question about focus (which receives little scrutiny)

Discussion in '35mm Cameras and Accessories' started by David Lyga, Mar 3, 2012.

  1. David Lyga

    David Lyga Subscriber

    Messages:
    1,873
    Joined:
    Nov 25, 2007
    Location:
    Philadelphia
    Shooter:
    35mm
    I have a 'kit' Ricoh lens that is K mount but was made for use with the KR-5. Its specs are 2.2/50 and there is something different from this lens that confuses me. The rear element set (behind the aperture) is rigid, does not move when lens is being focused. Only the front element set (in front of the aperture) moves. The closest focus is about 25 inches, certainly not impressive. At infinity the lens is tack sharp, even at f2.2. At the CLOSEST distance, the viewfinder indicates that 29" is in 'focus' when, on the negative, 25" is in focus. At infinity, the viewfinder indication matches the reality on the negative. That discrepancy, alone, surprises me, as always test lenses for both infinity (I have wonderful, distant skyscrapers outside my window) and closest focus.

    The overwhelming number of prime SLR lenses are not like this, as all elements are moving as the lens is being focused (because the entire element set is housed in one set). My question is this: there has to be an advantage to the rear elements moving, right? But what is this advantage, in detail, and what is the disadvantage to the rigid rear element set that I have? Many early rangefinders lenses also had this 'fixed rear set' also. - David Lyga
     
    Last edited by a moderator: Mar 3, 2012
  2. E. von Hoegh

    E. von Hoegh Member

    Messages:
    3,925
    Joined:
    Sep 14, 2011
    Location:
    Adirondacks
    Shooter:
    Multi Format
    Focussing a lens by changing the element spacing disturbs the corrections of the lens. It sounds like you also have a screen that is not properly aligned.
     
  3. David Lyga

    David Lyga Subscriber

    Messages:
    1,873
    Joined:
    Nov 25, 2007
    Location:
    Philadelphia
    Shooter:
    35mm
    OK, I thought that the screen was properly aligned because infinity was properly matched. I will look into this. - David Lyga
     
  4. E. von Hoegh

    E. von Hoegh Member

    Messages:
    3,925
    Joined:
    Sep 14, 2011
    Location:
    Adirondacks
    Shooter:
    Multi Format
    The screen may be aligned well enough to be within the DOF at infinity, but the mis-alignment will show up at closer distances due to the restricted DOF at close up reproduction ratios. Try it with a different lens.
     
  5. ic-racer

    ic-racer Member

    Messages:
    7,473
    Joined:
    Feb 25, 2007
    Location:
    Midwest USA
    Shooter:
    Multi Format
    Your test for infinity focus is off. How did you test it?
     
  6. E. von Hoegh

    E. von Hoegh Member

    Messages:
    3,925
    Joined:
    Sep 14, 2011
    Location:
    Adirondacks
    Shooter:
    Multi Format
    Sometimes, SLR lenses will focus "beyond" infinity. But if there is a four inch discrepancy between the viewfinder and film plane at two feet, the problem is not in his infinity test.
     
  7. Rol_Lei Nut

    Rol_Lei Nut Member

    Messages:
    1,118
    Joined:
    Jul 2, 2006
    Location:
    Hamburg
    Shooter:
    Multi Format
    When a lens focuses by moving only some of its elements it's:

    1) a good thing ("floating elements"), aimed at correcting aberrations at closer distances
    or
    2) it's a cheap approach, making the lens easier to build but will probably negatively affect close up performance


    How did you actually measure the distance which gave sharpness on the negative?