What's the best way to take a picture of an LCD or CRT screen?

Question

I have seen many bad pictures of LCD and CRT screens. How do I get a good picture of an LCD or CRT screen?

Asked by daviesgeek

Answer

It's not very hard to take a good photo of an LCD or CRT screen. I'll add oscilloscope in here as a special case of both as I've taken meany such over the years. A few pointers. Soem of the following may sound complex at first glance but it's all fairly u=inyuitive taken a section at a time.

• Shutter speed is critical for a frame refreshed device due to the way the iamge is written to screen. Understand the nature of the image - it's a line scanned image refreshed in interleaved half images with non HD 525/625 line TVs, and may be something quite different if using a monitor or oscilloscope or eg Plasma screen. Play with exposure times to see what works.

  Whole screen refresh rate on a non-HD TV is 25 Hz with 50 Hz mains and 30 Hz with 60 Hz mains. It actually writes a screen at double that speed but with only half the data and then writes the second half interleaved with the first. Whether you can see that half is missing if you shoot at 50/60 Hz depends on the image and phosphor persistence and brightness and image content.

CRT framerate will vary with screen mode - you can look it up or check the computer video settings - or just try different shutter speeds until you get one or several that are best. Be aware that frame rate may change if you change mode.

Using exposure times that are not equal to these can give partial pictures with bright or dark areas or no picture at all if you are especially clever.

Take a series of images at rates from say 0.1 s to 0.01 and faster and see what "artefacts" appear. There will (probably) be a shutter speed that best suits and after that you use aperture and ISO and exposure time and lave shutter speed alone.

I just tried various speeds on a 1080p Plasma screen and anything BELOW 1/90th second is good. Above 1/90th the image artefacts are bizarre and almost inexplicable.

A CRT oscilloscope usually scans left to right BUT a modern LCD screen oscilloscope may combine trace write time with frame writes, so start playing.

Once you are happy with shutter speed effects ...

• Use minimal background light - have a desk lamp handy that you can turn off when taking photos. Washing out the image is easy.

• Tripod or solid support! Frame the screen and leave it there OR if that's not conveniemt have a setup where you can promptly put it back in exact position. For some work I'v used two drawer slides with a board on top that allows the camera to be "dollied" sideways out of the way with minimal loss pf position in return, given due care [tm].

• Small aperture but not so small as to get diffraction. Flat screen depth of field needed is minimal. Curved CRT needs only a little. f8 probably OK and f11 or f16 usually very usable - exposure time goes up.

• ISO - something that gives quality with your camera. Higher the better that gives acceptable quality. I have an SLT77 that probably would be OK at ISO800. My older 5D & 7D Minoltas maybe ISO 400. A700 Sony 400 to 800. ISO 50 or 100 if you must but quality usually does not justify it.

• Manual focus and exposure are not essential but help. If having to move and replace camera, if available set to "Manual focus with press to focus when needed" and NOT with shutter button.

• Set LCD/CRT/Scope scope picture and camera exposure to what produces good results. Phosphor persistence will usually cause blurred lines at settings that look OK to the eye normally. Probably have scope trace dim and well focused.

Play.

With an oscilloscope, if it has an "illuminated graticule" adjust level to look good -not blazing bright wrt trace and not invisible.

LCD screens best viewing angle will vary with contrast setting. If contrast is not adjustable you MAY find that an angle above or below horizontal works best.

Answered by Russell McMahon

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