge problem – just refocus the lens a bit closer. If the lens is further away than the its designed register then it will focus a little closer, which is a much bigger problem as it means you can’t achieve focus on an object at infinity as you can’t focus the lens further away than that. Mounting a lens further away from the sensor plane than designed limits the maximum working distance of a lens, and even a small difference has a dramatic effect. The EF mount register is 2mm longer than the FD register – 44mm instead of 42mm – but for a standard 50mm lens that reduces the maximum working distance from effectively infinity to around 1.3m.
The solution is to add a slightly diverging lens element enabling the FD lens to focus further away than it was designed to, but also slightly magnifying the image. Hence the lens element in the adapter – in essence it’s a mild tele-converter with an EF mount on side and an FD mount the other. Unfortunately, while it solves the infinity focus problem it introduces problems of its own. The lens is a simple doublet, not cemented and completely uncoated, with no blacking applied to the edges. That’s four extra air-to-glass interfaces so it’s hardly surprising that there’s flare. You can see the scale of the problem in the test sequence I took.

EF-FD Adaptor investigation - 50mm

Even at the small size here, you can see the flare in the left most pane. Stopping down the lens abates the problem, but it doesn’t disappear until f/5.6 at which point the fast prime has lost one of its main advantages over the kit lens. Another problem is that the lens element in the adapter is less than half the diameter of the adapter itself, and so when using a fast lens, the adapter acts as an additional stop. Not only is it limiting the quality through flare when using a 50/1.8 wide open, it’s also preventing it actually operating at f/1.8.

So what’s the answer? Well sadly it seems to be that if you want to use FD lenses to shoot at infinity on an EOS body, you can’t use an adapter. However, if you want to shoot macro, then take the optics out of the adapter, or get one without any to start with, and as I mentioned yesterday you have a pretty good macro set up.

If you’re feeling more ambitious or handy, then a final option is to physically adapt lenses to fit the EF mount. I still use my FD lenses on the A-1 so mostly that route doesn’t suit me, but I did have a spare lens, with a faulty aperture that wouldn’t stop down so I took it apart and bodged a lens mount by using an excess EOS body cap. It’s messy, but it works. Other people have done much neater and more solid conversions.

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