well for me short is 336mm. At 30 seconds ISO800 my sensor is pretty much swamped (histogram almost touching the right side).
so there's no point going longer than say 60 seconds ISO400.
in general under 800mm is short. a cat is looooong (and heavy) not to mention slow (f/10). of course if you have hyperstar you'll be very happy (short -- 400mm, and fast -- f/2).
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