Something I couldn't explain

[Kyle M.]

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.

[Bill Rolph]

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.

The longer lens (with a larger diameter opening) will cast more total light than the shorter lens, but over a proportionally larger surface area. Thus the light intensity is the same for both lenses.

[Kyle M.]

The longer lens (with a larger diameter opening) will cast more total light than the shorter lens, but over a proportionally larger surface area. Thus the light intensity is the same for both lenses.

I sat and thought about it for a bit after that post and it started to make sense. But there's still a lot of stuff when it comes to lenses and their design that totally baffles me. So I generally confine myself to just using and enjoying them.

[Wayne]

I sat and thought about it for a bit after that post and it started to make sense. But there's still a lot of stuff when it comes to lenses and their design that totally baffles me. So I generally confine myself to just using and enjoying them.

That's what they are for.

[Mike in NY]

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.

Kyle, I think you've either contradicted yourself in your response to my post, or maybe you misunderstood mine or the original poster's (unless I'm missing something). When speaking about the two lenses, you stated that "technically, the apertures will have a different diameter..." But the OP described both the Rapid Rectilinear and the Petval as having the same focal length of 200mm, set at the same f stop. Based on the formula you provided, which is correct, the diameters would be the same if the two lenses were set at the same f/stop, right?

[Kyle M.]

Kyle, I think you've either contradicted yourself in your response to my post, or maybe you misunderstood mine or the original poster's (unless I'm missing something). When speaking about the two lenses, you stated that "technically, the apertures will have a different diameter..." But the OP described both the Rapid Rectilinear and the Petval as having the same focal length of 200mm, set at the same f stop. Based on the formula you provided, which is correct, the diameters would be the same if the two lenses were set at the same f/stop, right?

Yes, I was thinking the OP was talking about two different focal length lenses in which case the apertures would be different diameter but a specific aperture would still give the same exposure with both lenses.