khan@romulus.rutgers.edu (Farrukh Shah Khan) (06/03/91)
I need to take color pictures of the screen. Could anyone help me out with this problem. What camera, film etc. to use and any other useful tips. Thanx in advance Khan
morris@ucunix.san.uc.edu (Ted Morris) (06/04/91)
We've had good results with the following: turn off all of the lights and measure the light off the face of the CRT at about 8", and again at the camera lens. Use a bit longer than normal lens and shoot from ca. 3' to get full frame=full screen; using a 50mm even with macro will tend to give you some bending. Figure about a 1/4 sec. exposure to minimize diagonal banding problems from the screen refresh, and set aperature accordingly. If you need to take a picture of someone USING the screen, as well as have what's ON the screen show up, use a flash w/slave setup to light the scene-and-person, set flash distances/etc. to give proper exposure with the aperature you're using for the screen, and shoot at 1/4 sec in total darkness, letting the flash and slave illuminate the scene for the proper time at your desired aperature, and the screen to "burn through" during the rest of the 1/4 second. Hope this helps; we publish these regularly in an in-house offset magazine and the work out pretty well. Theodore Allan Morris, U. Cincinnati, Med Ctr Info & Comm, Info Rsrch & Dev, 231 Bethesda Av, ML#574, Cinti, OH 45267-0574, 513-558-6046V, 513-558-0758F, MORRISTA@UCMCIC.OA.UC.EDU / MORRIS@UCUNIX.SAN.UC.EDU, NTS WB8VNV, AppleLink U1091 | No Good Deed Ever Goes Unpunished.
3ksnn64@cidmac.ecn.purdue.edu (Joe Cychosz) (06/04/91)
In article <Jun.3.12.11.48.1991.3513@romulus.rutgers.edu> khan@romulus.rutgers.edu (Farrukh Shah Khan) writes: > >I need to take color pictures of the screen. Could anyone help me out >with this problem. What camera, film etc. to use and any other useful >tips. Well this question hasn't come up in a while, so I guess I post my usual response. Film: I use Kodak 100 print film for prints and Ektachrome 100 (Ektachrome 64 is no longer available) for slides. I have not yet shot Ektar 125 but plan to on the next roll. The film density is much greater and the color is better. It also makes excellent enlargements. It shouldn't be used for photographing people but I think it will do quite well for computer graphics. The exposure/f-stop numbers should be close to those for 100. Setup: This is the longest and most crucial step. It is important to get the lens as perpendicular as possible to the screen. Things to watch for are monitors which tilt (like Suns or SGIs). I also try to get the lens as close as possible to the screen and still be able to focus. This helps reduce the effects of the curvature of the screen. For a 70-210mm Macro Nikon this is about 2 ft at 70mm. This contradicts what one might think in that one would expect the flattest photo to appear by getting as far away as possible and shooting with as long of a lens as possible. The problem is in the distoration in the lens system at 210mm. Keep in mind that a macro lens works on a series of approximations. What results is concave pillowing which is oppisite of the convex pillowing caused by the curvature of the screen. The optimal flat picture occurs when the convexed pillowing of the screen is balanced with the concaved pillowing of the distortion in the lens system. Since I do not have a straight 200mm lens, I have not been able to study and measure the distortion in the macro. One final note: I tend to shoot images that are either 640x480 or 512x 512 off of the SGI. I have shot full screen images, but the curvature of the screen gets a bit difficult to control, especially with the 70-210 lens problems. Exposure: I use a 1/2 second exposure at f8 with half a stop added in. To bracket the exposure I also shoot a f5.6 and a f11 exposure. With 100 speed film the exposure can be fairly forgiving. That is to say, you'll get a reasonable picture as long as the exposure and f-stop are within reason. Never shoot faster than the refresh rate of the monitor, 1/60th. A wide black bar will appear on the screen. This even goes for taking pictures of monitors sitting in rooms. If you do you will endup with black bars on the screen. For room shots I use 1/8 second or longer. I have shot 1 second exposures at f8 and have had similar results. Again, 100 speed film is pretty forgiving. I have found the exposue metering systems in cameras too unreliable when determining the exposue. This is especially true when shooting vector displays. Also, you want to avoid full white areas such as menu areas and the like. Change them to a 70 - 80% grey. The pictures will look much nicer when printed. Processing: For prints I usually tell the people that process my film that the roll contains computer graphics. Many film printers are computered controlled and are setup for printing people, sky, grass, trees and other things which you would find in normal photos. These printers usually lose it when the get a picture with lots of black and fully saturated colors. Vectors are the hardest to process. I use a Noritsu and a Fuji machine for my processing. Usually one or two units of density must be added (i.e., +1, +2) for computer graphics shots. The process you are going through is to balance the quality of the color with the quality of black. If you don't do this you will end up with brown instead of black. The Noritsu and Fuji will will print on the back of the photo what settings were used. So if you do reprints bring the old photo in so the processor can see what settings they used last time. Examples follow below: Noritsu: 896 45 N N N 6 2 896 = Sequence number 45 = Film code (45 = Kodak 100) N N N = Yellow, Magenta, Cyan print levels. N = neutral, there will be numbers -1, +1, etc. for non-neutral settings. 6 = Density setting. 2 = Ignore. Fuji: 79 010005 28 +01 +0 -01+02 NN -5 0 79 = Sequence number. 010005= Film/vendor/speed code (Kodak 100) 010205 for Kodak 200 Gold 28 = Daily density setting, results from calibration. +01 = Density setting. +0 -01+02 = Cyan, Magenta, Yellow print leves. +0 = neutral. NN = Ignore. -5 0 = Ignore. One other problem the film processing machines have is alignment. Lets say you are shooting pictures of some square 512x512 images. The film processor will more than likely assume that the left edge of the image is the left edge of the picture. I will not center it for you. This will also happen with slides. To solve this problem two things can be done. One: shoot a few fully exposed pictures at the beginning of the roll so the machine can set where the left edge is for the rest of the roll. Or two: us an alignment background pattern like I do. This looks something like this: ---------------------------------- ! ! ! ! ! ! ! ----------------- ! ! ! ! ! ! ! ! ! !-------! Image ! ! ! ! ! ! ! ! !--------! ! ! ! ! ! ----------------- ! ! ! ! ---------------------------------- I am in the process of working on a calibration image which will help the process people make there settings. I am also working on reducing the yellow edges which appear (especially at red and green boundries). I don't expect to have any of this done sometime soon though.
srnelson@nelsun.Eng.Sun.COM (Scott R. Nelson) (06/04/91)
From article <Jun.3.12.11.48.1991.3513@romulus.rutgers.edu>, by khan@romulus.rutgers.edu (Farrukh Shah Khan): > > I need to take color pictures of the screen. Could anyone help me out > with this problem. What camera, film etc. to use and any other useful > tips. Having just taken about 150 slides for a Siggraph course with good results, I would like to add one more long description of what works well for me. This is an update of a posting I made in January. You will (obviously) need a tripod to hold your camera. A cable release is helpful in reducing camera vibrations, but it is not absolutely necessary. A good daylight film produces best results on a color monitor because it is sensative to the same colors as the phosphors on the monitor. For slides, Ektachrome 50HC works very well. For prints, Ektar 25 is probably the best you can get. The slower the film speed the better the results you can expect. Slower films have a finer grain as well as allowing you to shoot at slower speeds which gives you more even light distribution. You are more likely to get a good color match with slide film because the photo processing lab does not attempt to do any color adjustments on slide film. Nearly all prints have some color adjustments done on them which can change your results. Before you start shooting pictures, make sure the color CRT is totally clean. The darker the room the better. Be sure there are no reflections off of the CRT from any other light sources. It is also a good idea to make sure no other bright CRTs are nearby. Align the camera and screen so that the screen is centered both vertically and horizontally relative to the lens of the camera. Contrary to what other posters have said, the further back from the screen you can get the camera, the better results you will have. If you have a zoom lens, zoom in all the way and move your camera back to get the right sized picture. I use the 70 mm setting on my camera. Moving back reduces the distortion caused by the curvature of the CRT. Also, if your camera has autofocus as mine does, get the focus right once, then turn the autofocus off. Because the electron beam sweeps over the face of the CRT somewhere between 50 and 76 times per second, you want the exposure time to be very slow. I prefer to catch at least 10 sweeps of the electron beam to guarantee even light distribution. I never set the speed faster than 1/4 second. Most of my best pictures with dark backgrounds were shot at 1.5 seconds. You can't always trust the light metering system to get an accurate reading from a CRT, but my camera, a Canon EOS Rebel, comes quite close. If you want the very best pictures from a CRT screen it is recommended that you bracket the exposure by shooting half an F-stop to either side of what your camera recommends, in addition to using the setting the light meter selects. If you need to shoot a lot of images, you should use up one roll of film just getting the settings right. Get it developed and look at the pictures before shooting your important images. I took 10 shots of one text slide varying both the CRT gamma setting and the exposure to get one slide that I was happy with. It turned out in this case that raising the gamma setting and reducing the exposure looked best when the slide was projected. I was then able to shoot 36 slides without worrying about bracketing each one, because I had the right setting. If you must have prints and want the best color reproduction, you will probably have to take something to the lab that they can use to match colors. The image called "lenna" is an excellent one for getting good color matching, since any good photo lab can produce a correct flesh tone. If you are shooting many different images, keep in mind that brighter images will need a different exposure setting than darker images. This should be obvious, but I ran into that problem once. One other hint for your film processing: make sure the first photo on your roll of film is bright to both edges. The film cutting equipment usually aligns to the first shot and lines up the rest of the photos based on where the edges of the first one are. I had about five slides with the edges cut off a few weeks ago because all images on the roll had black backgrounds. I had to hurry and shoot another roll of film to get all of the pictures I needed. --- Scott R. Nelson srnelson@eng.sun.com Sun Microsystems "Proofread carefully to see if you any words out."