Is it worth it to buy an FD-EOS adapter, or is it better to exchange a misbought lens?
I recently bought a manual-focus (since manual doesn't bother me) 100mm prime macro lens. However, I didn't notice at the time of purchase that it was an FD lens. (I'm a hobbyist; I didn't know Canon had actually changed lens mounts at some point.)
On the one hand, I kind of want to experiment with this guy. And quality FD lenses are relatively cheap on the used market, so if the adapters were worth it, I could probably get more (and more varied) glass. On the other hand, everything I read about the FD-EOS adapters is highly contradictory. As in, some people swear by them, and others think they're outright tripe.
I am still within the exchange period, and can easily exchange this lens for this 50mm prime that I'd been tossing up about buying anyway, as they are the same price.
Which would be the wiser choice?
Generally I'd say it's not worth trying to adapt an FD lens for the EF mount. The reason for this is that the EF mount has a larger registration distance, that is distance from the sensor the mount so that any simple FD to EF adaptor will act like an extension tube and you wont be able to focus beyond a few meters!
Canon produced an adaptor with a glass element which corrected focus distance but increased focal length by a factor of 1.1, could only be used with telephotos and decreased optical quality. This adaptor was mainly produced to placate those with a significant investment in long Canon glass and is quite rare. This is probably the reason people consider this type of adaptor to be "tripe". There are also third party versions of this adaptor which are comparable to the Canon one (i.e. still not that good).
However, as the lens in question is a macro lens, you could use a simple mechanical adaptor and the result would be a decrease in minimum focus distance, increasing magnification. So if you plan to use this lens for macro work then you should be able to adapt it no problem, however it wont be useful for anything else. A glassless adaptor wont compromise the optics so this is probably responsible for the mixed opinions on adaptors.
At the very least you need an adaptor with that can engage the aperture lever so you can stop down (almost essential for macro work) I think most of the glass ones do this, if you can't find a mechanical adaptor which will work the aperture for you, you can remove the glass from one of the other adaptors.
Incidentally, that 50 f/1.8 is well worth the price so I would leave it on the shopping list even if you keep the macro, for when you can afford it.
FD lenses *have* aperture rings--no need for the adapter to give control of the aperture, (vs. Nikon G-series lenses, or Canon EOS-> mirrorless adapting).
Yes, FD mount lenses have aperture rings, but they also have a spring connected to a tab that needs to be compressed by the adapter (or the FD mount camera body). This is so the aperture can be held wide open for metering and then, when released via the tab connected to the adapter/body, the spring will force the aperture closed until "stopped" by the setting of the aperture ring.