Determine f/stop for telephoto lens

[Randy]

Am guessing this has been asked but I can't find the answer through any search.

I have a Wollensak 15" Tele Raptor with a disabled aperture (yes, I buggered it up all by myself). The aperture ring will not operate the aperture but I can remove the rear cell and with my finger, rotate the mechanism and close down the aperture somewhat.

Max aperture is f/5.6. I would like to just close it down to f/8 and leave it there since I don't know how well the lens performs wide open. Problem is - I don't know how to measure. The lens is 15" (380mm) but infinity focus is at about 10" (254mm from lens board to film plane) because it is telephoto.

With the aperture wide open, measuring the opening through the rear element it is 38mm. Through the front it is 55mm.

I just want to know how to measure the aperture opening with this lens to get to f/8, and perhaps f/11. Do I divide the diameter into 380mm or 250mm? Do I measure through the rear or front?

Or, if someone has this lens, if you could remove the rear cell, set it at f/8 and f/11 and measure the diameter of the aperture for me and reply.

[cowanw]

F8 about 40mm and f11 about 29mm on mine. I expect it should be 380/8=37mm for f8 if life were perfect.

[Randy]

Thanks a bunch Bill. I'll make a note of your figures and start playing with the lens. BTW, I knew a Bill Cowan when I was a kid in Poway, California.

[cowanw]

Hope he made a good impression.

[Mark Sawyer]

I don't know much about telephoto lenses, and I don't have one to play with, so I may be sticking my foot in my mouth here...

Telephoto designs are largely based on a strong negative element at the rear. If you turn it around so the negative element is at the front, does it even function as a telephoto any more?

Also keep in mind, the entrance pupil is now fronted by a negative element instead of a positive one, so the marked aperture values would be waaay off...

Quick, someone who knows more tell me I'm wrong! :confused:

[tih]

I just want to know how to measure the aperture opening with this lens to get to f/8, and perhaps f/11. Do I divide the diameter into 380mm or 250mm? Do I measure through the rear or front?

The aperture number is the ratio between the focal length and the apparent entry pupil. Your lens is a 380mm with a maximum aperture of f/5.6, which means that when seen from a long distance out in front of the lens (theoretically infinitely far), the aperture opening should be seen to be 380mm / 5.6 = 68mm in diameter. To see this, you might place the lens in front of a light background, tape a ruler with a mm scale across the front, and then look at it from a number of yards away - maybe using another camera with a long lens to see the scale clearly. :)

At f/8, the entry pupil should be 380mm / 8 = 48mm.

[Carsten Wolff]

since you buggered up the aperture barrel anyway, find an Alphax, or Betax #4 shutter; should screw straight in :); I have a jpeg with the matching aperture scale for it somewhere.

[Randy]

since you buggered up the aperture barrel anyway, find an Alphax, or Betax #4 shutter; should screw straight in :); I have a jpeg with the matching aperture scale for it somewhere.I have a Betax #4 and have used the lens with it, but the shutter is being used with the lens that came with it - and my use for the 15" lens will be on my Graflex Series D with a focal plane shutter.

tih - thanks for the information.