Photographing video screens ... a question about how they work?

[Keith]

The other evening while shooting a gallery opening of video art I ran into a problem that I've sort of had before but not to this extent. Most of what I shoot at these things is in on monitors and projection screens and while they can be very hard to meter because they're constantly changing I take plenty of shots to cover my arse so to speak! 😀

I struck a screen the other night that sent my poor D700 into a fit ... every time I took a shot of it and reviewed the image it would be a different colour ... bright orange, blue, green and then occasionaly (very seldomly in fact) completely normal. I never managed to get a successful image of the damned thing the entire night and in fact I'm going back to the exhibition later today to have another go at it. I was talking to the tech who sets these things up and he concluded that maybe some of these digital projectors pulse alternately between the primary colours at a rate that the human eye doesn't notice but the camera singles out these phases at faster exposures ... but why just this screen?

Does anyone have a technical explanation of what may be happening here?

 

[kzphoto]

Keith,

I'm not an expert here, but I think a lot of it will have to do with the technology involved in the projector. Have you thought about shooting a slightly slower shutter speed to get the full scale of colors?

As a side thought, is the camera set to auto WB? Could be the camera can't decide how to balance the scene.

 

[Keith]

Keith,

I'm not an expert here, but I think a lot of it will have to do with the technology involved in the projector. Have you thought about shooting a slightly slower shutter speed to get the full scale of colors?

As a side thought, is the camera set to auto WB? Could be the camera can't decide how to balance the scene.

Definitely not a white balance issue as the areas around the screen remain fine.

But I think you may be right about the shutter speeds because I've just realised it's a problem that I'm getting more with the Nikon than the M8 when I had it. The M8 was restricted to 640 ISO max so shutter speeds were correspondingly lower ... looking at the exif data from the Nikon I'm shooting at 1/500 and faster on some of the really big screens because I'm using the f2 lens wide open all the time at ISO 3200.

The first thing I'll be trying this afternoon when I bo back is shooting the offender at 1/30 off a tripod and see how that looks.

 

[Brian Legge]

You probably need a shutter speed closer to its refresh rate - around 1/30th or so. That would maximize the chance of exposing for the duration of a frame.

If you expose for less, well, its a bit like a florescent light cycling. You're only getting a portion of a cycle. In the case of a cathode tube TV, that would mean only part of the TV screen will have an image - the rest will be blank.

On the other hand, if your exposure is too long, you'll get multiple frames. You'll probably have to play around a bit to figure out what shutter speed is closest to the subjects refresh rate.

 

[tlitody]

yes but because it hasn't happened elsewhere I suspect there is more to it than that. Probably the way the video has been put together and possibly a very slow frame rate but if that was the case, then you would see the video flickering by eye. Once frame rate gets below around 20 frames per second you'll really notice it becoming poor quality and flickering.

 

[Keith]

You probably need a shutter speed closer to its refresh rate - around 1/30th or so. That would maximize the chance of exposing for the duration of a frame.

If you expose for less, well, its a bit like a florescent light cycling. You're only getting a portion of a cycle. In the case of a cathode tube TV, that would mean only part of the TV screen will have an image - the rest will be blank.

On the other hand, if your exposure is too long, you'll get multiple frames. You'll probably have to play around a bit to figure out what shutter speed is closest to the subjects refresh rate.

Because the matrix metering is so uncannily accurate, even in this extreme environment, I use it exclusively so I'm not really conscious of shutter speeds until I'm post processing the files.

It sounds like I might have the answer. 🙂

 

[Keith]

yes but because it hasn't happened elsewhere I suspect there is more to it than that. Probably the way the video has been put together and possibly a very slow frame rate but if that was the case, then you would see the video flickering by eye. Once frame rate gets below around 20 frames per second you'll really notice it becoming poor quality and flickering.

I noticed at the time that this particular video was very crappy quality ... it was shot in a busy street in Beijing of some nut case building a besser (cinder) block wall in the middle of the road and progressively moving it across the street from one side to the other by moving blocks from one end to the other while cars and trucks flashed past! 😱

Art! 🙄 😀

 

[lynnb]

given the description of the video I wouldn't be surprised to see a big splash of red!!
I believe Brian is correct about the refresh rate. You need to shoot slower than the refresh rate used in the original. Too slow and you get motion blur.

 

[alfredian]

Showing my age (59)

Showing my age (59)

Slow down - it's the "refresher rate" that's hobbling you. Try 1/30th sec. or slower. Back in the Sixties I had to photograph a television appearance (on "educational TV") by my younger sister. B&W film in the SRT101, and the Kodak guide-thingy said 1/30th second or slower because the TV refreshed the "rasters" on that cycle or a bit faster. I lost track of the technology, but analog TV used "scan" every second line, then go back to the top & scan the ones in between. Help smooth out the flicker-effect.

This is why cats don't watch much TV - their retinal renewal rate is so fast that they see an image with nothing smoothed out or filled in. Typical predator!

 

[Particular]

With old crt video signals the frame rate would be 60 fps for ntsc (US-Japan), or 50 fps (rest of the world) interlaced. The interlacing makes it that half the image lines are drawn, and then alternately the other half making the real frame rate 30fps and 25fps. In order to capture a complete frame, your shutter speed needs to be below this. 1/30 may work on some shot, but you probably need 1/15 to be safe. You would not really want to go slower than this because it will cause image blur.
As this video was shot at an unknown frame rate, and then possibly computer processed to a different frame rate, and then projected from a device which may have a frame rate of its own, all kinds of things could be going on. You will need to experiment, but a tripod and a slow shutter have always worked for me shooting video.

 

[tlitody]

I'm assuming this "Art" is being projected from an LCD projector. So there is the "Refresh Rate" of lcd projection panel which is probably around 60Hz so it redraws itself between 2 and 3 times for each "Frame" of a 30 frame per second video. Except LCD panels are continuos display. i.e. They don't switch on and off like a crt does. So the refresh rate is just fast enough to keep pace with fastest video frame rates. So in this case I strongly suspect it is the Video frame rate that is the problem and nothing to do with refresh rates unless the LCD projector is faulty.
But if projection was from film using a film projector, then it is entirely possible that the projector shutter was faulty or not in sync with the frames of the film.

Dunno because we don't know what type of projection was being used but I'd guess it wasn't a CRT.

Another possibility is that film or old video has been poorly converted to digital video so that you have a lot of bad frames in it.

 

[Thardy]

Video is either 30 frames per second (NTSC) or 25 frames per second (PAL). Use a shutter speed of 1/30-1/15 with no flash and the screen can be photographed. CNNY has a more detailed explanation.

Wow, the irony.

 

[steamer]

Always something new to learn here, just tried 1/30 on my NTSC TV and it worked perfectly. Thanks.

 

[Keith]

Well ... I went back to the gallery and experimented with the D700 on a tripod in front of the offending screen. Anything below about 1/80 sec was fine!

The strange thing is some of the screens and monitors seem ok at higher shutter speeds and some not!

Every time I do one of these things now I'll have to go around the gallery just prior to the opening and figure out which ones are going to cause me a problem. They always let me in for a scout around anyway when I first get there ... so I can take a few test shots in the black holes they always manage to create! 😀

I think this is what I really like about photography ... you never stop learning.

Thanks for all the help. 🙂

 

[filmtwit]

That's true for NTSC, but in Australia they use PAL, which is at 25fps, so you'll want to at that increment.
You can use higher speeds, but it's a crap shoot where the frame refresh will be at.

You probably need a shutter speed closer to its refresh rate - around 1/30th or so.

 

[Particular]

That's true for NTSC, but in Australia they use PAL, which is at 25fps, so you'll want to at that increment.
You can use higher speeds, but it's a crap shoot where the frame refresh will be at.

My Leica M3 has a 1/25th speed.

Modern HD/computer monitors/projectors run at all kinds of different speeds. Many have higher refresh rates to be easier on the eyes. That would explain why Keith could shoot at 1/80th.

 

[JoeV]

I suspect that DLP projectors would give this funny color problem, since each of the three primary colors are projected as a separate raster scan, using the same DLP light chip. These projectors use a rotating color wheel that filters the halogen light during each color frame to the correct hue.

Too fast of a shutter speed on your camera would only see one of these color fields.

LCD and plasma screens would have different artifacts, as do CRT displays.

~Joe

 

[_larky]

As everyone is saying slow down. Depending upon the screen technology w]being used, you'll need to use anything from 1/5th (some old CRT's) through to 1/60th (latest LED based panels, like Samsung 550EX etc). Projectors depend upon if it's DLP technology or not, but the same thing can be applied, and as you are shooting digi you can test it.

If you are getting lines, you are basically shooting too fast for the screen to fully render the image. The refresh rate is slower than the shutter speed, so slow yourself to suit. If you get odd colours, your shutter is going to fast for the projector to go through it's cycle of pushing it's colour range, so again you need to slow down. The thing is, you'll never get it perfect, because you'll never get your speed in perfect sync with the display equipment.

So don't worry, and fiddle. 🙂

I have to do this stuff a lot for my job, or previous job. OK, I USED to have to do this a lot!

 

[Keith]

Just when I thought I had the subject movement blur under control thanks to the D700's wonderful high ISO capabability I now have to back off to get the screens under control again ... sigh!

It really is the most bizarre environment to take photos in! 😛

 

[tlitody]

Those Leica M9 designers obviously knew that high ISO is not the B all and End all of low light photography.😀