From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 03/11/03-06:04:24 PM Z
----- Original Message -----
From: "Christopher Lovenguth" <zantzant@hotmail.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Tuesday, March 11, 2003 12:19 PM
Subject: figuring out f-stop on old brass barrel lens
> 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
>
The lens takes what are called Waterhouse stops. You can
make them from brass shim stock or even from heavy Kraft
paper.
Here is how to determine the focal length and size of the
entrance pupil of any lens.
First, determine the focal length by autocollimating. This
requires a mirror capable of covering the front of the lens.
It can be done using a view camera. A first surface mirror
is ideal but a standard makeup or shaving mirror will do.
Just be sure to use the plane side, not the magnifying side.
Place the mirror over the lens. Distance does not matter
but the mirror should be parallel.
Place a small light, a pencil flashlight is ideal, against
the ground glass, near but quite at the center. The mirror
will reflect an image of the light back to the ground glass.
For easier focusing you can draw a small cross on the ground
side with a pencil (will come off again). Focus the image as
sharply as you can. This focuses the lens _exactly_ at
infinity. Mark this distance at some convenient point on the
camera.
Now, set the camera up for an exact 1:1 image of some
object. A small ruler is handy since you can tape a similar
one to the ground glass to match it. When adjusted for 1:1
the diference between the focus point and the infinity focus
point will be _exactly_ the focal length of the lens.
To measure the _effective_ size of the stop set up a
pinhole source at the exact infinity focal plane (which you
determined in the first measurement). Place a sheet of some
translucent material over the front of the lens, and record
the diameter of the circle of light. This is the effective
size of the aperture. Devide this into the focal length to
get the f number. The size of the effective aperture may not
be much different than the physical aperture in many lenses
but the difference will be significant for some. This method
takes into account the magnification of the stop by the
lens.
If you make a test stop of known size you can determine
the difference between the physical size and effective size
and use that to calculate the size needed by the actual
stops.
Since you now know the focal length you can determine the
locations of the principle points by focusing the lens
exactly on infinity and measuring back one focal length
toward the lens from the focal plane. That is the location
of the principle point for that end of the lens. When the
lens is in its "normal" position this is the second or rear
principle point. To get the first or front principle point
turn the lens around and refocus it. Knowing the principle
points is sometimes useful.
The internal structure of lenses which are not too
complicated can often be determined by shining a small light
into each end of the lens and counting the reflections.
Easier if you can examine the cells separately. Glass/air
surfaces, even when coated, are bright, cemented surfaces
are dim. Sometimes its possible to determine the power, or
rather the sign, of the surface this way, but it can be
tricky.
My guess is that this is probably a Petzval portrait lens.
--- Richard Knoppow Los Angeles, CA, USA dickburk@ix.netcom.com
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