Juan Buhler mused: > > On Wed, 9 Mar 2005 15:14:32 EST, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote: > > > Obviously a small (higher) f stop shows up fine detail that might get > > blurred > > with a shallower depth of field. > > > This is precisely what is not obvious to me. If the dust is on the > front element of the lens yes, it will be more visible at smaller > apertures. But we are talking about sensor dust, which is right on the > sensor, without a lens to "focus" it.
It's not "right on" the sensor; it's a little way in front of it. And you're not photographing the dust spot; it is casting a shadow. The sharpness of the shadow of the dust spot on the sensor depends on how large a light source is illuminating it. For example; consider an object (such as a fork) held a little way above your kitchen surface. If you have a directional light source (such as the sun, or a single bare tungsten light bulb) the edges of the shadow are crisp. If, however, you have a fluorescent light fitting (especially one with multiple tubes behind a diffuser) the shadow of the fork has fuzzy edges; in fact you might not get sharp shadows of the tines at all, just a slightly darker area. A lens stopped down to f22 acts almost as a point source of light; one at a much wider aperture acts as a diffuse light source.
