Color TV Screen Shots

[old_tv_nut]

All good advice above.

Reasons why:

The human visual system has parallel processing for average brightness and details, so you can see that there are color dots or scan lines and still see the average of these with the black spaces in between. Cameras with good resolution see the dots or lines and the black spaces individually, but set exposure according to the average. The lines or dots, if resolved, then get overexposed when the average is normal. The solution is to deliberately underexpose when taking the photo, then blur it slightly by reducing the resolution, and finally adjust the brightness/contrast (actually the level, as stated above). In Irfanview, there is no level control as such, but you can get the same effect by adjusting both contrast and brightness as required.

This problem showed up in early kinescope recorders (which made movies off a TV monitor for delayed broadcast) and in electron beam recorders that recorded TV directly to film. This was fixed by careful control of the CRT spot size (line width) and/or quickly wobbling the scan up and down at a very high frequency to fill in the space between scan lines.

Regarding lens zoom - in general it is not necessary to use a particularly wide zoom setting. Less geometric distortion will usually be had by using a mid zoom setting and moving the camera to a point where the TV screen fills most but not all of the viewfinder. It may be good to experiment with different zoom settings (filling the viewfinder more or less) to see if some give more or less moire' patterns.

Regarding shutter speed: you need 1/30 of a second to capture a full NTSC frame. This is still too fast for good results because the start point and end point of the shutter opening will not be exactly synced to the TV and will show as a light or dark bar across the screen. This bar can be horizontal, diagonal, or other, depending on what sort of shutter mechanism (mechanical or electronic, vertical-moving or horizontal-moving) your camera has. Practically, you need to use 1/15 second or slower.

If your camera does not have manual settings, you can fix the dot overexposure by using the exposure compensation, which nearly all cameras do have, and setting it towards darker (minus). Unfortunately, the camera may decide to make this adjustment by shortening the shutter opening to less than 1/15 of a second, which is needed for a good capture of a complete frame (see next paragraph).

If your camera does not have manual settings, it will be difficult to get good results, because your camera will set the shutter speed and aperture based on regular scenes, where it tries to stop motion if possible - it doesn't know you need 1/15 of a second for a TV picture. In this case, your experiments with zoom setting are also inadvertently affecting exposure settings because you are changing the percentage of the frame containing relatively bright TV picture vs. dim room surround. You will have to experiment until you find a setup where the camera picks the shutter speed and exposure you need (if you ever do).

[Electronic M]

Originally Posted by old_tv_nut

All good advice above.

Reasons why:

Regarding shutter speed: you need 1/30 of a second to capture a full NTSC frame. This is still too fast for good results because the start point and end point of the shutter opening will not be exactly synced to the TV and will show as a light or dark bar across the screen. This bar can be horizontal, diagonal, or other, depending on what sort of shutter mechanism (mechanical or electronic, vertical-moving or horizontal-moving) your camera has. Practically, you need to use 1/15 second or slower.

My Nikon Coolpix S3700 and other digital cameras I've worked with DO sync their shutters to ~60Hz light. I can point my camera at my TV screen with the basement fluorescent lights on, and it will synch to the flourescents (which are slightly off frequency from vertical scan frequency) instead of the TV and a rolling bar on screen will be seen with the camera...Kill the florescents and the bar instantly disappears as the camera syncs to the TV...If I use lights that do not flicker at 60Hz such as incandescent lighting or the sun the camera will always sync to the only source of flicker: the TV.

[old_tv_nut]

Originally Posted by Electronic M

My Nikon Coolpix S3700 and other digital cameras I've worked with DO sync their shutters to ~60Hz light. I can point my camera at my TV screen with the basement fluorescent lights on, and it will synch to the flourescents (which are slightly off frequency from vertical scan frequency) instead of the TV and a rolling bar on screen will be seen with the camera...Kill the florescents and the bar instantly disappears as the camera syncs to the TV...If I use lights that do not flicker at 60Hz such as incandescent lighting or the sun the camera will always sync to the only source of flicker: the TV.

Cool!

[dtvmcdonald]

Originally Posted by Electronic M

My Nikon Coolpix S3700 and other digital cameras I've worked with DO sync their shutters to ~60Hz light.

Now that's a real feature! Expensive SLRs should have that as an option
in the menus. They should not be so hoitsy-toitsy as to not do it!

[Dave S]

Originally Posted by old_tv_nut

...Reasons why:...

Another 'graduate-level' explanation from Wayne. Once again, I am less ignorant today than I was yesterday. THANKS!

-- Dave Sica