<p>With all the FD knowledge on this site, I am a little surprised that no one has yet mentioned the FD 300mm f2.8 flourite with a dedicated 2X extender or nFD 300mm f2.8L with extender 2XB, to get the best 600mm lens from Canon. The 2X extender supposedly had no image degradation when used with the 300mm f2.8 fluorite lens and could be used with other lenses as well. IIRC, the dedicated 2X extender later became the 2XB extender, available separately.</p>
<p>It seems to me the 600 f4.5 was made available as a cheaper (consumer?)alternative to the 300mm fluorite/2X extender lens, just as the 300mm f4.0 non-L lens was to the 300mm f4.0 L lens...</p>