The starting points in designing a lens hood for a fixed focal length lens are:
1. The position of the first nodal point of the lens relative to the lens assembly.
2. The lens’s coverage angle.
These are obviously known to the lens designer. They are also generally available in the published data for enlarger, large format, and process camera lenses. We can find the coverage angle and focal length in the maker’s published data. But camera makers didn’t usually supply nodal point data for camera lenses.
If you were resourceful, the lens could be placed onto the holding fixture of a nodal slide mounted on an optical bench with the required associated equipement so that the position of the fist nodal point could be determined. Then, the design of a lens hood would be straightforward.
For most of us, this is impractical. So, we’d have to experiment to determine an appropriate hood design.
If you know the focal length f and diagonal d of the format rectangle, then the coverage angle is approximately
θ = 2*arctan(d/2f)
For example, for the standard 35 mm format (24 mm x 36 mm), the diagonal is about 43.3 mm. For the focal lengths cited in Post #1 we have focal length, and coverage angle to the nearest degree for a conical hood:
28 mm, 75º
40 mm, 57º
85 mm, 29º
For a rectangular cross section hood, we have focal length, vertical angle, horizontal angle:
28 mm, 46º, 65º
40 mm, 33º, 48º
85 mm, 16º, 24º
It’s safe to assume that the front of the lens barrel doesn’t intrude upon the coverage angle. So, from that point forward a hood that maintains an angle of θ/2 from the lens axis won’t interfere with the lens’s view.
Although I find this interesting to discuss, the answers given in Posts 2 & 5 are the practical way to approach this.
