number of rings in a zone plate

[sven]

It's a bit theoretical, but after seeing Yves Gentet's holograms I started to wonder something.

How many rings can a zone pate have? Seeing the clown for example, I think it must have many zone plates of which the centre (bullseye) just falls outside the plate itself.

I found an expression on the web that says:

d = 1.86*SQRT(f.n.l)

d = diameter of the n-th ring of the zone plate
f = focus = 50 mm
n = ring number
l = wavelength = 560 nm

So, if n = 100,000 the ring is 98421.949 um in diameter (about 10 cm) and the line is 492 nm thick. Now this is starting to get smaller than the wavelength of the light itself. Is this a problem or may de lines be even thinner than that? (I'm not well into diffraction theory yet).

Also I've red the glass plates that Yves Gentet uses can resolve up to '10 nm giving a resolution of 10,000 lines/mm'.

So if n = 100,000,000 the zone plate has a diameter of well over 3 meter. The thickness of the line then is about 16 nm.

So, what really limits the size of the zone plate? The wavelength or the resolution of the photographic plate?