----- Original Message ----- From: "Richard M. Koolish" <[email protected]>
> 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
