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Other Five Parameters
The anamorphic model has five other parameters that are more physical in nature, rather than polynomials in the coordinates.
HCTR and VCTR are the position of the optic axis (center) on the image plane, each ranging from -1 on one edge to +1 on the other. The entire image expands, shrinks, and distorts about this point. Ideally it is at the center of the image, but this is rarely the case, as it is difficult to locate the sensor chip that accurately. HCTR and VCTR can be difficult to determine exactly because they often don’t affect the image significantly. Longer shots where the trackers remain visible as the camera moves closer and further or especially rotates makes the center position more important to locate. See Lens Workflows for Anamorphic
Shots for important information about whether or not you should solve for the lens center.
HSCL and VSCL are image scaling parameters used to adapt to lens focus breathing effects. It’s important that the lens field of view be locked whenever HSCL is being computed, because the two values are fundamentally equivalent. Be sure to read the section of lens breathing for details.
The ROT (rotation) parameter addresses any setup error in the lens mount, where the anamorphic shrink axis of the lens doesn’t align with the coordinate axes of the camera sensor. If the distortion is otherwise symmetric, ie a merged model (or identical CX and CY parameters) is used and the only nonzero values are C02, C04, and/or C06, and HSCL and VSCL are 1.0, then the rotation angle does not matter. With different CX and CY parameters, or non- zero values in any “additional” lens coefficient, or HSCL or VSCL not 1.0, then lens rotation misalignment causes subtle asymmetric distortions that the ROT parameter can correct. The lens mount should always be aligned well enough to keep the rotation misalignment under a degree or two.
Note: The rotation angle parameter ROT is not well supported in common downstream applications and may need to be implemented by additional compositing nodes. See Maximizing Lens Model Compatibility and Performance for more information.
Note: Anamorphic distance and rolling shutter are not part of the lens model. They are separate issues, discussed elsewhere (in the main user manual for rolling shutter), that happen to be best placed on the lens solving control panels.
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