Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!spool2.mu.edu!news.cs.indiana.edu!msi.umn.edu!vixvax.mgi.com!keeney From: keeney@vixvax.mgi.com (Richard Keeney) Newsgroups: comp.graphics Subject: Re: HELP NEEDED IN PHOTGRAPHING SCREENS Message-ID: <1991Jan28.010515.708@vixvax.mgi.com> Date: 28 Jan 91 07:05:15 GMT References: <1991Jan27.232910.22575@ms.uky.edu> Organization: Management Graphics, Inc. Lines: 116 In article <1991Jan27.232910.22575@ms.uky.edu>, munish@s.ms.uky.edu (Munish Mehra) writes: > Does anyone have some tips on photographing computer screens. > There was a discussion about a year or two ago, does anyone have > a copy or pointers to it. > I'd appreciate any help. > I have a Cannon EOS Rebel with a 35-70 Zoom, so I guess that should be good > enough. > Thanks. If you are going to be doing a lot of that sort of thing, I would recommend purchasing a high quality digital image recorder like those sold by Management Graphics (my employer). If you only need a couple-few images, I would consider sending screen dump files (in some form of well used image file format like PICT, TIFF, GIF, etc) to one of the many services that make a business of imaging other peoples graphics on film using high quality image recorders like the ones we sell. Many of these businesses charge as little as $5.00 per slide for two or three day turn around. I can supply the name of one of our customers near you who do this sort of thing if you are interested. In either case the quality of the image recorder produced slides will be far superior to that of a picture of the screen. If you insist on photographing the screen yourself, here are a few tips: Turn off the room lights to eliminate reflections off of the tube. It is easy to overlook such reflections with the result being a ghost image of your face, the room, etc on every slide. Use a daylight balanced film - most CRT's are calibrated to have a white of 5500 deg. K or so. Use a long focal length lens (75 to 100mm) that can focus close enough such that the screen fills the frame. The longer focal length helps overcome some of geometric distortion introduced by the curved CRT. Some macro lenses designed for copy work may be appropriate if you can find one of reasonable long focal length. This type of lens is design to not introduce any of its own geometric distortion when photographing a flat subject (like a pice of paper of a print) while lenses designed for general photography will compromise geometric distortion for other priorities (like aperture size, zoom range, color correction, etc). Use a tripod and a cable release since the exposure will be on the order of a few seconds. Be sure the camera is positioned on a line normal to the center of the image area of interest on the screen to avoid any "keystoning". Use an exposure of at least 2 to 5 seconds to eliminate the horizontal bar that can be present if a shorter exposure is used. Most cameras will not let you select an exposure time of say 10 seconds unless you use the "B" setting. When using the "B" setting, the built in exposure meter cannot give you any useful information. To get around this problem, set the shutter speed to 1 second (or whatever the slowest speed the camera can still meter at) and take a reading of the required f-stop. Then open the lens one stop for each time you double the exposure time. For example, if the meter reading was f-16 at 1 second, you could properly expose at f-11 for 2 seconds, f-8 for 4 seconds, f-5.6 for 8 seconds, or f-4 for 16 seconds, etc. The f-stops marked on most lenses follow the sequence: .. f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32, f44, f64 ... Descending from right to left one of these steps doubles the exposure (the numbers are proportional to the inverse of the diameter of the aperture, the exposure being proportial to the area of the aperture). Be sure and "bracket" the exposure for your first few attempts. "Bracketing" in photographic terms means to expose the same subject at a variety of exposures both above and below that recommended by the exposure meter. This can be a tremendous time saver (at the slight expense of the cost of the additional film used) since it can ensure that the best possible exposure of an exposure critical subject is obtained on the first shooting session, and thus avoiding the delay associated with trial and error when each try has to be sent out for processing. You will most likely need to increase the exposure 1/2 to 1 stop over the meter reading due to reciprocity and stroboscopic effects in the film (typically, film is not a perfect linear integrator of light and tends to be underexposed when longer exposure times or intermittent exposures like those of the flickering light of a CRT are made). This desire for an increase in exposure can be offset somewhat by the fact that most slide films produce more saturated colors when underexposed, but at the expense of overall brightness of the slide when projected. This may be important when the subject on the screen is a chart or graphic with large areas of saturated colors, and is of less importance when the subject more closely resembles a natural scene. Print films, on the other hand, have more lattitude for overexposure, and print quality will deteriorate rapidly with underexposure. When you find a setting that works for your CRT, Lens, and Film combination, be sure and write it down as a starting point the next time you have to do this. Also note the brightness and contrast settings on the monitor. I myself find it hard to put too much faith in the monitor settings, and I would be tempted to bracket on subsequent shooting sessions anyways Use a fairly small aperture (large f-number) of at least f11 or f16 to make sure that the depth of field is sufficient to ensure that entire screen is in focus. With larger aperture settings (smaller f-numbers), the corners of the screen may be slightly out of focus compared to the center. Adjust the exposure time accordingly, keeping in mind the requirement of having an exposure time on the order of at least a few seconds. Physics is on your side here since you can always use a smaller aperture to achieve a longer exposure time, or a longer exposure time to achieve a small aperture. The only way to run into difficulty with this is if you are using too fast of a film (ISO 400 or higher). The faster film is more expensive anyways, so I would stick with something like Ektachrome 100 HC or Fujichrome 100 for slides, or Kodacolor 100 for prints. Good luck. -- Richard A. Keeney Senior Software Engineer Internet: keeney@mgi.com Management Graphics, Inc. Phone: +1-612-851-6126 1401 East 79th Street, #6 Fax: +1-612-854-6913 Bloomington, MN, 55425 U.S.A.