looking for the formula

Discussion in 'Exposure Discussion' started by CollinB, Oct 25, 2011.

  1. CollinB

    CollinB Subscriber

    Messages:
    661
    Joined:
    Aug 2, 2007
    Location:
    Ohio, USA
    Shooter:
    Large Format
    When I look on any lens or camera there is a focus scale that is logarithmic.
    How is that scale scaled -- how is it arrived at for each focal length?
    What I am wanting to do is consolidate all of my LF formulas into a single application --
    lens f/l + focus distance + exposure compensation.
    It will make a useful Android app.
     
  2. Ian C

    Ian C Member

    Messages:
    722
    Joined:
    Feb 8, 2009
    Shooter:
    Large Format
  3. Ian C

    Ian C Member

    Messages:
    722
    Joined:
    Feb 8, 2009
    Shooter:
    Large Format
    If you’re interested in the distance the lens must be extended Δ forward its infinity position to focus at a lens-to-subject distance s,

    Δ = sf/(s - f) – f

    Example: f = 90mm, s = 3 meters = 3000mm

    Δ = 3000*90/(3000 – 90) – 90 = 2.78mm

    You’d extend the lens 2.78mm forward of its infinity position to focus lens-to-subject distance s = 3 meters.
     
  4. CollinB

    CollinB Subscriber

    Messages:
    661
    Joined:
    Aug 2, 2007
    Location:
    Ohio, USA
    Shooter:
    Large Format
    Thanks!
     
  5. polyglot

    polyglot Member

    Messages:
    3,470
    Joined:
    Jun 12, 2009
    Location:
    South Austra
    Shooter:
    Medium Format
    Keep in mind that that formula applies only to lenses that are not telephoto or retrofocus, in which cases different extensions are required to achieve a given focus. Fine for most LF lenses though.