123 lines
6.7 KiB
Plaintext
123 lines
6.7 KiB
Plaintext
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Photographing TV and Computer Screen Images
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From the brochure "KODAK Scientific Imaging Products", pages 26 and 27:
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Photographing TV and Computer Screen Images
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It is a relatively simple task to record a color or monochrome image
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displayed on a television set, monitor, or computer screen. Although
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a 35 mm single-lens-relfex (SLR) camera is recommended for ease of
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focusing and framing the image, any automatic or adjustable camera will
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yield good resutls. Close focusing capability is a must if the screen
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image is to fill the film frame.
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Several options are available: use a camera with a close-focusing
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(macro) lens, use a lens with a fixed focal length of about 100 mm,
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use "tele-extenders' to increase the focal length of a normal lens, or
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use a supplementary close-up lens on a normal 50 mm lens. A lens of longer
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focal length, (e.g., one of 100 mm), minimizes the effect of screen
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curvature. You also need a stable tripod or platform because exposure
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times are too long to permit hand-holding the camera.
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Shutter Speed
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Becuase images on a television or computer screen are formed line-by-line
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by a rapidly moving electron beam, you can obtain a complete picture of the
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screen only if the camera shutter speed is slow enough to allow the moving
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beam to complete its scan. To avoid photographs with dark or light "banding,"
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the beam should complete many scans.
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TV images typically are composed of 525 horizontal scan lines (U.S.
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standard) or 625 scan lines (the standard adopted by other countries).
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For the U.S. standard, a complete scan takes 1/30 second (1/60th for the
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odd- and 1/60 for the even-numbered lines). Theoretically you should use
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a shutter speed no faster than 1/30 second, or 1/25 second for a 625-line
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picture. These times are for a camera with a leaf shutter; for cameras
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with a focal-plane shutter, reduce shutter speed to 1/8 second in order
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to avoid getting a dark band across your picture and to stop TV action
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scenes.
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For computer screens, however, the subject is not moving and the
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resolution requirements are much higher. Also, photographs are frequently
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made of charts or graphs with light-colored backgrounds, and which show up
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dark bands particularly well. When photographing computer screens, you can
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obtain the best results with exposure times of 1/2 to 1 second. This exposure
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puts many images of the screen on the film to minimize the effect of
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"banding"--a film speed of ISO 100 is ideal.
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Adjusting The Screen Image
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Reduce the contrast of the screen image to slightly below the setting for
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normal viewing. For a black-and-white image, adjust the brightness control so
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that there is detail in the highlights and also in the shadows. For a color
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image, adjust the color controls so the image is visually pleasing.
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If your TV, monitor, or computer has an automatic brightness control--
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one that varies image brightness in response to changing levels of room
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lighting--turn the control off and adjust the brightness manually. For
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computer screens, reduce the brightness so there are no scan lines visible on a
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black screen.
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Room Lighting
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Darken the room completely if possible, or to a practical level to reduce
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ambient illumination. Doing so helps make the area around the screen image
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appear black in your pictures. A black surround is usually more pleasing than
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a lighter one or one reflecting part of the room. If you photograph screen
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images frequently, consider using a specially devised black "tent" or cone that
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fits tightly around the screen.
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Do not use flash or other lighting to illuminate the screen; it will
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overpower the screen image. If you camera is equipped with automatic flash,
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disable it or cover the flash with a piece of cardboard or other opaque material.
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Film Recommendations
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Use daylight color film to photgraph images on color screens. Color pictures of
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color screen images may have a blue-green appearance because the sensitivity of
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the film is different from that of the eye. Use a slow film, e.g., KODAK
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EKTACHROME 100 Profressional Film. This film speed will call for a 1-second
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exposure at f-8 to f-11. A 400-speed film may be too fast; you will not be
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able to stop down to near f-11. Excellent photographs of computer screens are
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made using small f-stops (to account for screen curvature) _and_ long shutter
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speeds. KODACHROME 25 Professional Film has produced very good results.
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To improve color rendition with KODACHROME and EKTACHROME Professional Films,
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you may need to use a KODAK Color Compensating Filter CC10 or 20R over the
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camera lens and to increase the exposure by 1/3 stop. Use a CC20B filter with
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a color negative film such as KODAK VERICOLOR III Professional Film, which
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provides brighter colors than and increased color contrast over the "conventional"
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100-speed film. Experiment with this film, becasue it works better with some
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phosphors than others.
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You can also use the above color films to record the white, green, or amber
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images from monochrome computer screens (without the use of color compensating
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filters), or use black-and-white films if you do not need to retain the green or
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amber colors in the picture.
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[herein are listed a number of KODAK films. It is, after all, a KODAK document.]
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Exposure Determination
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Choose the correct shutter speed (1/2 to 1 second for computer screens and
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1/4 to 1/8 second for TV images), and leave the f-stop stetting to be determined.
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Consult your camera manual to determine how to set exposures with an essentially
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fixed shutter speed.
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Cameras equipped with a built-in exposure meter are useful, but the TV or
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computer screen should fill the image area when the reading is taken. For
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typical displays of "average" color and brightness content, you meter should
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yield correctly exposed slides. For predominantly bright displays, bracket
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exposures on the "overexpose" side; for predominantly dark screen displays,
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bracket on the "underexposure" side. If you have through-the-lens metering
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and you are using a color film with a color compensating filter over the camera
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lens, your camera should automatically increase exposure to compensate for
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attenuation by the filter.
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To be sure of getting a properly exposed picture, bracket your exposures.
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In 1/3-stop increments, overexpose or underexpose 1 full stop from any
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calculated exposure.
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Selected Reference
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Eastman Kodak Company 1986. Photographing Television and Computer Screen
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Images (AC-10).
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To order publications from KODAK, write to:
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Eastman Kodak Company
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Dept 412-SP
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343 State Street
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Rochester, New York 14650-0608
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Copied without permission, but then, it was a free catalogue, so I don't
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think they'll mind.
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Alan Berenbaum adb@research.att.com
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AT&T Bell Labs
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Murray Hill, NJ
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