hummm... Was used to a Sigmar 300mm f2.8 and a focal duplicator.... some time ago i've bought a cathadyottric lense and i've found that are really funny to use. i've bought a 500mmm VIVITAR months ago... and now i'm planning to buy a Sigmar 600mm to replace the low-cost low-quality VIVITAR. The cost of this kind of lenses is really low... (my vivitar costs 90$) ... with the possible exception of Nikon ones... The Interesting side effect of this kind of optic is the "rippling" effect you get in OoF regions of the pic.... i like it...