Re: Zone Plate Focus stringency

----- Original Message -----
From: "Richard M. Koolish" <koolish@bbn.com>

> A zone plate focusses like a lens so you should adjust the focus for
> objects at different distances. On the other hand, they tend to have
> large f numbers (like f/32) so they have large depth of field. They
are
> also hard to focus because they are so dim.

Right.

And if you ever decide to focus them, it is best to do it on the aerial
image. My suggestion: don't bother focusing, mount it at a distance from the
fillm plane equal or as close to the focal length it was made for and that
should be sufficient to get good results.

BTW, I don't think making ZP faster than f/64 is a good thing. A f/32 ZP
would require a large number of rings/zones and that augments the
noise/signal ratio considerably. For instance, a 90mm ZP f/32 would need 79
rings/zones (total), f/64 would just need 19.

Guillermo
Received on Mon Oct 29 10:23:42 2001