Originally posted by newarts Clearly
I'm not clear on what "at that specific point along the sensor" means; I need to think about it a bit and would appreciate an analytic reference.
your diagram is not really correct, which is why you are not understanding the point here. In your diagram the lines indicating the "total light admitted into hood" is actually the correct representation of the lens , hood and field of view. Remember the image is inverted through the lens. Now, looking at those lines, consider a smaller image circle, which you would have as yoou move alonmg the top or bottom of a frame, from the corner towards the sensor center. The lines will still cross at the same point and therefore, as the image circle reduces the length of the hood would increase, this is why the petal shape evolves.
Quote: I'm not clear on what basis optimum is meant. I think I can make a rectangular prism hood the same depth as the total depth of the petaled hood that excludes more unwanted light.
Dave
note again you mis interpret the use of "Unwanted" light, but a rectangular hood could also achieve the same but not better effectiveness, just that since lenses are round it is much easier to attach a round , but petaled hood, as opposed to a rectangular pyramid, that you need to somehow, make circular.