I have already realized several green and bluescreen projects and in my early days I sometimes experienced bad surprises in post-production. The reason for a bad key is almost always that the material was not shot properly.
Of course, budget affects the quality of the shot: camera, studio size, screen size, lighting units, etc. But that doesn't mean you can't get good results on a small budget. I've also shot greenscreen footage with a Canon 5D Mark II in a small photo studio, and the footage was great. However, tests in advance are important.
Successful green or bluescreen shots depend on many factors, which I would now like to explain in more detail:
Camera:
The choice of the right camera is crucial and should therefore not only depend on the budget. The sensor is the heart of every digital camera. The two most commonly used sensors are the 3CCD and the CMOS sensor.
In principle, a camera with a 3CCD sensor is the ideal choice for a blue or green screen. The prism separates the light coming through the lens into separate red, green and blue rays. Each colored ray then gets captured by its own sensor. This way, each sensor receives the full color information of its spectrum. The captured light is optimally utilized – unlike with the CMOS sensor – resulting in good color quality reproduction and low noise. The problem is that this 3CCD system is a lot bigger and more expensive to build for cinema cameras than the CMOS sensor.
That´s why almost all new digital cinema cameras have a CMOS sensor. This sensor is usually equipped with a Bayer filter, half of which consists of green-sensitive pixels and a quarter each of blue- and red-sensitive pixels. Which is why green components generate less noise in the image than blue components. A camera with a CMOS sensor is therefore rather unsuitable for bluescreen images. However, the amount of noise varies here as well, depending on the generation and the manufacturers' efforts to optimize the sensor and the Bayer filter.
After my last test in December 2013 with the RED Epic, the ARRI Alexa XT and the Sony F55, the latter clearly emerged as the winner: The Epic attracts with its high frame rates and pixel numbers. However, compared to the F55 it is merely a deceptive package and not competitive with its supposed "RAW" that is actually a compressed signal and its "5K". Even with only having a 2K Image, the Alexa works great. Provided you shoot at 400 or 200 ASA and not on the base ASA of 800 which would result in heavy noise. RAW is clearly the more suitable recording format, but you can also achieve good results with ProRes444 thanks to the high dynamic range of the camera.
The F55 with its 4K makes for incredibly sharp images and has by far the lowest noise. Here I recommend shooting in 4K RAW format and if 2K is desired, down sample the footage to 2K before keying. The resulting additional sharpness will help with keying and can be removed afterwards. This also applies to motion blur: Since it is easier to add motion blur in postproduction than to key a blurry image, use shutters smaller than 180°. Especially for 4K footage at 25fps or 24fps, where motion occurs over a larger number of pixels, I recommend shuttering smaller than 90°, otherwise you lose the advantage of the higher pixel count and just get more blur in the frame.
Shooting with an HD or 2K camera in portrait mode to have more pixels available for postproduction can make sense too. Especially for full-body shots of individual actors, this method is ideal to really take advantage of the entire sensor.
Blue or Greenscreen: The subject determines the choice of screen. The more complementary the background is to the foreground, the better. If the actor is wearing a saturated green jacket and yellow pants, you are better off with a bluescreen. But image composition also plays an important role. If the character ends up swimming in the open ocean and the background with sea and sky is blue, you can save yourself a lot of keying problems with a bluescreen. This is especially true for full body shots where the figure is standing directly on the area to be keyed out. There, spill is unavoidable and therefore it´s an advantage if the screen and composition background are in the same color direction. The aforementioned sensor structure is another decisive point in the choice of screen, which is why greenscreen is more often shot as bluescreen nowadays.
For the film "Die roten Schuhe" we shot with bluescreen. I decided in consultation with the postproduction house Cinegrell. Mainly because of the composition background and the skin tone of the actress. Also, the given costumes were completely unproblematic for the use of a bluescreen.
On set, however, there was a last-minute costume change in the ballet dress: the new peacock feathers, the semi-transparent fabric and the blue tones gave me a hard time. While the end result is respectable, the costume change required additional rotoscoping in post-production.
This shows how incredibly important it is to test and keep agreements when planning and later shooting Keys.
Lens:
It´s an advantage if both the subject in the foreground and the background inserted later are shot with the same lens, but it is not a must. Flares should be avoided at all costs when shooting greenscreen. Lenses that produce little flares are therefore best suited. Sharpness also plays an important role: the sharper the image, the more accurate the key. This also applies to the aperture. A wide open aperture is not recommended. In addition, the lens should neither exhibit vignetting nor distort towards the outside. If such effects are desired, they are better created in post-production. Depending on the key program, vignetting can usually be eliminated with a plate of the screen. If you shoot a greenscreen digitally in high resolution and with the new lenses designed for it, such as ARRI Master Primes or Leica Sumilux, you have no problems.
Light:
This is where most things can go wrong. Even the best camera and the most expensive lenses can't save an incorrectly lit scene. What does it come down to? Tests and good preparation are just as indispensable as the right equipment.
Light meters are certainly of great use. However, in my view, they definitely do not replace the waveform monitor, if only because many light meters give incorrect readings with monochromatic light, especially blue. The screen should be lit at around 80% on the green / blue channel waveform monitor in relation to white in the scene being at 90%. The background brightness depends on the aperture, not on the scene. So day and night scenes are filmed with the same screen brightness.
After all the important camera settings have been determined (shutter, aperture, ASA and frame rate), I recommend first lighting the foreground of the scene and only then the green or bluescreen. This way leads to the goal much faster and you save time and money.
KinoFlo units are the best way to illuminate a screen. Tubes like "525nm Green" and "420nm Blue" are specially made for green and bluescreen and have a big advantage over the normal 3200 Kelvin tubes: While the latter illuminate with a wide color spectrum and, for example, pollute the green of the green screen with blue and red, the others are almost monochromatic and provide a pure result.
If no special tubes are available, gels from LEE and Rosco such as "Primary Green" and "Primary Blue" can help. But you lose light intensity accordingly.
To prevent the screen and foreground from ruining each other, the distance between them must be large enough. Five meters or more is desirable. Such a distance prevents the actor from casting shadows on the screen and reduces the strength of the spill light bouncing of the screen on to the actor. To further minimize spill, it helps to flag off the areas of the screen that are not in the frame. A highlight from the screen direction also eliminates much of the spill. Care should be taken to ensure that this highlight matches the background and its light directions in the composition. If a green screen shot looks fake in the film, it is usually not due to a bad key or a cheap camera, but to incorrect light sources and their quality and direction. In order to light the set appropriately, the background which will be used for the shot later, should preferably already be available for reference. This way you can adjust the light perfectly.
I hope this listing helps you shoot great green and bluescreen shots and avoid mistakes so that a "fix it in the post" is no longer necessary.
Comments