A question of curiosity only.

Is there any advantage in using a ND filter rather than stopping down a lens, or vice versa?

For instance using a ND8 filter on a f5.6 lens. Does that change the lens to the equivalent of a f11 lens, or does it merely allow a longer exposure without changing the optical properties of that f5.6 lens.

I知 wondering, for example, of the depth of field of a f11 lens versus that of a f5.6 lens with ND8 filter attached. Assuming no structural differences in lens type etc.

Also, for the sake of this query I知 ignoring the extra glass element between the subject and the recording media; which could affect the quality of the recorded image.

Regards
Tony