Re: Correction !: ULF lens wanted-cirkuts

-----Original Message-----
From: Jamie Young <jamiehy@tds.net>
Sent: Feb 16, 2005 6:32 AM
To: alt-photo-process-l@sask.usask.ca
Subject: Re: Correction !: ULF lens wanted-cirkuts

Cirkut cameras are a lot of fun to use but are also the most
challenging cameras I've ever used too. I started out with a 8"
camera, shooting negs up to 6 feet long. and thought that was large and
hard to handle. I then bought a 10" camera which I've shot up to 8 plus
foot negs. That made the 8" by 6' negs seem small. After a while I
ended up with a 16" camera that could shoot negs up to 18' long, though
I'm only set up to do up to 10'. I made my own trays, print
frames,etc. A lot of work and fun. I like woodworking and do a (very)
little metal working as well. I've found the view that a cirkut camera
gives is unique , allowing one to choose the horizontal cropping, as
the cirkut allows view over 360 degrees, depending on film length
available. My 16" camera has a 15-24-36 t-r convertible, and being in
the midwest, I tend to use the 36" element the most. I had the 36"
element measured and found the actual focal length to be about 33", but
that's as long as the gear head allows, and only above a 60 foot focus
point.
I've done mostly van dyke prints with the larger negs, and have started
doing cyanotypes and then tea toning them. it a lot of fun. I'll try
to update my web site with cirkut stuff at some point soon, but there
are a few examples up right now.
http://www.jamieyoungphoto.com/cirkutmain.html
Jamie Young
On Feb 15, 2005, at 9:20 PM, Sandy King wrote:

   Its not too difficult to measure the actual focal lenght of a lens with simple equipment. You need a plane mirror large enough to cover the front of the lens. While a first surface mirror is ideal a good quality shaving mirror will do. You need a white card with a small hole in it and a pencil flashlight. Its even possible to do this with out the card.
    First step is to focus the lens exactly at infinity by autocollomating it. This is done by placing the mirror over the front. Then put the card in back of the lens and the flashlight behind the small hole. Focus the lens until the reflected image of the light is on the card, near but not on, the hole. The lens is now at infinity focus. On a view camera you can get close enough by just putting the end of the flashlight against the ground glass and focusing its image, this again places the lens at infinity focus.
    The second step is to set the camera up for exactly unity magnification. This is best done by focusing on a small ruler and having a similer ruler taped onto the ground glass. The movement of the lens from the infinity position to the 1:1 position is exactly one focal length. The distance from the image to the object at 1:1 is also exactly four focal lengths but is harder to measure accurately than is focus displacement.
     Since you now know the focal length and have means of focusing exactly at infinity you can also determine the locations of the principal points. By definition they are exactly one focal length from the focal plane at infinity focus to the lens. Turn the lens around to find the first principal point.
     You can further measure the locations of the pupils. You ned another camera for this. The pupils are defined as the images of the stop as seen from outside the lens. If you have a single cell behind the stop the pupil on that side will be the actual location of the stop and its size is the actual stop size. To find the pupil location with a lens in place first focus a camera that can focus at a very close distance on some convenient place, say the rim of the lens cell. Now, without changing the camera's focus adjustment move it so that the image of the iris is in focus. The distance the camera had to be moved is the distance from the reference point to the pupil on that side of the lens.
   To measure the f/stop use the card and light described above. Using the mirror-autocollimator focus the lens exactly at infinity. Now place a tanslucent screen (thin paper works fine) over the front of the lens. A circle of light will be projected onto it from the small light source. The diameter of this circle is the diameter of the entrance pupil, which is the value to use in calculating the f stop from the focal length.

--
Richard Knoppow
dickburk@ix.netcom.com
Los Angeles, CA, USA