Add Terms and Definitions and add entry for Nominal Image Plane

[JGoldstone]

There are two definitions that express positions that happen to be on the optical axis. One is the position of the lens entrance pupil. One is the position of the intersection of the optical axis and the object on which the camera has been focused. In both cases, an exact definition of the place on the optical axis from which those two positions are measured is needed.

This definition (from the Terms and Definitions of SMPTE RDD 55, "Material Exchange Format — Carriage of ARRI Camera System Metadata", p. 5) uses the term 'Reference Plane'. We could keep 'Reference Plane' or we could just as well change it to 'Image Plane'; a possible reason to keep it would be to leave 'image plane' as the plane on which a pinhole camera would have formed its image.

If the group thought it would be less confusing, one could add to that 'such as' phrase the typical IR filter. I am not sure whether the original author of this definition meant 'sensor' to include or exclude any microlens array or color filter array material but will ask.

[JGoldstone]

Speaking with our optics group, they suggest a different definition that avoids mention of any hardware between the lens and the "photosensitive material", whether that be electronic or photochemical.

There is no conflict per se with the RDD 55 definition above.

Here is what I was given:

"Nominal Image Plane
The nominal image plane is an imaginary plane, which represents the expected location of the camera's image plane, given the location of the mount flange plane and the specified nominal mount flange focal distance. For LPL mount, the nominal image plane is offset by 44mm from the mount flange plane (see flange focal distance).

Since the nominal image plane is often used as the origin when specifying points in space (e.g. object distance, entrance pupil position) its location is indicated on a camera body."

Given that the Cooke Optics' "Camera and lens definitions for VFX" document is the only one of its kind, as far as I know, I suggest we stay closer to its terminology than the terminology used in RDD 55 (to read which you must part with a cool $240). Now, the Cooke document sometimes says "image plane" (2X on p. 5, 2X on p. 6, 2X on p. 7, 3X on p. 9, 3X on p. 11, 1X on p.12), and sometimes says "nominal image plane" (4X on p. 11, 1X on p. 12). That last one is important, though: that is the definitions section:

I suggest we add a Terms and Definitions section to camera-lens-tables.md, and that as a start, it only have one definition: Nominal Image Plane. I suggest it read thusly:

The nominal image plane is an imaginary plane, which represents the expected location of the camera's image plane, given the location of the mount flange plane and the specified nominal mount flange focal distance.

Though one could jump straight into adding definitions for Mount Flange Plane and Nominal Mount Flange Focal Distance, I suggest we do not, until pressed for them.

Once the definition for Nominal Image Plane exists, we can (e.g.) define the location of the object in focus and of the entrance pupil as offsets from the intersection of the optical axis and the nominal image plane; but altering the existing definitions (and the name of the defined thing, if need be) is something for a separate Issue.

[revisionfx]

Image Plane - imaging plane or focal plane, more common than Reference Plane, that would be my preference. I think in film days we were discussing middle of film emulsion versus film surface but my interpretation of Ian's original 2018 paper was that the digital camera side optics (sensor side filters) were designed so they can be ignored here and the sensor surface can be used as location. That is flange mounting distance is to that. Not quite clear what this means with modified cameras (e.g. scraping the mosaic to get black and white camera) and use of sensor-side filters... or what happens with cameras that support different mounts... I talked to a lens designer who created special nano-coatings for their fisheye lens (to reduce chromatic aberrations) and in that case that does not support lens adapter (is totally locked to a particular flange distance) so there are exceptions to that (not every lens make sense to be mounted on any cameras).

Flange Distance or Flange Focal Distance- although a slight offset (e.g. require shim) is a callibration process outside of scope here, seems useful info because with so many mounts many people end up using adapters and focal reducers. Maybe need for an optional effective something, also some cameras offer different mount options also, not quite clear it's information complete with just camera and lens model (e.g. designed for PL Mount).