In the near future on a lowbudget film project I will be shooting a green screen for the first time. The only thing I really know is that they must be lit entirely even as stop and quality of light. Because of the budget it looks like we will have access only to tungsten based lights. Any advice would be great appreciated.
New York, NY
I recently did some density tests with green screen. Without going into too much detail:
There are many many variations of this approach as you can imagine.
Mark Woods, Director of Photography
>In the near future on a lowbudget film project I will to shooting a green screen for the >first time.
I would be interested in seeing what the cost comparison is with tungsten lights and green gel compared to Kino Flos with green tubes. The tungsten units, cabling, green gel and manpower can be more depending on the circumstances. 8 four bank Kinos can do a nice job lighting good size green screen at one stop under exposure. Curious...
> Light the green to about 1 stop less
Why do you light the screen to one stop less than your subject key?
If you have the budget to get kino's to light the screen, then that's the way to go. The best tungsten lights to use, if it's a medium to large screen, are 2k or 5k skypans. Very even fields, and it's easy to blend the edges. Most likely, 2k skypans will be sufficient. You have to light the screen 1 to 1 1/2 stops under key, and using any more than one scrim in the fixture will cause a moire, which is not good of course. Generally speaking, it is better to use tungsten light than green kino's or gel. Unless it's a very large screen, and you're never going to be shooting the floor, the spill from the green light becomes much more difficult to control. It absolutely cannot hit the actors because any green light on them will dissapear just like the screen. I would also recommend you get at least a couple of Image 80's that you can keep on stands to walk around and fill in holes where needed. The screen needs to be lit evenly to within 1/2 stop. Make sure you talk to the viz effects guys to make sure everything you're doing will work with their particular requirements. Also, keep in mind that they can always use a garbage matte to remove lights and equipment from the frame if needed, as long as the key elements (the actors) have nothing but green touching them.
Exposing the green screen about 1 stop less increases the saturation of the green and decreases the bounce back onto the subject. Both of these factors help the ability of the compositors to matte the work. That said, if you're going to use a luminescence key with the color key, the color key should be about 2 stops (reflected reading) below the stop that you shoot the subject. I'm going to publish an article about this in ICG sometime in the next few months with graphs and more extensive explanations. BTW, shooting with the green Kinos will help separate the green density from the blue density that results from normal lighting of the subject. That is why blue artifacts sometimes occur in the digital matting environment with green screen. If more green isn't added the green and blue negative densities will be about the same.
Mark Woods, Director of Photography
I'd agree with Mark here - although, I prefer not to use the green gel if possible because you need to be much more careful not to allow any green light onto the talent. It helps to back light the subject, perhaps with a different color such as a 1/4 or 1/2 CTO to minimize the green spill. The important thing is to light the talent so that they seem like the talent is in the environment the background suggests. Also, shiny objects in front of the screen can be troublesome - look out for anything that might reflect the green for the obvious reason that if it does, it will key. Of course, there are those who don't mind that and it can work in your favor if you have the right compositer and situation.
A few musings regarding Jim's question....
20 x 20 green screen on a frame:
4 4' 4-banks at the top, 4 at the bottom, say 2 on each side. 12 x $75/day = $900. Cable to power this set up: Total amperage about 60amps. 350amp generator. 1 run of banded, 1 distro box, 2 25' 100amp extensions, 2 lunch boxes, say 4 25' stingers. 34 + 75 + 18 + 40 + 12 = $179/day. $900 + 179 = 1079.
4 5k sky pans with skirts & gel (and let's throw in some nook lights top and bottom, say, 4 2k nooks each, not that I think it's necessary, but let's have them there just in case). 4 x $65/day + 8 x $30/day = $500. 1 roll of gel: $140 list price. Cable for this setup: Total amperage about 235 amps. 350amp generator. 1 run of banded (three phase), 1 distro box, 2 100 to (2) 60 splitters, 4 100amp extensions, 2 lunch boxes, say 4 50' and 4 25' stingers. 34 + 75 + 16 + 36 + 40 + 12 + 12 = $225. $500 + 140 + 225 = $865
I'd want the same amount of manpower - the tungsten lights aren't that heavy, and the cable isn't any different. Kino Flos have smaller parts that always seem to take more time than you'd think to put them together.
The real difference is in the amount of power being drawn - we could probably get away with a Honda 6500 for the fluorescent setup, but then we'd have no amperage left for the rest of the set.
If we didn't use the gel for the tungsten setup, we could get away with just the 4 sky pans without the skirts (subtract $380) - that would definitely tip the scales financially toward the tungsten setup. Of course, if we think that we only need 8 KinoFlos, then we cut $300 out of that budget, and the minimzed fluorescent setup ($779)is less than the full on 4 5k's and 8 2k's ($865) - but not less than the minimized tungsten setup ($485).
It's a close race...
Lighting and Photography for films and other projects
Los Angeles, CA USA
There's quite a lot about this on the website, together with screen grabs and density readings of different ratios of foreground/background exposure.
Geoff Boyle FBKS
Director of Photography
>perhaps with a different color such as a 1/4 or 1/2 CTO to minimize the green spill.
To minimize any color's spill the complimentary color should be used to get "white" light. That would be magenta on a green screen or yellow on a blue screen. The CTO would just become olive and if too much back light were used the edge would approach Dmax and appear as a bad matte when it actually is too much back light on the subject.
Mark Woods, Director of Photography
On the subject of green screen lighting:
I made a discovery born of invention while shooting a heavily VFX laden science fiction picture (on a horrendously low budget) for Scott Billups recently:
We were scheduled to shoot an exterior scene in the alley behind the studio, but in the "active combat" of guerrilla filmmaking, it would have been a logistical nightmare to attempt it, because of actor availability and costume changes, so Scott decided to shoot the sequence against the green screen, for composite with background plates. (It's easy for Scott to be so cavalier about such decisions, having a fully equipped suite in his guestroom.... and a VFX wizard for a spouse!).
I had originally lit my green cyc with 2k softlights. However, since we were only modestly equipped lighting wise, I had to pull them off the grid for use on another set. So here I was, faced with a "Head to toe" green screen shot, no available soft lights and ten minutes to get it "in the can."
The green screen was a corner cyc, with perpendicular walls about 15 feet long, and the floor was painted green about 20 feet out from the wall. (roughly 900 square feet)
The only un-deployed light left in my kit was an ARRI HMI 400 Par with a "Chinese Lantern" style Chimera. I hung the lantern from the grid, equidistant from the two walls and the corner.
Striking the walls and the floor from the same source, at approximately the same distance, gave me the softest, most even light I could have ever prayed for!
Since it was supposed to be a "night shot," the same source also acted as a "Moonlight" rim light for the actor!
All I used to complete the lighting was a 650 watt fresnel with a square Chimera (dimmed down to 30%) and I was "Lit to go!" with two minutes to spare.
From now on, I'm going to light ALL my green screens this way.
(I don't work for Arri, nor Chimera, although, if I don't get another gig soon, I might come knocking!)
Joe Di Gennaro
Director of Photography
Sherman Oaks, California.
>in my kit was an ARRI HMI 400 Par with a Chinese Lantern" style Chimera. I hung >the lantern from the grid, equidistant from the two walls and the corner."
Space lights work well for this trick. You can control them a little better and they have a higher output if you need it.
Randy Miller, DP in LA
When you did the test which post package did you use for pulling the Matte?
>Use an even and extremely soft back light with 1/2 minus green on your subject (I >use foam core on the sides and top for a back bounce)
It has been my understanding and experience that in digital keying packages that using minus green as a back light is counter-productive to the process. This is a departure from the analog keying systems where using the minus green was an effective tool. In training with Ultimatte, they made it a very strong point to never use minus green on backlights when using a digital keying system. And my testing in the past with the Discreet keyers seem to also favor not using minus green.
So I would like to know if you where able to test both sides of this variable and if you have some still grabs of your keys both with and without the minus green back light.
> When you did the test which post package did you use for pulling the Matte?
We pulled the mattes on a modified Flame, although I've done this very same approach using the Ultimatte. I set up the blue screens for Pee Wees Playhouse on the second season, and if I remember correctly, the post house used an Ultimatte system. Clearly that was analog.
>It has been my understanding and experience that in digital keying packages that >using minus green as a back light is counter-productive to the process.
On a package of recent commercials for Disney where I was filming children on a green screen, I used the same approach and the mattes cut like butter. I don't know if the post house was using Ultimatte, but it was a digital system. I've done a number of jobs for the same agency who asks the production company specifically for me because of the ease they have with the mattes. I also did a luminescence key for the for a Mickey's Christmas DVD where I created the shadows of the characters by silhouetteing them against a white BG exposed 5 stops over the stop on the lens.
>And my testing in the past with the Discreet keyers seem to also favor not using >minus green.
It could be the quality of the backlight, I don't know how you approached it. I only know my experience using the complimentary colors. Many years ago I used CTOs, but they never worked as well. As I learned color theory, it became extremely clear why they didn't work.
>So I would like to know if you where able to test both sides of this variable and if you >have some still grabs of your keys both with and without the minus green back light.
The short answer is no. I was testing something else with the green and blue screen I was filming. The Flame artist loved what I gave him.
Mark Woods, Director of Photography
> I have to shoot a blue screen work in two weeks .
There is an excellent recent thread on one of the cml lists that answers your questions in detail, but here are my thoughts in a nutshell:
1. Yes, light the screen evenly. But don't stress too much about the edges where the actors don't "cross." These can be eliminated with "garbage mattes" in post.
2. No, the actors don't need to be lit flatly. They should be lit consistently with the background you will be keying in. But be careful to not let your eye get fooled by the bright background; it is easy to think you have more fill on the actors than you actually do.
3. Shoot a "clean plate" of the background with no actors. The compositer can load that image into the Ultimatte software (maybe others?) to create a reference that will improve the key quality.
4. Backlight is unnecessary if you are far enough away from the blue screen that you are not getting blue spill reflected on the actors.
5. Some people advocate putting blue gel on the lights that you light the blue screen with. The theory is that this improves the purity of the reflection from the screen. The downsides are these: 1. Blue gel sucks up two and a half stops from a tungsten fixture. 2. If you have a little spill from a white light getting on the actor, no big deal. If you get spill on the actor from a glue light, it can cause big problems in post.
6. It is important to get a saturated blue on your screen. Most people seem to advocate that you light either to the same key level as your actor, or one stop lower. All seem to agree no hotter than your key level!
7. Don't let your colorist do secondary color correction to "help" the color saturation of the screen in post. This can ruin the clean edges that your compositor will need.
>3. Shoot a "clean plate" of the background with no actors. The compositer can load >that image into the Ultimatte software (maybe others?) to create a reference that will >improve the key quality.
This only applies if the camera is locked off, or if you're using motion control and can repeat your move.
> If you get spill on the actor from a blue light, it can cause big problems in post.
I don't think it's possible to shoot a blue screen shot with no spill whatsoever, at least not under "normal" shooting circumstances. Having said that, all modern keyers have very effective spill suppression and in most cases minor spill is not really a problem. That's not to say one shouldn't try to clean up what they can in production.
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