stops translated to IRE

Sjur Pollen · September 5, 2006

Hi. I'm going to do a greenscreen shoot on DVCPRO HD, and I have no practical experience with this. I've read alot about it though, and want to light it one to two stops below key. However, I'm a video guy and I don't have a lightmeter... I do however have a vectorscope, so my question is: Is it possible to translate stops to IRE values? Or does anyone have some good numbers for IRE levels for a geenscreen shoot?

Nathan Milford · September 5, 2006

While it's more complicated than this, I generally go by 20 IRE = one stop. Middle grey is around 55 IRE, white skin tones are generally at 75-80 IRE. But that's all ballpark. Shoot tests >8)

Laurent Andrieux · September 5, 2006

You're supposed to double the signal level when you open up one stop, but that dosn't take the gamma in count...

But since you're a video guy, just apply the same technics you would in SD, better than working with a lightmeter.

Have an HD monitor on the set and set your stop on the monitor by eye, as to get as close as possible to the result you're looking for.

If you have a postproduction timing process, you can consider being a bit over on the set so that you can darken the image in post, with more details in the black...

John Mastrogiacomo · September 5, 2006

Hi. I'm going to do a greenscreen shoot on DVCPRO HD, and I have no practical experience with this. I've read alot about it though, and want to light it one to two stops below key. However, I'm a video guy and I don't have a lightmeter... I do however have a vectorscope, so my question is: Is it possible to translate stops to IRE values? Or does anyone have some good numbers for IRE levels for a geenscreen shoot?

You want a waveform monitor, not a vectorscope. :)

Michael Collier · September 6, 2006

Well, you could use a vectroscope to tell you that the green screen is actually green!

(it also helps to show your not blowing out the chromanence, leading to softer edges around contrasting colors, but if your exposed properly on a well set up camera, that shouldn't be an issue)

Bob Hayes · September 6, 2006

I think it is trickier then you might think. The set up on the camera determines a lot of the sensitivity of the camera. One setup might have a stop equal to 20 ire while with another setup it might be more or less. Also a when you add gamma correction and clipping to blacks and whites the correlation gets even more convoluted. The best thing to do is test you camera with a light meter.

Sjur Pollen · September 6, 2006

You want a waveform monitor, not a vectorscope. :)

Yeah... I know. I was in a hurry when I wrote my first post. I have both available though. Anyway, I knew there wouldn be a very precice translation and was just looking for some aproximates, really. Thanks fot the replies.

Tim J Durham · September 6, 2006

I think it is trickier then you might think. The set up on the camera determines a lot of the sensitivity of the camera. One setup might have a stop equal to 20 ire while with another setup it might be more or less. Also a when you add gamma correction and clipping to blacks and whites the correlation gets even more convoluted. The best thing to do is test you camera with a light meter.

It would be tricky. If you are in a totally dark room, opening the iris may produce no change in IRE. If you are shooting the sun, the lens closed down to f16 may already be at 100 IRE and any wider apertures may be clipped at 105 or whatever. I think it would be tough to come up with any useable formula as the situations would fall along some dificult to define point between those two extremes.

Brian Dzyak · September 6, 2006

Hi. I'm going to do a greenscreen shoot on DVCPRO HD, and I have no practical experience with this. I've read alot about it though, and want to light it one to two stops below key. However, I'm a video guy and I don't have a lightmeter... I do however have a vectorscope, so my question is: Is it possible to translate stops to IRE values? Or does anyone have some good numbers for IRE levels for a geenscreen shoot?

As others have mentioned, IRE levels can't really translate directly to stop readings in quite the way I think you're asking.

I don't know what equipment you'll have to work with, but when I light a greenscreen, I typically start by just working on the green itself to get it evenly lit. For what I do I usually can get away with two 4' Kinos. My stop is roughly a 2.8/4 split (for an IRE of about 70) at that point so I then know what I have to do with my key to get an IRE of around 70 on the subject's face.

I would highly recommend using a light meter to check the levels on the green. I prefer my old Spectra with the needle. It's very helpful to be able to just watch the needle move (or hopefully not) as I "drag" it across the borders of my frame. Trying to use the zebras in the viewfinder to judge hotspots is very unreliable. But once I know that my green is evenly lit, I take a glance at the footcandles and can very quickly set my key and backlights wherever I want them in relation.

One other note, I'm not sure why you want to light the green two stops below the key. I certainly wouldn't overexpose the green, but underexposing isn't necessarily right either. I try to talk to the post people who will be dealing with the shots to see what they like to work with and then go from there.