Okay, so using these old manual FD lenses and other lenses with passive adapters will leave a lot of Exif data empty in your files.
David over at http://photolifetoys.blogspot.jp have written a very nice article o its use, so I will not in to detail explaining it here.
I will however just briefly mention a few pros and cons.
As David mentions in his article;
It is a very simple solution which changes just those three metadata elements (LENS TYPE, FOCAL LENGTH, APERTURE) - nothing more, nothing less.
Which is great for say, my Canon 135mm f2 L lens because that lens is fixed at f2 for me... but the QuickExif.app applies the exif info from the naming of the app itself.
I.e. Lens_type ; Focal_length ; Aperture
So in Davids case, he renamed the app to "Samyang 8mm T3.8;8;5.6"
Which translates to:
Lens type - Samyang 8mm T3.8
Focal length - 8mm
Aperture - 5.6
Any of the FD lenses have a manual aperture control ring, so that would mean an QuickExif app for each of your apertures(and for every manual lens), or change the name of the app for each lens and remembering what shots where made at what aperture. I certainly forget right away so I find that I don't use it too much, and simply ignore that some of the info is missing from the images. Instead I stick vigorously to my aperture library structure to keep things in order and simply do without. Moving photos in and out the library is also too much trouble for me.
But if you have a manual lens and want to give it a try, why not try it out? it might be a good solution for you :)