focal length changed for a prime lens ?

Discussion in '35mm Cameras' started by Steven Woody, Jun 3, 2006.

  1. Steven Woody

    Steven Woody Guest

    today i looked though my prime lens and was astonished. while i
    changing the focus, the subject changed its size a alittle in the
    viewfind! actually, i found that when focus changes nearest distance
    to infinite, the subject size reduces a very small scale. i am not
    sure if it was that before. is this a problem?

    my lens is a Ziess 50mm prime. thanks.

    Steven Woody, Jun 3, 2006
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  2. Steven Woody

    Alan Browne Guest

    No. All lenses exhibit this "effect" as focus is achieved by moving
    lens elements fore and aft causing a very minor change in magnification
    between the subject plane and the focus plane. (I might not be
    explaining it in the best technical manner...).

    Alan Browne, Jun 3, 2006
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  3. Steven Woody

    Mark² Guest

    This happens in lenses that focus by extending the front element. This
    means the extension is altered, which literally changes the focal length.
    This is especially noticeable in true macro lenses that extend a great
    length during close focus. For this reason, focus rails are used to achieve
    focus (a focus rail is designed to move the entire camera/lens set-up
    forward or back) without changing the focal length...and the

    Internal focusing lenses help this problem because there is no over-all
    extension change.

    If you compare the older Canon 100mm 2.8 macro lens with the newer USM
    version of the same lens, you'll find that the older one extends in this
    way, while the newer version has internal focusing. I've considered selling
    my older macro and buying the new...just to avoid this very issue.

    Mark², Jun 3, 2006
  4. Steven Woody

    Paul Furman Guest

    Ah, interesting. I have tried merging bracketed focus for macros and
    it's very tough with this effect. Not only the enlargement changes but
    the overall perspective and proportions of things.

    Also, I was doing some closeups with an (internal focus) 70-200 zoom
    with closeup lens attachment and I noticed there are a variety of ways
    of achieving the same enlargement using the zoom and focus. I want to
    study this more.
    Paul Furman, Jun 4, 2006
  5. Steven Woody

    Steven Woody Guest

    thank you!
    Steven Woody, Jun 4, 2006
  6. Except all internally focusing lenses _do_ change focal length as
    they focus. The effect is most notable at macro and near-macro

    A lens system which focuses by extension alone keeps the same
    focal length, but you can still see some change in subject
    Michael Benveniste, Jun 4, 2006
  7. Steven Woody

    Mark² Guest

    Whatever you claim...the fact is that the internal focussing 100mm 2.8 USM
    macro doesn't behave the way the older extending 100mm macro does in this
    Mark², Jun 5, 2006
  8. See:,
    which states, "The effective focal length decreases a bit
    toward closer focus..." By my calculations, at 1:1 it's about
    Michael Benveniste, Jun 5, 2006
  9. Steven Woody

    BC Guest

    The effect is called breathing, and it afflicts essentially all still
    camera lenses. It afflicts lenses whether or not their focal length
    changes slightly during focusing. In your case, the focal length is
    clearly constant for all focus distances.

    Its fairly easy to understand what happens with a lens having simple
    focusing, as in your 50mm Zeiss. As you focus to a close object you
    move the lens further from the image plane. Therefore, the field of
    view must become smaller since the film/sensor subtends a smaller
    angle. Conversely, as you go from close focus to infinity the field
    angle gets bigger, and therefore the subject size would get smaller,
    which is what you noticed.

    The effect is more pronounced in wide angle lenses.

    Breathing can be corrected in the optical design, but typically this is
    only done for cine lenses, where dynamic focusing effects can be very
    critical. Its one reason why cine lenses are so expensive.

    BC, Jun 5, 2006
  10. Steven Woody

    Bandicoot Guest

    He's not 'claiming' anything - simply stating a fact. Actually, lenses that
    focus solely by moving the whole lens, en bloc, relative to the film plane
    don't change their focal length at all, though the image size does change.
    Lenses that use internal focus pretty much always alter focal length, though
    how much varies - FL seems to _decrease_ as they are focused closer, at
    least in every case I've encountered.

    Bandicoot, Jun 6, 2006
  11. Steven Woody

    Steven Woody Guest

    very clear explaination! i fully understand. thank you BC!
    Steven Woody, Jun 6, 2006
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