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Thread: How do I calculate the F stop for this lens?

  1. #1

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    How do I calculate the F stop for this lens?

    I have just received this lens along with a Thornton Pickard shutter, and I'm not familiar with it's design. The lens appears to be a single Plano-convex lens at the rear, and just a black disk at the front. There are no markings anywhere on the lens, or any method of setting an aperture eg waterhouse stop etc.

    Does the hole in the front glassless, black disk act as an aperture? If so, I assume that it the same calculation to work out the F stop once I know the focal length?

    The lens is 4cm wide by 4cm long, and the flange is integral to a sleeve in which the lens can move backwards and forwards as a crude method of focus.

    Can anyone tell me what sort of angle of coverage this design of lens gave?

    Thanks
    Bruce

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  2. #2

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    Re: How do I calculate the F stop for this lens?

    Yes, the hole at the front is the aperture - measure that, and divide the focal length with that figure and you've got the working f stop. For example, should the focal length be 20cm, and the aperture 15mm in diameter 20 / 1.5 = f13.3.
    Peter

  3. #3
    Randy's Avatar
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    Re: How do I calculate the F stop for this lens?

    Looks similar to a Wollaston Meniscus. I use a modern version, a 190mm, on 4X5. I am guessing you should be able to calculate the f/stop as with most other lenses once you determine the F/L by dividing the F/L by the aperture diameter.

    edit: What pjd said...
    https://dl.dropboxusercontent.com/u/52893762/bigger4b.jpg

  4. #4

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    Re: How do I calculate the F stop for this lens?

    This is a basic meniscus landscape lens which was the price-right lens for the enormous boom in photography sales from 1890 onwards. This is a nice example, because the whole front aperture can be removed and the lens used for "pictorial" images.
    There was probably a set of disc/washer round apertures (with diminishing sized holes) which fitted in front giving larger depth of sharp focus. There was a compression ring which held them in place. I doubt this is completely plano/convex, unless the lens has been changed. It should be a cemented achromat, with the most convex surface facing towards the camers. These are found in both longer and shorter focal length so coverage is difficult to guess. F is approximately focal length divided by the aperture diameter.

  5. #5
    Mark Sawyer's Avatar
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    Re: How do I calculate the F stop for this lens?

    Before we get too far carried away with calling it a Wollaston meniscus,

    Quote Originally Posted by brucep View Post
    I have just received this lens along with a Thornton Pickard shutter, and I'm not familiar with it's design. The lens appears to be a single Plano-convex lens at the rear...
    The Wollaston had pronounced curvature front and rear. The English and French Landscape Lenses were also meniscus shaped, but the rear surface was only very slightly curved, easily mistaken for being planar. Sounds like it could be one of those; you'd have to look for signs of cement failure at the edges, or remove the lens from its mount to inspect the sides for a seam to be sure.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  6. #6

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    Re: How do I calculate the F stop for this lens?

    Thanks for all the info. I'll get it on a lens board this weekend and give it a try.

    Bruce

  7. #7

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    Re: How do I calculate the F stop for this lens?

    Probably a 5x7" plus coverage. 13x18cm was the popular size at the time in Europe.

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