Hi folks Sorry to send this message to the list, but I'd like to contact Guillermo and I lost his email address. Please, could you contact me Guillermo jribe...@greco.com.br Thanks folks
Joao ----- Original Message ----- From: Guillermo <pen...@rogers.com> To: <pinhole-discussion@p at ???????> Sent: Monday, March 10, 2003 8:02 PM Subject: Re: [pinhole-discussion] 120 mm zone plate > > ----- Original Message ----- > From: "Daniel Bouman" <daniel_bou...@hotmail.com> > > > > have never used a zone plate before. I've just received a 120 mm f/80 > for > > use on a field camera and I have a few questions which I hope some one > will > > enjoy answering. It seems to me that I should set the camera's front > > standard 120mm from the film plane. > > Yes, a 120mm lens, be it Schneider, Nikon, Canon, etc. or Zone plate, is > made to focus "sharply" subjects at infinity when the lens is placed 120mm > from the film plane. > > > If so, what are the near and far > > limits of the field of "sharp" focus. > > Those are subjective limits with glass lenses, more so with pinhole and even > more so with ZP or pinhole sieves. Having focal length and f/stop you can > find those limits for your subjectivity by applying the lens formulas > available in the lens faq (google find the faq) > > > I've read that zone plates don't have > > the universal field of focus that pinholes have and can be focused. > > Yes, that's 'cause they do focus light. > > > If I > > extend the bellows for a close in shot then I'm not using the zone plate > at > > the focal length it was designed for, right? > > If you extend the bellows and your subject is not at infinity, you are not > using the lens at the focal length it was designed for, and it doesn't > matter if it is zoneplate or your expensive Rodenstock lens. When you > extend the bellows, you are increasing the "effective" focal length of the > lens, therefore you have to reduce the distance lens to subject, they have a > reciprocal relationship, you increase one you'll have to decrease the other. > In short, it is OK to use the ZP for a distance lens to film longer than > 120mm, provided your subject is closer to the camera. Again, refer to the > lens faq to find out how much you should extend the bellows for a particular > distance lens to film. Having said that, I wouldn't worry too much, I mount > my > zoneplates at the "calculated" distance and use them like that no matter > where the subject of my picture is. > > > Or is it that the depth of > > field at f/80 is big enough to cover a subject at a foot or two from the > > camera? Any takers? Cheers, Daniel Bouman > > In practical and for "lensless" imaging purposes the answer is yes. In > mathematical terms, you would have to extend the bellows beyond the focal > length distance the lens was calculated for. > > For instance: for your 120mm ZP and a subject at 2 feet (609.6mm) from the > lens, the bellows should be extended > > F=focal length the ZP was calculated for > S=distance lens to subject > I=bellows extension > > I = (S x F) / (S - F) > I = (609.6 x 120) / (609.6 - 120) > > I = 149.4mm > > Hope it helps, > > Guillermo > > > _______________________________________________ > Post to the list as PLAIN TEXT only - no HTML > Pinhole-Discussion mailing list > Pinhole-Discussion@p at ??????? > unsubscribe or change your account at > http://www.???????/discussion/ >
