So what are FD lenses? Canon has changed the part (aka a bayonet), which connects the lens to the camera a few times in it's history. Currently Canon is using an EF mount. Prior to the EF mount, Canon used a FD mount. There are two obvious differences between these two mounts. 1, the FD is smaller than the EF. 2, the EF contains electronics; and the FD does not contain any electronics, such as auto focus. FD lenses are fully manual. This means the aperture is located on the lens, as show in the image to the left. And you must focus. No auto focus.
When using a modern camera along with a FD lens, the camera's aperture controls are blank. You will need to rotate the aperture ring on the lens and adjust the speed on the camera. Now the modern Canon camera will meter the subject; however the metering process will be off. If your camera uses "Live View," then you may notice that the "Live View" feature will not give you a "Real" idea of how dark/light the final image will be. To overcome this problem, you will have to take multiple shots until you have your desired exposure. Or after knowing how the lens shoots; you can adjust the exposure compensation in your camera as soon as you turn on your camera (with the FD lens attached).
I've found that FD lenses with my Canon 5D mark II offers a darker image when utilizing "Live View." And when using the view finder the metering is darker (as I mentioned above). As a result I know before hand to compensate camera speed to expose a slightly lighter image. I've found that you can loose up to two stops.
OMG! I'm loosing two stops of light?!? Yes you can. Why on earth would I ever buy a FD lens? Easy... I paid $20.00 for a 28mm f2.8 FD lens! (Enough said...)
Learning how to use the FD manual lenses with your modern camera may seem like a lot. But the learning curve is incredibly low, in my opinion. Like everything, you have to learn how to use it. The more you use it the quicker you will become, using it.
WARNING: At first you will notice you are not taking photos as quickly as you would with an auto focused EF lens. THIS IS GOOD!
I have really nice EF lenses. However you will find, I almost always shoot without the use of Auto Focus. Cameras are tools. Yes, you should use auto focus. However in my case. I also view it as a skill which should be sharpened constantly. For me, the biggest reason I shoot in manual focus is I can manually focus faster than most can with Auto Focus. The bee in this image was shot with an EF lens, using manual focus.
Below are examples of the image quality achieved with FD lenses.
Far left: Shot with a Canon 5D mark II and a Canon 100mm Macro 2.8L EF lens. The lens in this photo was about 2 feet away from the flower. Obviously I cropped into the image. The bee was quickly collecting pollen. This image is NOT one of a series of rapidly shot photos. I took 3 different and single shot photos.