I have an FD 100-300 zoom 1:5.6, my question is, because it only has 1 "f" number (5.6) does that mean it is a "prime" lens? I put my adapter to use it on my Xsi ,and turning the aperture ring has no affect on the irising in the lens. All of my other FDs you can see the aperture open and close when turning the ring. The ring has from 5.6 to 22 on it, the description on the barrel has only 5.6. When I take multiple photos with it of the same subject at the same settings (other than the "aperture) I see no difference in brightness or anything else...
Thanks in advance RMBuck