----- Original Message ----- From: bendur...@aol.com >My names Ben >I have been a member of the discussion group for a few weeks, but have never posted anything, >I just wondered, what is the diameter of the 20 foot tube. Also I dont understand what reciprocity is. >Cheers Ben
Ben, phographically speaking, EXPOSURE is given by a pair of exposure values, they are TIME and APERTURE and they have a RECIPROCAL relation between them given the same EXPOSURE, in other words, if you increase one value by one stop and reduce the other one by one stop, the EXPOSURE does not change. That is why if your meter tells you need f/8 and 1/8 secs, you could opt to use f/11 and 1/4 secs or f/16 and 1/2 secs, this reciprocity tho fails when the exposure time is (geneally speaking) 1 sec or greater or 1/1000 secs or smaller, some films fail later than others. For instance, if f/16 and 1/2 sec is the indicated exposure, the reciprocity law tell us f/22 and 1 sec is its equivalent, unfortunatelly, for a film like TRI-X, failure kicks in at 1 sec and according to Kodak we should give 2 secs instead of just 1, so for TRI-X for an indicated exposure of f/22 and 1 sec, the reciprocity failure corrected exposure would be f/22 and 2 secs. Now, reciprocity failure doesn't fail arithmetically, otherwise for TRI-X it would be just a matter of doubling the indicated exposure time. Reciprocity fails geometrically, the larger the indicated exposure time the larger the failure. Here is a graphic showing the reciprocity corrections needed for TRI-X: http://www.kodak.com/global/en/professional/support/techPubs/f9/f002_0062gc.gif If the above does not help you, just read some of these sites: http://www.google.ca/search?q=reciprocity+photography&hl=en&ie=UTF-8&oe=UTF-8 Guillermo
