IT WAS GREAT GOING BACK TO THE CLUB AND SEEING YOU ALL AGAIN on the green screen evening this week.
As you may have noticed, I was using a light meter, but there wasn’t time to explain.
It would be really useful to have a copy of the greenscreen clips shot so we can all practice chroma keying in our various packages, and discuss problems found and tips etc.
Posted by: Declan Smith <firstname.lastname@example.org>
It’s important not to overexpose or underexpose the greenscreen in relation to the actors. If overexposed, this will cause more fringing and can increase the chances of green spill. Also, when lighting a greenscreen, it’s important to get it as evenly lit as possible. Ideally, it should be lit by very soft light sources, such as with softboxes. The screen on Tuesday was lit by two openface hard lights which we applied diffuser on to make the quality of the light softer which helps with minimising shadows and hotspots.
Once the lights were in place, I used the light meter to measure various points around the screen to see how even the light was. Where it was too hot, I would physically move the lights, then re-measure. The picture below was taken from my Canon 7D running magic lantern software. The overlay I used was one called “false colour”. This is a useful tool which shows the exposure of the shot by colouring it. What this shows it that the main green screen was lit evenly, show in grey, with it going slightly darker towards the edges. Patrick also had an iPad app that did something very similar.
The trick is to set the exposure of the greenscreen around middle grey. You may have also seen me using a grey card (as pictured at the top this post). This was used to set the middle exposure on the cameras used to film the greenscreen shots. It’s important to know that your camera’s built in light meter is trying to expose the shot for what’s called “middle grey”. In order to set your camera exposure correctly, you need to have a grey card filling the screen, in your scene where the main action will be happening, and then set the exposure so that your camera’s in built light meter shows it in the middle. When you reframe of course, you may want to tweak this depending on the shot. Or, use an incident light meter which shows what light is falling as middle grey without having to hold up a grey card.
Once the green screen had been lit, the actors were lit separately. Now the light meter was tuned to my camera and the exposure it read out on the green screen meant that I had to set my aperture to F/4.5. When lighting the actors, I used the light meter to measure the light falling on the two actors and adjusted the lights until I got an aperture reading of F/4.5 where they were. If you look at the picture above, you can see that the skin tones are largely grey, or rather, exposed at the same level as the screen. Of course there are variations because what you are looking at is reflected light, which is what the camera sees, Fo a constant colour, such as the greenscreen, we want a constant reading.
As the evening progressed and we were against time, and new setups were needed, the setup would have varied as we didn’t have time to finesse, however modern software should be able to deal with it.