Effective Aperture of a Petzval

[Shangheye]

I have an unmarked Petzval of approximately 320mm focal length. The iris diaphragm the blades of which which has been removed has markings that start at f2 and then work their way up at 4, 8, 16 etc up to 64. The actual measured opening is 75mm inside the lens....by the usual standard of dividing the focal length b the aperture this is not an f2. However from what I have read in the literature, this simple sum only a applies to simple lens designs and not to Petzval lens designs where the front element has large condensing power. for such complex lens designs the effective aperture is larger than the actual aperture, in some cases up to one or two stops at the wide end of the iris opening.

I guess my question is, is there any reason to mistrust the markings on the lens that look original, even if the lens is of an unmarked brand? F2 seems damn fast

As an additional point, when I received the lens the rear meniscus (concave/convex lens in the rear group....not the rear most Plano convex one) was the wrong way around with the concave side facing the front of the lens....given this would spread light outwards towards the edges of the ground glass from what I understand of lens design would this result in a different effective aperture than the original design...a kind of loss in condensing power?

Appreciate any night you may have.

Rgds, Kal

 

[Jim Jones]

Several systems of labeling lens apertures have been used over the years. Yours might be the old U.S. system:
U.S. f/
2 5.6
4 8
8 11
16 16
32 22
64 32
128 45
256 64

You can verify this by measuring the diameter of the aperture as seen through the front elements of the lens and calculating the actual f/number. Your eye should be some distance from the lens when measuring to reduce parallax errors.

 

[Shangheye]

Thanks Jim...since the lens came from the US that would be consistent. Do you have any idea when that nomenclature went out of fashion? It would help date the lens.

Rgds, Kal

 

[Ian C]

About 70% into this article is a good account of the old US aperture system:1, 2, 4, 8, 16,…and how it relates to the current system. It includes a useful table relating the various aperture numbering systems.

http://en.wikipedia.org/wiki/F-number

 

[Shangheye]

That"s a great link Ian. It does also refer to the issue of working vs measured aperture under the Dallmeyer section. I have measured it as per Jim's way, since the Dallmeyer route was cumbersome and the difference is probably minimal. With the blades removed the lens clearly has a greater working aperture...I can not see the aperture retaining ring (so maybe a tad wider than the front element) On that basis, with the focal length and using the front objective diameter as the guide (the aperture is at least as wide as this) this is now an f4.2 lens or so with the iris removed.

Rgds, Kal