How to make sense of Canon FL/FD/FDn/EF mounts:
If you are the type of person that looks for deals on camera equipment (like me) you may have considered attaching old manual lenses to your modern dSLR. If you shoot Nikon, you are in good shape, since the basic F-mount is still compatible with autofocus cameras almost 50 years later (almost -- I'm not going to get into that here; do some reading before you attempt to mount an old lens on a new body because some aren't compatible without modifications). Similarly, the Pentax K mount, has maintained the same basic lens mount through the years.
Not so with Canon.
When Canon introduced the EOS system in 1987, it scrapped the previous FD mount (which was also backwards compatible with the FL mount ['64-'71]) and began again with a completely new EF mount, which includes gold plated contacts for electronic autofocus. The FD mount and EF mount are completely incompatible, but by doing so, Canon was able to streamline the EF mount and avoid a lot of difficulties keeping compatibility.
Also, the FD/FL mounts were breech lock, meaning you have to push the lens onto the camera then twist a ring to lock the lens on (which prevents the lens flange from wearing out the camera ring, changing the film distance and hurting image quality). The later FD mounts (called "new" FD, nFD, or FDn) were bayonet mounts like the EF series -- insert the lens into the mount and then twist the lens itself to attach. Realistically, bayonet mounts are just as good as breech-lock mounts and more convenient to use. See more here.
Canon made a TON of FD lens which have now been orphaned by the new EF mount. For us cheap photographers, this is both good news and bad news. The good news is that there are a lot of old lenses on the used market which can't be used on new cameras, meaning high quality glass for cheap. The bad news is that attaching the lenses to an EF mount is a real hassle which I'll get into in a bit.
First though, what lenses are out there? How about this awesome list of FD lenses by Denis Baron. It lists all the major FD lenses made by Canon and gives specifications. A great resource if you are looking for old lenses online or on eBay. Note that FDn lenses are listed as 'new' FD, of which most have the S.S.C. coating even though the designation was dropped; S.S.C. is a better coating than S.C. Also, Photography in Malaysia has a great FD lens archive with reviews and tons of images. Don't forget about third party manufacturers, especially Sigma, Vivitar, and Tamron, because many of them supplied a lot of FD lenses.
Also, I should note that the most popular FD mount camera by far was the Canon AE-1 (review). The AE-1 sold over 5 million units due to a strong advertising drive, like the commercial below...
The Challenges of Mounting FD on EF:
In 1987, when the EOS system was introduced, the Canon photography world was thrown into a temporary upheaval. The new mount, while it included autofocus and a number of other advancements, caused most photographers to have to decide whether to stay with FD lenses or move to the new system. Most amateurs stuck with the old system a while longer, while most professionals moved quickly since they had the money and could make the investment in a better autofocus system. The good news is Canon pushed hard to get photographers to make the switch, and the brand was strengthened because of it.
The new EOS system did catch on relatively quickly with amateurs too, in large part to the big push to sell the Rebel series. If you were born before 1985, you probably remember the commercials centered around Ain the 70s or 80s, you probably remember the commercials centered around Andre Agassi like this classic:
At the time, I didn't do anything with photography, but I definitely remember the commercials!
I alluded to the significant hassle of mounting an FD lens on an EF camera -- the primary reason is the new EOS mirror box is deeper than FD with a longer distance between the lens mount and the film plane. This made it impossible to insert any sort of adapter between the mount types and preserve the lens to film distance. The end result, if you want to use an FD lens on an EF body with an adapter ring, is you will lose infinity focus. Essentially, you are adding a small extension tube between the camera and the lens.
This isn't just a problem for FD lenses either. Photonotes has a page about adapting manual focus lenses to the EOS system which mentions other lenses and what is needed, plus many of the other little issues you'll encounter. Some lens types you can adapt, some you can't. Bob Atkins has a great page up outlining the flange to film distances and which may allow an adapter (without optics) to EOS. POTN also has a good thread discussing FD-EOS converters which outlines which lens mounts work and which don't. I've been focusing on FD lenses because there are so many out there and they seem to be inexpensive compared to many alternatives.
Solutions for FD-EOS Attachment:
Of course, Canon realized that this would be an issue, so they did supply two versions of FD-EOS adapters. The macro adapter was similar to many of the existing adapters on eBay like the Bower/Hartblei item I reviewed in my last post in that it was thin and had no optics. This page shows both the Canon macro converter and the Hartblei I reviewed. For macro purposes, losing infinity focus is not a big deal. There's a good thread on the Photo.net forums too about the different options with lots of pictures.
The non-macro adapter was only supplied to those professionals who had a supply of very expensive multiple wide telephotos so that they didn't need to immediately update their expensive lenses. It included a lot of glass, acted like a small teleconverter, and is very rare and expensive now (PIM review). In the past month, only one has shown up on eBay and it didn't sell for the asking price of $740. Needless to say, it isn't practical for a budget photographer like myself.
The good news is that the Hartblei and equivalent adapters are available on eBay for good prices and the quality is quite good as I reviewed. My recommendation is to get one with a removable optical element so you can improve image quality if you aren't concerned about infinity focus. It is also possible to make an adapter yourself, but I don't recommend it because if a homemade adapter breaks it will drop either your lens or your camera on the ground, which won't be desirable. The commercial adapters are cheap enough on eBay that you don't really have an excuse not to go with one of those.
I've set up a search widget below which should show some the current auctions for the EF to FD adapters.