shooting agains a video wall

Started Apr 2, 2012 | Discussions thread
kb2zuz Veteran Member • Posts: 3,202
Re: shooting agains a video wall

The 1/30th of a second was needed for Cathode Ray Tube monitors, which only displayed one pixel at a time and you had to give enough time for the entire screen to draw. If the screens are old CRT screens/monitors, then you will need to use longer exposures. Fortunately, these are few and far between today, most likely you'll be dealing with and Liquid Crystal Display

An LCD everything is drawn all at once, so shutter duration is not generally a problem... with a could caveats. If the LCD is fluorescent backlit, it will have a flicker. Generally the ballasts in these are relatively fast because people don't like flicker on the screen, however if your shutter speed exceeds this, the brightness of the screen will vary from shot-to-shot because you may catch the tube when it's darker in one shot, and lighter in another. LEDs also pulse (it's how they reduce the brightness) but they can pulse as extremely fast rates that the camera will not be able to detect, so an LED back-lit you should be able to shoot at however fast a shutter speed you desire.

If the screen is drawn via projection, it's probably a tungesten lamp projecting through an LCD or reflecting off of a Digital Micromirror Device in a DLP projector... if it's an LCD, it shouldn't be a problem. I haven't actually been able to test this with a DLP projector, but I do know some DLP projectors use a spinning color wheel, so you get a red frame, followed by a green frame, followed by a blue frame. In many better DLP projectors it may be 240Hz, so if you shoot at 1/120th of a second with these projectors, you should be fine, though some older or cheaper DLP projectors may cycle through the colors more slowly.

There are some other things to think about with LCD (including LED backlit LCDs) is that they work by polarization... so if you need to use a polarizer for something, they can darken or black out the display. You should always test first, because you're not always certain if something is fluorescent backlit or LED backlit, and if it's fluorescent you may need to stay below a certain speed (which can vary depending on the screen... could be as low as 1/60th, could be 1/300th). If it's projection, it could be DLP and if it's a color-wheel based DLP, if you do too short of an exposure, you'll have weird colors.

In any case it's best to test ahead of time by used the fastest possible exposure you think you might use (over estimate for the purpose of the test) and fire off several shots in a row (that way you can tell if there's an issue where the screen might be darker or brighter between shots if your shutter speed is too fast.

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