The lens “entry pupil” (http://en.wikipedia.org/wiki/Entrance_pupil) is the location where we have to put the virtual camera in order to match up properly to the optical properties of the lens. I’ll get into this more later, but the important points are:
1) it does not correspond to a physical part of the lens — you can’t measure it
2) in zoom lenses, it moves around a lot — many tens of centimeters — as the focal length of the lens is increased.
Finding the correct entry pupil is critical for proper CG line-up, and it’s why we had to custom build our own lens calibration system to solve this for each combination of zoom and focus barrel position.
Reading Lens Data
All cine type lenses have the same gear pitch on the barrels, about 0.8M/32 pitch. This is a pretty coarse gear, which is good for moving big lenses. The various types of ENG lenses all have their own pitch by manufacturer (Canon, Nikon, etc.) and have smaller gear teeth, so it’s a real pain to use an external focus system on these.
Fortunately, the ENG type lenses are usually encoded internally, so we don’t need a Preston type lens controller on these lenses to read what position the barrels are at. We already read the Fujinon lens protocol, so pretty much any Fujinon ENG lens with a serial port works with Previzion.