I asked a photographer about studio the other day. She said we needed 1/200th sec, to freeze motion. After all, you cannot shoot moving things at, say, 1/40 second. Right?

Wrong. And right.

That is: if you use only flash (i.e. your settings make ambient light go away) then you effective shutter speed is the speed of the flash—which is 1/1000 sec or faster.

So this shot was taken at 1/40 sec while the subject waved her hand quickly:

What you see is:

1. The hand is substantially sharp: that is the flash part of the exposure.
2. There is some “ghosting”: that is the ambient part.

If we had gone to a smaller aperture, say f/11, that part (2), the ghosting, would have disappeared. Even if I had shot at 1/10 second.

This is ONE reason that flash gfives you sharp images: it “freezes” everything.