Scals of Zeiss Protar aperture

[Cheng Lu]

This is a Zeiss Protar VII trible convertable lense. Its aperture scale is 5, 10 , 15, 20, 25, 30mm. Who knows how to use it, and how to convert to normal aperture like 5.6, 8, 16, 22, 32?

Thanks.

Cheng Lu

[Sean Billy Bob Boy yates]

Mr. Lu,

The Protar aperture scale is calibrated in MM because of the number of cells (and therefore focal length combinations) available. To calculate the aperture, you must know the focal length of the cell you are using when focused at infinity. I suggest you get in contact with Steven Grimes, www.skgrimes.com. or Ron Wisner www.wisner.com. I'm sure there is a formula to convert mm to aperture but I don't know it off the top of my head.

[Franco Rallo]

Dear Cheng Lu, I have and use the same lens; the way to calculate the "normal" aperture values is very simple, if you follow this procedure: 1) With the lens combination you prefere mounted on the camera, focus the subject. 2) Mesure the distance, in mm, between the film plane and the aperture ring on the lens (wich is placed in the optical center of the compound lens, but is not very different when you use only one element). 3) Divide this distance by the "normal aperture value" you want to apply. 4) set the aperture ring at this value. For example, if after focusing the distance between film plane and aperture ring is 210 mm, and you want to shoot at "f/11" you calculate 210/11 = 19.1 So you set the aperture ring at 19 mm and this will correspond to "f/22". Conversely, if you find that the right aperture for your picture, looking in the ground glass, is for example 22 mm in the lens, this correspond to 210/22 = about "f/9.6" . In fact, ther numbers you read on the aperture ring of the Protar represent the diameter of the iris in mm, and the "f/numbers"' are the ratio between the distance "optical center of the lens/focus plane" and the diameter of the hole. The method I suggested works with the subject at every distance, either infinity or close up. If you use only one element and you want to be more precise, mesure the distance between the film plane and the center of the optical group. I hope having been clear and useful to you. Regards, Franco

[Franco Rallo]

Dear friends, in my previous answer I typed wrongly a number in the first numerical example. I wrote: "... you set the aperture ring at 19 mm and this will correspond to normal aperture f/22." The last number is wrong, and shold be read as:"...and this will correspond to normal aperture f/11". I apologize for the mistake. Franco

[Sean Billy Bob Boy yates]

Sheesh, where were my brain cells when I posted that? f/8 = focal length/8 So, a 12"/8 = 304.8mm/8 = 38.1mm. So, knowing the effective focal length you could calculate the aperture you need as Franco so abley pointed out. Someone hand me a towel so that I may wipe the yolk from my eyes.