Hi
well, right now I have here in the house a Oly 300 f4.5 and a FD 300 f4. I tried a side by side of them at wide open, f5.6, f8 and f11
I can
just see differences, however when I split the R G and B I see that the FD actually focuses all three layers better than the Oly does (but only at wide open and f5,6).
I'm now scratching my head as to which one I'll sell ...
I don't know if you saw my
FD to Oly 50 comparison but while the FD was not as good as the Oly I'm also sure that the gap can be closed quickly with a little tweaking of curves and local area contrast (Eg in photoshop unsharp mask set radius to 60pixels and amount to about 15%) brings them to within ball park.
So ... what I'm saying is that almost any 300 f4ish lens by a major Japanese maker will do very well.
I bought each of mine from KEH for less than US$200 ... so its not expensive to fiddle about.
I'm tending to keeping the FD lens as its more convenient to operate than the Oly is and I like the rotating mount better ... expect a blog article comparing them after the weekend.