jorge@bocklin.UUCP (04/05/85)
I want to try infrared film for the first time. I understand that infrared is focused differently from regular B&W film. Some modern lenses ( if not all ) have a red dot to show where the focusing should be set at. My lenses, Leitz from the M3 era do not have these markings. Any net-landers know of any rule of thumb to do this ? Or computations to determine corrected infrared focus ? Also, if people could suggest films, maybe there isn't a whole lot to choose from. Thanks in advance ! -- Jorge Daniel Ochoa-Lions jorge@arizona Computer Science Department University of Arizona
jans@mako.UUCP (Jan Steinman) (04/10/85)
In article <107@bocklin.UUCP> jorge@bocklin.UUCP writes: >I want to try infrared film for the first time. There are different flavors of IR film, the most popular are Kodak B&W and Ektachrome. Neither is suitable for "no-light" photography -- a cryogenic camera is needed for such use. Either film should be kept cold until use. (Bring to room temperature an hour before use to avoid condensation.) The B&W is the one used for survelience photography. Typically, the scene to be photographed is illuminated with an IR source, and the lens is covered with an IR filter. Kodak makes a Wratten (gelatin) filter specially for this purpose, although threaded filters may also be available. I once did some free-lance work for a detective agency -- I covered an ordinary strobe with the Wratten filter and shot away in the dark. Human eyes detect a faint purple glow if looking directly at the flash, otherwise, it doesn't attract attention. IR Ektachrome is fun to shoot. It simplyshifts the color spectrum, with IR coming out red, red coming out yellow, etc. This also must be used with a special filter, because the film is excessively sensitive to blue light. This film is useful for the special, off-world color scheme, and for cutting through haze. With the specified filter, striking black skies and red-to- magenta vegatation are produced. This film is useless for low-light work. >I understand that infrared is focused differently from regular B&W film. >Some modern lenses ( if not all ) have a red dot to show where the focusing >should be set at. This is critical for the B&W film, which only sees IR through the jet-black filter. You can focus normally, note the distance marking, then move the focus until the noted distance marking is opposite the IR mark. The best way is to forget visual focus (you've got an opaque filter on, anyway!) and either stop down, or use a wide angle lens in order to obtain enough depth-of-field to shoot fixed-focus. In this case, choose the middle of your range and simple set that distance over the IR mark. >(My lenses)... do not have these markings. If you don't have an IR focus mark, or are shooting the color film, you have to fudge. Guess at how much IR you are getting and "lean" the lens long if you suspect a large ammount of IR is illuminating the subject. (Vegatation is a strong IR reflector.) Don't overdoo -- usually the IR marks are very close to the normal focus mark. A general rule of focusing is to make damn sure the closest highlight of your subject is sharp. Focus on that point, note the distance on the lens, move that distance over the close-end depth-of-field mark for the aperture you have selected. For mid-aperture, IR should be fairly sharp using this technique. If you don't have depth-of-field markings, use the depth of field pre-view and focus out (i.e., starting at the closest possible focus on your lens) just until the closest highlight becomes sharp. I have succeeded in using Ektachrome IR in macrophotography using this technique, so it isn't too critical, given a reasonable stopped-down lens. Don't have IR marks, depth-of-field marks, or depth-of-field pre-view? Point and shoot and hope for the best! -- :::::: Jan Steinman Box 1000, MS 61-161 (w)503/685-2843 :::::: :::::: tektronix!tekecs!jans Wilsonville, OR 97070 (h)503/657-7703 ::::::