khj@ecsvax.UUCP (Kenneth H. Jacker) (03/19/88)
I need to make color slides of a Mac II screen. My
system uses Apple's High-Res RGB monitor.
Any suggestions regarding film type, shutter speed,
aperture, and/or exposure time?
--
Kenneth H. Jacker
Dept of Math Sciences UUCP: ...!{decvax,akgua}!mcnc!ecsvax!khj
Appalachian State Univ BITNET: khj@ecsvax
Boone, NC 28608jnh@ece-csc.UUCP (Joseph Nathan Hall) (03/19/88)
In article <4783@ecsvax.UUCP> khj@ecsvax.UUCP (Kenneth H. Jacker) writes: > > I need to make color slides of a Mac II screen. My >system uses Apple's High-Res RGB monitor. > > Any suggestions regarding film type, shutter speed, >aperture, and/or exposure time? > I've photographed a few CRTs. While I can't give you exact exposure info I can offer a few helpful hints. 1) The room must be DARK. Completely. Any light spilling onto the screen will ordinarily appear as a dim, putrid green (if your lighting is fluorescent). If you can't darken the room completely it will be necessary for you to construct some sort of shade around the screen. Also you'll want to remove fingerprints and dust, other crud, etc., from the screen. 2) Use shutter speeds substantantially longer than the display's refresh rate. I suggest 1/8 second or longer. I've usually used 1/2 second or more. Obviously you'll have to set your exposure manually. 3) Most electronic metering systems will not properly meter a flickering source (like a CRT). Older "electromechanical" meters will. (Luna-Pro, cheap handheld meters, most needle-type meters in general.) You can't use your camera's meter to read the exposure, unless it's of the electromechanical type. 4) When you meter the screen, turn off the lights and place the meter directly on the display, pointing it at a NEUTRAL, relatively dim area. If you're photographing a display of lines, you'll have to meter something else to get your exposure. Meters are calibrated for a reference subject of 18% reflectance. Ideally, you'd want to meter something that was 18% as luminous as your display's "white," but there's usually no practical way to do this. So you'll have to guess, and... 5) BRACKET YOUR EXPOSURES. A minimum of three exposures per display is advisable. If your exposure turns out to be 1/4" at f/8, you should take 1 picture at 1/4" as planned, and then one at, say, 1/6" and one at 1/3". If you're not too sure about what you're doing I suggest you fire off a test roll or two. Write down EVERYTHING as you conduct your test. Bracket by half- or third- stops in both directions from your estimated exposure, 1 or 2 stops each way. Record the settings of brightness and contrast controls on the display. Be sure when you look at the results of your test that you view the slides properly, either in a good viewer or as they will be projected. Most slides that are a half-stop under- or over-exposed look OK when just held up to the ceiling lights or whatever. 6) If your slides have a pronounced color bias you'll need to try filtration. Use CC ("color correction") filters, and experiment until you succeed. This is a complicated procedure for the amateur photographer, but basically you'll need gelatin filters and a gelatin filter holder, both of which can be ordered from a good photo supply store. 7) A mild telephoto lens (about "portrait length"-- 85-110 mm) is best unless you're photographing a particularly large screen. Then you'll want an even longer lens. You need to be far enough back from the display to eliminate perspective effects and to make sure the entire screen will be held in focus. In particular you need to be far enough back from the screen to eliminate "bowing" of the display due to the CRT's curvature. 8) A GOOD TRIPOD is essential. A remote shutter release is almost as essential, unless you have either a self-timer or have a very light touch on your shutter button. Films: Different films react different ways to different screen phosphors. You might have had great luck with Ektachrome 100 on your VAXstation but that doesn't mean it'll work as well photographing your TV. I suggest you try Ektachrome 100 or Kodachrome 64 at first. If you have problems with the color balance you can always try a couple of other slide films before proceeding to the (ugh) filters. If your results aren't good at first, keep trying. Good slides of computer graphics are a little tricky to make, but not impossible. -joseph (I thought I'd post this instead of replying directly, since I think it's probably of interest to a few other folks. Sorry it it wasn't.) /* * We all have our own opinions here. */
dave@onfcanim.UUCP (Dave Martindale) (04/04/88)
Some additions/corrections to the posted info: >3) Most electronic metering systems will not properly meter a > flickering source (like a CRT). Older "electromechanical" meters > will. (Luna-Pro, cheap handheld meters, most needle-type meters > in general.) You can't use your camera's meter to > read the exposure, unless it's of the electromechanical type. The Minolta Spotmeter M (and probably the newer Spotmeter F) do a fine job of measuring flickering light sources such as a CRT. The Pentax Digital Spotmeter definitely does not work. >4) When you meter the screen, turn off the lights and place the meter > directly on the display, pointing it at a NEUTRAL, relatively dim > area. If you're photographing a display of lines, you'll have to > meter something else to get your exposure. Meters are calibrated for > a reference subject of 18% reflectance. Ideally, you'd want to > meter something that was 18% as luminous as your display's "white," > but there's usually no practical way to do this. So you'll have > to guess, and... Well, you can display a grey patch of 18% of full brightness and meter that, but unless you've done very careful "gamma correction" of your monitor, this won't be accurate. Instead, display a white patch in the centre of the screen (since intensity falls off at the edges) and measure it with the spotmeter, or a standard meter up close. Then give 2.5 stops more exposure than the meter indicates. Since the log to the base 2 of 0.18 is -2.47, this is equivalent to measuring an 18% grey patch but doesn't depend on gamma correction. Another way of thinking about it is that 2.5 stops is the appropriate exposure increase to place the measured area of the picture (full white) at the upper end of the linear response curve of a transparency film. Negative films can generally take even more exposure. So, for example, if the light meter read f/11 for the white patch, you would set the lens diaphragm halfway between f/4 and f/5.6. >5) BRACKET YOUR EXPOSURES. A minimum of three exposures per display > is advisable. The first time I tried the above technique of using a white patch for reference, I bracketed the exposure widely with tests spaced every 1/2 or 1/3 stop. The exposure calculated by the meter turned out to be the best. Since then, I never bracket, I just trust the meter, and all exposures have been fine. >6) If your slides have a pronounced color bias you'll need to try > filtration. Use CC ("color correction") filters, and experiment until > you succeed. This is a complicated procedure for the amateur > photographer, but basically you'll need gelatin filters and > a gelatin filter holder, both of which can be ordered from a good > photo supply store. If you don't have a gelatine filter holder, you can always use adhesive tape on the corners of the filter... >7) A mild telephoto lens (about "portrait length"-- 85-110 mm) is best > unless you're photographing a particularly large screen. I second this. I use 135mm. >8) A GOOD TRIPOD is essential. A remote shutter release is almost > as essential, unless you have either a self-timer or have a very > light touch on your shutter button. Yes, and also make sure that the camera's lens is as close as possible to the centerline of the CRT face, both vertically and horizontally, to avoid distortion. Focus very carefully. One final point: I always use a shutter speed of 1 second. Long exposure minimizes any effects of the shutter not being synchronized with the CRT refresh, and 1 second is the longest exposure my camera will time for me.