That fd 500 works well with the fd 2x B TC as well. Here is a 100% crop of a shot with the fd 500 and 2x B attached to an M6 II (that's 1000 mm with a pixel density equivalent to an 82 MP FF sensor). I have two copies of the lens and this is the best one. Note the fine lettering in the bottom right corner of the label. The 1000mm F/11 Nikon and the 500mm Minolta MD are also quite good and the Tamron 500 is the best at close focus distances. The 500mm Nikon is also good, but heavier than the other 500s and it has a very long minimum focus distance. I have a couple of the Sigma 600s and they are quite soft, particularly in comparison to the others mentioned. I do think cats with IS and AF would be truly useful as the primary dings on all the older lenses are difficulty of focus and stability. For situations where you can overcome those two problems, all but the Sigmas make quite decent images.With stabilization and AF these could be immensely useable for long-lens type stuff (not all telephoto photography has specular out-of-focus highlights).
I wonder how these will work with the extenders. The FD 500mm f8 accepts the FD 1.4x.
Honestly for 500mm it's incredible how compact this setup can be...
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