Handheld shutter speeds? Whats safe?

[DRZ]

Yes, you do. Admittedly most people are OK with the inverse of the focal length on a crop body, though.
The best example of this is comacts - due to teeny sensor sizes they have v. short focal lengths - but there's no way you could handhold a compact at 10mm for 1/10th of a second.

So, I take a shot on a full frame DSLR, 600mm lens, 1/600th and look at it in PS and decide it is tack sharp. I then decide it needs a tighter crop - to APS-C proportions.

Do I suddenly see motion blur?

EDIT:

We will dance in circles unless there is some sort of rationale put to this. I am pretty certain that the 1/f shutter speed guide just happens to work for the 35mm format. Its only a guesstimation but it just happens to work. I dont think that it is a linear thing though, you cant change the coc and just expect the rule to work. Ultimately, my intuition is making me think that its related to the CoC and has nothing whatsoever to do with the sesor size. It is the only way that it makes sense of the mathematics of the situation.

So, for any given field of view due to sensor size where the circle of confusion happens to be the same as that for the 35mm format, you get the 1/f relationship.