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If you take that 600 and stack a 1.4x and 2x, you can get out to almost 1700mm. I've tried it with my 500 and, if the light is good, the results are
surprisingly good. On the other hand, if the light is bad, so are the results.
Also, amazingly, with a 1 series body, with the stacked converters you will
maintain AF because the camera only knows that the converter closest to
the lens is on board. With the 500 and stacked converters, AF was a bit slow, but it did lock in.