Re: figuring out f-stop on old brass barrel lens

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From: Robert Newcomb (Robert.Newcomb@gactr.uga.edu)
Date: 03/11/03-02:52:14 PM Z


To fine the f-stop number you take the focal length of the lens and divide by
the size of the opening in the lens. To give anything more then a rough idea,
you'll need accurate focal length number and a good measure of the diameter of
the opening where the stops are. From the photo you can see that this lens
uses the slide in waterhouse type stops, I would measure the diameter of the
lens at this point. Hope this helps!
Robert N.

Christopher Lovenguth wrote:

> Hi all, it's been awhile since I've thought about this and can't remember
> how to do it. I'm looking at buying an old barrel lens that doesn't have any
> markings on it and I can't get to it to see it in person. I've seen pictures
> of it and it's very very very low in price so I'm willing to take the chance
> of buying it without seeing it. However I want to try to figure out what the
> lens f-stop is first.
>
> The person selling it thinks it's a 16inch lens that is 4 1/2 inches long
> and 3 inches diameter across the front.
>
> With that info is it possible to estimate an f-stop or do I need the
> diameter of the back of the lens as well?
>
> Have any of you seen old brass barrel lenses that were higher then F8
> anyway? I would prefer something 5.6 of under because I'd be using this for
> making daguerreotypes. Anything over F8 forget about it, we're talking 30min
> to an hour in full sun to get an image.
>
> Here is a link to a picture of the lens, highly unlikely since it's only a
> side view but maybe someone will recognize it. Plus does anyone know what
> that slit is for? Is this one of those lenses that you used to drop in an
> aperture and if so can it be used without them?
>
> http://www.chrisportfolio.com/upload/lens/
>
> it's the barrel lens.jpg on that page
>
> Thanks in advance,
>
> Chris
>
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