Shutter Efficiency – mental ray 3.11
Starting with 3.11, mental ray supports a shutter shape function ability for applying arbitrary exposure shapes to the shutter interval. This allows for motion blur that better matches film. While programmers can write their own functions, the built in trapezoid shape is good enough for most production needs.
Motion picture cameras use a rotary disc—a semicircular disk that spin in front of the film gate—as the shutter device. Light is only captured when the missing part of the disk exposes the film gate. This is referred to as the shutter interval. The portion of the complete circle that the disk is missing is referred to as the shutter angle. A shutter angle of 180˚ is considered normal for a motion picture camera. A larger shutter angle results in a longer shutter interval and more motion blur effect.
A shutter that is completely open for the entire interval would capture the most light possible and would be said to be 100% efficient. The physical nature of rotary disks means that the time it takes for shutters to fully open and close is non-zero. This results in less light being captured at the ends of the shutter interval. This softens the appearance of motion blur at its edges.
You can conceptualize shutter efficiency with a graph that plots exposure vs. shutter interval. An idealized shutter would correspond to a box shaped function on this graph. Real-world cameras have a more rounded shape that can be approximated as a trapazoid. The minimum efficiency of a trapezoidal function is 50%. Most real-world cameras are around 80% efficient.
Why do Maya cameras have a default shutter angle of 144˚ if 180˚ is more ‘correct’?
This is because renderers typically—and incorrectly—describe shutter exposure as being 100% efficient. Using a shutter angle of 144˚ with a 100% efficient shutter matches the total exposure of a real-world camera. That is:
144˚ × 100% = 180˚ × 80%
If you decide to apply an 80% efficient trapezoidal shutter shape to your scene, you may also want to adjust your render camera’s shutter angle to 180˚. This will give you a similar result to the idealized case, except now with more physically accurate (and visually pleasing!) motion blur.