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View Full Version : Single Meniscus lens - how do I determine f stop?

Jim Becia
24-Jun-2011, 08:43
Here are two photos of an inexpensive lens I bought. I have a project of sorts I would like to try using something old and "unsharp." I have figured that it focuses at infinity at about 5.5 inches. So do I measure the lens diameter to determine f stop? And seeing that the shutter is in front of the lens, do I measure the rear element? Also, the f-stops are notated as 1, 2, 3, and 4. If I can get the opening figured out, would each f-stop be one stop down? Any help is appreciated. Jim

edp
24-Jun-2011, 09:10
Here are two photos of an inexpensive lens I bought. I have a project of sorts I would like to try using something old and "unsharp." I have figured that it focuses at infinity at about 5.5 inches. So do I measure the lens diameter to determine f stop?

Measure the aperture in inches and divide by 5.5 to get a good enough approximation.

And seeing that the shutter is in front of the lens, do I measure the rear element?

The widest aperture you can get is the hole in front of the shutter. You'd get a lot more lens, and probably a lot more 'unsharpness', if you can unscrew the metal piece with the hole in from the front of the shutter and use the full diameter of the glass.

Also, the f-stops are notated as 1, 2, 3, and 4. If I can get the opening figured out, would each f-stop be one stop down?

Maybe, maybe not. Measure the size of the aperture at each setting.

Jim Galli
24-Jun-2011, 09:24
I think you'll find most of the old Kodak's to be f12 - f16 - f22 - f32

You may also want to be clever and try to remove the lens group and use it somehow without the f12 restriction. Most of these are about f6 uncorked, and to me at least, that is their most pleasant use.

Jim Becia
24-Jun-2011, 13:29
Jim and EDP,

Thanks for the responses. It gives me a starting point at least. I'm afraid that if I try to take the front housing off, I'll screw something up. I'll try it like it is for now and then see. Thanks again. Jim

Steven Tribe
24-Jun-2011, 13:49
I think you can unscrew the front plate without the shutter mechanism falling out. This must be a standard shutter - identical for a wide range of lenses with or without front glass. It may even be possible to "tweak" the iris control to make measureable higher f's. You could bore and file the removed plate to this larger aperture and screw it in again.