✔ 最佳答案
Because the FD lenses are manual lenses. autofocus won't work. Even when you are using the EF lenses in manual mode, when the object is in focus while you press the shutter halfway, you'll hear a beeping noise that the object is in focus.
There are adapters to fit the FD lenses into the EF mount. (FEDTIC). However, in order to test the focusing accuracy of the FD lenses on your 5D, you need to make sure to make a series of test shots on a tripod to find out the lenses focusing point and then you check the view finder's eyepiece adjustment. It is a trial and error process.
By the way, since 5D is a full frame camera, good lenses are extreme important to produce good result. It is much less forgiven than crop sensors camera.