Ernie: I respect you and your opinion very much, and I hope you know that my own postings that may be contrary to yours are in the area of honest debate and in no way personal. I am not trying to pick a fight with you. 🙄
It seems we have much in common in this issue...
As I understand it we both agree that the terminology creates a much more confusing, and frankly, that creates most of the issues with where we are coming from, and expressing our positions. We need a terminology that precisely expresses the difference between the physical focal length of a lens - which (I think we agree) remains a physical constant no matter what body they may be attached to.
The issue, in the end, is what we capture from the result of combining a lens of a particular focal length with a sensor of a particular size. The fact is that that this varies. If the lens is a constant, the smaller sensor captures a smaller area of what is delivered by the lens.
So, to try to create as distinct a dialogue as possible, I use the following terminology.
Focal Length (FL): Measured in mm and printed on the lens. The distance from the optical centre of the lens to its centre of focus - that is a constant physical attribute for a given FL and does not change. Lens FL metrics are linked to the model that it is projecting onto a 24x36mm surface, given that is what became a de facto standard in camera production in the latter part of the last century.
Angle of View (AoV): measured in degrees, the cone of light transmission generated from the lens as it projects and image towards the sensor plane. That too does not change if the focal length of the lens remains the same.
Sensor Size: usually measured in mm x mm of a rectangular shaped flat plane, but often referred to by its diagonal relationship to that of the 24x36mm film that became a de facto standard during the second half of the 20th century - associated with the rise of 35mm rangefinder and SLR cameras that dominated the market in those decades.
Full-Frame : a colloquial term, that has become a de facto standard, commonly used by manufacturers and other professionals within the industry referring to a film format and now digital sensor with dimensions of 24mm x 36mm.
Crop: Another colloquial term, again used commonly in industrial nomenclature that refers to any film or digital sensor format that is smaller than the so-called FF sensor.
Medium and Large format: Blanket terms for extremely large recording media including such formats as large plate or view cameras, 6x4 etc. Basically anything bigger than FF format.
I submit the following taken from the web as an example of the common use of the names FF and Medium format.
Angle of View (AoV): The cone of transmitted light delivering the image projected via the lens. Being an angle, it is measured in degrees of arc (not mm) and is directly tied to the FL of the lens.
Field of View (FoV) or Field of Capture (FoC): (I am preferring FoC these days as it is not used in other contexts and is a more descriptive term for what is occurring). The area that is recorded by the medium (film or sensor) from the cone of light that is projected by the lens.
Usually such medium is rectangular, so it fails to display or record the full amount of the image projected by the lens' AoV that is not within the rectangle of capture - essentially, it crops the image projected from the lens. If one DID have a sensor that was completely round and was able to record the whole of the projected image there would be no crop, but that is not the case for commercially-available cameras. The degree of crop varies with the size of the recording medium.
In the image below this is demonstrated as the identical circles are the projected image from the lens AoV. However the relationship with the sensors shows how they fail to capture (i.e. crop) that projected image.
Crop Factor: The relationship between the diagonal dimensions of a recording medium, relative to that of a FF (24 x 36mm) medium, as shown on the table above. It is a constant for a sensor size that calculates how much the diagonal of a sensor would need to be multiplied by to achieve the same size as the FF medium. Noting that medium and large format cameras will have a value < 1. This is used to as a lineal metric to establish an Equivalent value for FoC, and is squared when calculating the number of pixels captured by a sensor relative to a FF sensor.
Equivalence: A means to quantify the FoC with the lens FL. Given the FoC is the result of this chain of elements: FL--- AoV---Sensor or Film SIze --- FoC. The problem is that what is captured by the lens has been erroneously directly linked to the FL, thus causing major terminology issues. The resolution of this is to multiply the actual FL by the Crop Factor to establish a means to express FoC in FL terms as an Equivalent FL. THIS is a major cause of confusion as already mentioned.
I have outlined all of this and demonstrated the physics behind it in the article I wrote, for which I provided the link HERE . As to the usefulness of the concept, I added at the end of the article several examples of where addressing the issue of Equivalence is relevant. Certainly, for many people, especially those starting out, it may not be necessary to know this, but when one is buying lenses for different camera formats, it is useful because it is referred to in specs and in reviews and can have an influence on a specific lens choice.
So, I don't think that the discussion of these issues is irrelevant or unnecessary, it depends upon how the terminology is defined and how it is applied - and that is certainly not consistent - hence my effort in trying to be as precise as possible.
