Page 1 of 2 12 LastLast
Results 1 to 10 of 17

Thread: How to determine focal length of brass lenses from length and diameter measurements?

  1. #1

    Join Date
    Aug 2013
    Posts
    4

    How to determine focal length of brass lenses from length and diameter measurements?

    I have been researching/looking at brass lenses on ebay and have found that many give only the length and diameter measurements.
    How do I find the focal length from these measurements?

    Thanks.

  2. #2
    Mark Sawyer's Avatar
    Join Date
    Jul 2004
    Location
    Tucson, Arizona
    Posts
    5,136

    Re: How to determine focal length of brass lenses from length and diameter measuremen

    You don't. Physical size has nothing to do with focal length. The closest you could get would be if the maximum f/stop and diameter of the aperture were known. You could multiply one by the other to get the focal length. Mind you the aperture was often marked in systems other than f/stops.

    Or, if you had the lens in hand, you could just measure from the aperture to where the lens focuses an image at infinity...
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  3. #3
    Name: ______William Booth
    Join Date
    Feb 2012
    Location
    Alabama, USA
    Posts
    193

    Re: How to determine focal length of brass lenses from length and diameter measuremen

    You'll have to ask the seller (usually un-knowlegeable if they didn't give a focal length in the first place) to estimate it for you. Tell them to point the lens toward a window in a dark room and measure the distance from the middle of the lens to the wall when an in-focus image is projected.

    Other than that, the diameter of the glass will only help you determine the speed of the lens, given the focal lenth, IF you know the lens design.

    In my experience, very large diameters (over 3.5-4") are 10" FL or longer for petzvals, but I don't believe there are any surefire methods of divining this information without asking the seller to make an estimate for you.

    -William

  4. #4
    Mark Sawyer's Avatar
    Join Date
    Jul 2004
    Location
    Tucson, Arizona
    Posts
    5,136

    Re: How to determine focal length of brass lenses from length and diameter measuremen

    Quote Originally Posted by fecaleagle View Post
    Other than that, the diameter of the glass will only help you determine the speed of the lens, given the focal lenth, IF you know the lens design.
    f/stop = Aperture/Focal Length, so if you know the aperture and f/stop, you can extrapolate: FL = Aperture x f/stop. For all designs.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  5. #5
    Name: ______William Booth
    Join Date
    Feb 2012
    Location
    Alabama, USA
    Posts
    193

    Re: How to determine focal length of brass lenses from length and diameter measuremen

    Quote Originally Posted by Mark Sawyer View Post
    f/stop = Aperture/Focal Length, so if you know the aperture and f/stop, you can extrapolate: FL = Aperture x f/stop. For all designs.
    Deferring to Mark. My bad...

  6. #6
    Andrew's Avatar
    Join Date
    Jan 2008
    Location
    Sydney, Australia
    Posts
    367

    Re: How to determine focal length of brass lenses from length and diameter measuremen

    as a questimate... if you take a half inch off the barrel diameter it gives a ball park for aperture.
    Multiply x8 for a rapid rectilinear and x4 for a petzval
    then wait for the surprise of finding out what it really is after it arrives

  7. #7

    Re: How to determine focal length of brass lenses from length and diameter measuremen

    For Petzval lenses, here is a very rough guide using the lens' diameter:

    1.50-2.00 inches = 1/4 plate which typically is a 6-7 inch focal length lens
    2.00-2.75 inches = 1/2 plate which typically is a 8-9 inch focal length lens
    3.00-3.75 inches = whole plate lens which typically is a 11-12 inch focal length lens

    Dan
    Last edited by CCHarrison; 2-Sep-2013 at 05:55.

    Antique & Classic Camera Blog
    www.antiquecameras.net/blog.html

  8. #8

    Join Date
    Aug 2013
    Posts
    4

    Re: How to determine focal length of brass lenses from length and diameter measuremen

    Nice. Very helpful everyone. Def cleared up some misconceptions and reaffirmed some general observations. THANKS!
    I recently learned the trick with the window and measuring from a sharp image with a 48" lens.

  9. #9
    8x10, 5x7, 4x5, et al Leigh's Avatar
    Join Date
    Dec 2010
    Location
    Maryland, USA
    Posts
    5,028

    Re: How to determine focal length of brass lenses from length and diameter measuremen

    Quote Originally Posted by Mark Sawyer View Post
    f/stop = Aperture/Focal Length, so if you know the aperture and f/stop, you can extrapolate: FL = Aperture x f/stop. For all designs.
    However...

    Focal length is the distance from the rear node to the film plane.

    Unless you know the lens design, you don't know where the rear node is relative to the elements or surfaces.

    Also, the "aperture" diameter used in the quoted calculation is the diameter of the entrance pupil, which may
    differ substantially from the measured diameter of the diaphragm blade opening in some lens designs

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  10. #10
    Mark Sawyer's Avatar
    Join Date
    Jul 2004
    Location
    Tucson, Arizona
    Posts
    5,136

    Re: How to determine focal length of brass lenses from length and diameter measuremen

    Quote Originally Posted by Leigh View Post
    However...

    Focal length is the distance from the rear node to the film plane.

    Unless you know the lens design, you don't know where the rear node is relative to the elements or surfaces.
    Oh no, nodal points! I must confess, I'm getting to the age where "close enough is good enough", and, (barring telephoto and retro-focus lenes), the aperture location is close enough to go by. So while Leigh is quite right, I've never heard of a photographer who knew exactly where his nodal points were for a particular lens, much less use them. (And just try explaining nodal points to a seller who doesn't even know focal lengths!)

    Quote Originally Posted by Leigh View Post
    Also, the "aperture" diameter used in the quoted calculation is the diameter of the entrance pupil, which may differ substantially from the measured diameter of the diaphragm blade opening in some lens designs
    The aperture would be the diameter of the opening of the lens measured through the entrance pupil (front element). Which designs does this not hold true for?
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

Similar Threads

  1. Zoom lens focal length and back focal length relation
    By raghavsol in forum Lenses & Lens Accessories
    Replies: 7
    Last Post: 16-Mar-2011, 03:00
  2. Close Up Lenses and Focal Length
    By Fragomeni in forum Lenses & Lens Accessories
    Replies: 20
    Last Post: 15-Jan-2011, 13:31
  3. Aperture: diameter for given focal length
    By Nicholas F. Jones in forum Lenses & Lens Accessories
    Replies: 4
    Last Post: 28-Jan-2008, 02:45
  4. How to determine focal length when not marked?
    By bill zelinski in forum Lenses & Lens Accessories
    Replies: 3
    Last Post: 13-Jun-2000, 21:26

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •