television

[julie32]

Also--- how can I take a decent photo of a television program without the thick bands that are viewable across the screen..is it possible?

thanks

 

[LaFoto]

Yes. It is possible. I think your exposure time must not be any slower than 1/60 sec. I have taken photos off the old screen, and taken them off the new one (flat television, I don't know if the picture creates itself in the same manner on the new flat ones), and even those off the old screen mostly came out without the black band across the screen.

 

[Crosby]

I don't think so unless you use a longer shutter speed.

Shoot an LCD TV screen. I don't think that will produce those dark bands.

 

[Valethar]

The problem is going to be the refresh on the screen. The bands you are seeing are a result of the photon guns in the back of the picture tube updating the image on the screen, which is done in a strobe-like fashion. This is what creates the moving wavy lines you see when someone has a video camera aimed at a TV screen, or the banding on a still picture.

As Crosby mentioned, LCD screens may not give you this headache. I've never taken a picture of an LCD, so I don't know for certain.

I did a quick Google search and found an article that may help. It's a bit dated, but if you're taking a picture of a standard glass tubed television, is should still give you something to work with.

http://www.halfhill.com/archive.html

 

[Snyder]

you can record what you want onto miniDV import onto your PC then take a screen cap of it using adobe premier. native resolution for SD is 720x480 but is perfect picture quality.

 

[ThePup]

It's actually pretty easy to take a photo of a telly - The shutter speed needs to be SLOWER than the refresh rate of the television. For PAL this is 50Hz, NTSC is 60Hz. (So, SLOWER than 1/50th or 1/60th of a second.)

 

[totalmajor]

you got it!

 

[Early]

They used to tell us 1/30th of a sec or slower. Please don't ask who they are.

 

[Valethar]

They used to tell us 1/30th of a sec or slower. Please don't ask who they are.

The Men in Black?

 

[Helen B]

It's actually pretty easy to take a photo of a telly - The shutter speed needs to be SLOWER than the refresh rate of the television. For PAL this is 50Hz, NTSC is 60Hz. (So, SLOWER than 1/50th or 1/60th of a second.)

It's better to do it at twice that exposure for CRTs at the very least - four times is better. PAL is 25 frames per second, NTSC is 30 frames per second. There are two interlacing fields per frame, hence the 'refresh' rates of 50 Hz and 60 Hz. You need to allow both fields to refresh fully as an absolute minimum (for CRTs).

Best,
Helen

 

[Judge Sharpe]

Avoid multipals or divisors of 60. because of the 60 cycle/ second refreash in the tube. Think of the wagon wheels in an old western when they turned at 60 rpm the looked to stand still. More than 60, they semmed to spin forward, less than 60 the looked like they were spinning backward.
Judge Sharpe

 

[JamesD]

For NTSC video (In the US) and also PAL video (I think) there are actually two complete sweeps required to get the whole image. Although one sweep takes 1/60th sec (for NTSC), that only paints every other line on the screen--the odd-numbered lines, if you will. The next sweep paints the even-numbered lines.

Set your shutter speed to 1/30th sec. As Helen B says, you might want to set it for longer, (in factors of 1/30... ie, 1/15, 1/10 etc); however, this will expose several video frames, giving you essentially a multiple exposure. Not a problem if the image on the screen is fixed, but for action scenes, it effectively gives you incremental motion blur.

 

[Helen B]

...and also PAL video (I think) there are actually two complete sweeps required to get the whole image.

Yes, PAL is also an interlaced format, as I mentioned in my previous post. What you are describing as 'complete sweeps' I am describing as interlaced fields.

...however, this will expose several video frames, giving you essentially a multiple exposure. Not a problem if the image on the screen is fixed, but for action scenes, it effectively gives you incremental motion blur.

This can be a problem even if you only capture one frame of interlaced video because it is made up of two interlaced fields, each of which was taken at different times. If you do a frame grab of interlaced video it is often very obvious - you can either smooth the two fields together, or simply duplicate one of the fields. Videos that are made from film may not have this problem at all because one frame of film may be used to produce both of the fields that make up a video frame, particularly with PAL. It's a little different for NTSC.

Avoid multipals or divisors of 60. because of the 60 cycle/ second refreash in the tube. Think of the wagon wheels in an old western when they turned at 60 rpm the looked to stand still. More than 60, they semmed to spin forward, less than 60 the looked like they were spinning backward.
Judge Sharpe

It's safe to go longer than the frame rate (not the field rate) but not shorter, as has already been explained a few times in this thread.

Edit: it's not so much the speed of the wheels themselves, but the passing frequency of the spokes - ie a combination of the rotational speed of the wheel and the number of spokes. Whether they appear to go forwards or backwards is dependent on the degree of mismatch between successively-recorded spoke positions, not whether the wheels were going faster or slower than the frame rate. You can see this effect when wagons are speeding up and slowing down - the wheel could appear to be going backwards when going fast, then it appears to go forwards when it slows down a little, then it appears to go backwards again as it gets slower, then appears to go forwards again just before it stops.

Best,
Helen

 

[JamesD]

This can be a problem even if you only capture one frame of interlaced video because it is made up of two interlaced fields, each of which was taken at different times.

True; but with analog interlaced video, each scan field is going to be at a different time (film-capture excepted, as you noted). (Edit: Which is exactly what you just said... lol) So it's sort of "as good as it gets."

I would imagine that newer video systems, especially if they're digital, would capture a whole frame, then separate in software before transmission, similar to film-capture. Do you know if this is correct? That would make a difference, even if it's a program recorded on VHS and played back.

 

[Helen B]

True; but with interlaced video, each scan is going to be at a different time

Oh. I thought that was what I had written - I was just expanding on your answer, not disputing it. ("two interlaced fields, each of which was taken at different times.")

I would imagine that newer video systems, especially if they're digital, would capture a whole frame, then separate in software before transmission, similar to film-capture. Do you know if this is correct?

Yes, there are 'progressive' video formats that do exactly that. The format usually ends in an 'i' if it is interlaced and a 'p' if it is progressive (ie not interlaced).

Best,
Helen