1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Range of movement of a lens?

  1. Apr 25, 2012 #1
    1. The problem statement, all variables and given/known data

    In a 35 mm single lens reflex camera (SLR) the distance from the lens to the film is varied in order to focus on objects at varying distances. Over what range must a lens of 45 mm focal length vary if the camera is to be able to focus on objects ranging in distance from infinity down to 1.4 m from the camera?

    2. Relevant equations

    1/f = 1/p +1/q

    3. The attempt at a solution

    First, the thin-lens formula is applied for the case of focusing on objects at a distance of infinity:

    1/45 = 1/∞ + 1/q
    q = 45 mm

    Then, the same formula is applied for the case of focusing on objects at a distance of 1.4 mm:

    1/45 = 1/1400 + 1/q
    q = 46.49 mm

    Therefore, the range of movement is 46.49 - 45 = 1.49 mm.

    I actually had help from a tutor with solving this problem. I understand the problem itself, but what I don't understand is how q = 45 was derived from the equation 1/45 = 1/∞ + 1/q since the equation contains ∞ as a variable.

    Generally speaking, how do you solve equations with infinity (∞) as one of the variables? I am really stumped...
  2. jcsd
  3. Apr 25, 2012 #2


    User Avatar

    Staff: Mentor

    1/∞ is a very tiny number, called zero. :smile:

    So 1/p = 1/q, giving p=q
  4. Apr 25, 2012 #3
    OH... so 1/infinity basically just cancels out, then? Duh... thanks!
  5. Apr 26, 2012 #4


    User Avatar

    Staff: Mentor

    I wouldn't use the phrase "cancels out". mVVtk.gif  Perhaps "fades away", "disappears", or "vanishes", would be appropriate though.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook