How to shoot a 2 element composite!

"If an actor can’t travel to a distant or exotic location."

The following musing will review the step by step process most experienced visual fx supervisors follow to obtain the best elements possible for a two element green or blue screen composite. These rules applicable all to film, video and still photography shoots.
In addition to a camera, lens and a sturdy tripod, you will need the following tools to accomplish these kind of shots: an incident light meter, compass, tape measure, line level, a notebook for measurements and a "stand in" that approximates the size of the actor that will be photographed against a process backing, (ie blue/green screen) and composited later.
Critical data to notate: 
date, time, location, weather condition, compass direction, height, tilt, distance to subject-(reference stand in), elevation of subject relative to the camera height, focal length, color temperature, shutter speed, frame rate, shooting stop, key side reading, fill side reading.

Before you can shoot - the locations must be scouted! Always look at more than one area because you might not get permission to shoot there or it isn't affordable or practical to shoot there. Once you like a place, do a light study* to ascertain the optimal time to shoot for light and shadow conditions. In some cases, say a canyon location, you may have a very brief time to shoot the background so plan accordingly and set up early to be a 100% ready. *A light study is where you take a still picture every half hour from your probable camera angle. Remember - there is always enough time in a light study to shoot multiple angles - so cover yourself and don't guess. Review the photos and plan to shoot at that time. For best results, photograph your light study as close to the shoot date as possible. Now we shoot! And since it's a day exterior we must be prepared to accept the lighting conditions that exist on the day. Hopefully the conditions will support the intent of the shot in support of the story. Filmmaking is all about compromises and some are more painful than others. And none more so than to a DP who doesn't get the lighting conditions planned for on the shoot day. Oh, but it makes us strong! Frame up your shot and commit to the composition including the placement of the stand in. Lock off the camera - in other words - don't pan or tilt until after the shot is exposed and the aforementioned measurements are recorded. Shoot a pass with and without the stand-in. The pass with the stand-in is called a reference pass. These are useful because you get to see an example of how the natural light looks on a person in that environment. This will provide the foundation for your lighting on stage. The direction of the key should be replicated on stage and the amount of natural fill can be adjusted based on the look you're after aesthetically. If you have extra time and the weather conditions are changing, wait for the change to occur and shoot an additional pass. It's always good to have these elements in a variety of different lighting conditions because you never know - the script could change or the intent of the shot could evolve into something else. In other words – shoot versions.

Measurements are critical because they are needed for the second element that will be photographed in front of the process backing. If your stand-in is placed off center in the image, try not to triangulate his position relative to the camera. Rather, measure straight away from the lens and then pace off to either side to his position and measure that distance. Also, if the stand-in is uphill or downhill relative to the lens height, you should attempt to ascertain that elevation difference with a line level. This measurement will affect the lens height on stage unless that discrepancy is built into the set.

Take two important meter readings on location if you have to match that light exactly later on. Those two readings are the “key” and the “fill” in sunny conditions, and top light and a hooded fill reading in overcast conditions. A hooded reading is one where you cover the top of the ball on your meter to protect the fill reading from top light contamination. If there is some natural bounce that is reflecting off of sand, stone or buildings - take an incident reading of that source as well if that should become part of your lighting scheme on set.

If at all possible, shoot both the background and foreground elements with a prime lens and preferably the same prime lens. Though I have shot some plates with zoom lenses, I'm not certain you can ever get the exact size from background to foreground elements. Also, spherical distortion varies more dramatically between zoom lenses and you can encounter some real problems down the road if those imperfections are wildly different. The average viewer won't notice the difference but I insist on proper methodology for all visual effects shots. Why not? I like turning over elements that look good and fit perfectly. And by using the same lens throughout - lens color, contrast and sharpness will be identical on both elements. And try to shoot each element at approximately the same f stop. Use ND's outside to knock the aperture setting down to around an f 4 or any reasonable setting that can be achieved on stage.

By following these guidelines you will have the best chance of creating excellent elements that should comp together with a minimum of fuss. You can test this methodology very simply with a still camera and photoshop or your camcorder and final cut X. Whenever I do a simple green screen comp with my canon XA 10, I'll go to the widest or tightest end of the lens so I know the size will be the same between each element. In other words treat my zoom as a fixed focal length lens rather than guessing that the zoom setting between elements will be exact. This really does matter - do a test and see for your self.

One final note - if your shot requires a background plate with a pan, tilt, boom, dolly, focus change or zoom, you'll need a motion control rig to record and then playback all of those adjustments while shooting the second element at a later date. Also, in motion control photography, the camera and lens have to be nodal for all of the successive elements to fit together perfectly. Look up nodal point when you get a chance.

Good Luck!

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