Where is the error in this explanation of light collection/etendue?

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SKersting66
SKersting66 Contributing Member • Posts: 708
Where is the error in this explanation of light collection/etendue?

I have tried to create a simplified graphical explanation of how everything works by creating two diagrams which illustrate the difference in etendue/light collection between two different focal lengths. These diagrams are essentially irrespective of the lens' aperture and camera's sensor area.

This first image shows a shorter FL/wider FOV lens, and a source with a given angle of projection (FOV). Where the two FOVs intersect is the point at which light collection begins. The dashed lines represent the angular extent (relative size) of the source w/in the FOV.

This second image shows a longer FL lens with the same source, the lens' FOV intersects the source FOV, and light collection begins, closer to the source. The source's relative size increases as does the qtty/density of light from that source area; exactly the same as moving physically closer w/ the shorter FL would change things.

Up to this point I have ignored the aperture/entrance pupil factor in regards to the lens' etendue/light gathering capability, with the understanding that aperture is a constant of exposure. I.e. all lenses at f/1 are transmitting ~ 20% of the light available. The primary factor of concern is the point at which light collection begins and the resulting change in angular extent/available light/light density. 
However, the role of aperture is to manage/restrict light density at the focal plane. And in order to do that the aperture increases/decreases the lens' etendue and the qtty of light collected/transmitted by increasing/decreasing in diameter. The blue dashed lines in the second image represent the physical limit of the possible lens aperture/entrance pupil, and the point at which the light available from the source would become the limiting factor. I.e. a larger aperture would be of no benefit.
The only thing that really bothers me about this explanation/understanding is that it basically (roughly) has the ISL influencing the light from the point of origin to where it intersects the lens' FOV, and from that point the light is being described more as a flat wave front. I have no reason/explanation for that change; I do not see why the light would "bend" at the point where the FOVs intersect. Yet it does seem to exactly match/describe the way in which everything works together.

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