Quote Originally Posted by Mike in NY View Post
Since your son said, "assuming they are set at the same f stop," did you assume the opening for an f/8 Waterhouse stop for a Petzval lens would be larger than the opening of an iris stopped down to f/8 on the small RR lens? That wouldn't be the case. To build on what Emmanuel said, the reason the Petzval lens is so much bigger than the RR lens is because its maximum aperture is so much larger. The two lenses "start" with different apertures. If you built an RR lens with the same maximum aperture as the Petzval, it would become a much bigger lens.
Technically the apertures will have a different diameter, but aperture is aperture regardless of lens focal length. Aperture diameter is focal length divided by the f stop number. So f8 on say a 240mm lens would be a 30mm opening, on a 24mm lens it would be a 3mm opening. Different size iris opening but same exposure. As to how that all works I can't explain it.