Focal length of lenses

Discussion in 'Digital Cameras' started by Gerrit 't Hart, Jun 1, 2006.

  1. Seeing as the focal length of a lens doesn't seem to mean as much anymore
    what with different size sensors etc, wouldn't it be more sensible when
    selling and discussing lenses to use the angle of view?

    Or am I missing part of the argument?

    Gerrit
     
    Gerrit 't Hart, Jun 1, 2006
    #1
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  2. Gerrit 't Hart

    Charles Guest


    Familiarity. My 70-300 lens on a Canon 300D, how many steradians,
    off-hand?
     
    Charles, Jun 1, 2006
    #2
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  3. Given that the one lens can be mounted on cameras of at least 3
    different sensor sizes (Canon EF, 1x, 1.3x, 1.6x), I'd reach the
    opposite conclusion -- that we *can't* talk about the field of view,
    and *must* fall back on the actual lens focal length to avoid
    confusion.
     
    David Dyer-Bennet, Jun 1, 2006
    #3
  4. Gerrit 't Hart

    RW+/- Guest

    You need both or you cannot talk sense. One lens may be better with certain
    sensors than others. Because of the circle of light fast Fstops just might
    not be feasible with various sensors.

    Part of why at this stage dSLR's suck. Not saying that great pics cannot be
    had, just that choices in purchasing components with various bodies are
    best determined in actual usage, and partially via word of mouth.
     
    RW+/-, Jun 1, 2006
    #4
  5. Gerrit 't Hart

    Matt Ion Guest

    Yes: focal length has always meant the same thing. 35mm film users have
    simply misused it for too many years as an equivalent for various other
    measurements, such as field-of-view, magnification factor, etc.

    Focal length has only EVER been relevant to FOV when you're talking the
    same size of film - a 100mm lens will provide a different FOV on a field
    camera than on a medium-format camera than on a 35mm camera than on an
    APS camera.

    The only reason it's become a big deal with digital cameras is because
    of greater variations in sensor sizes vs. the common film formats.
     
    Matt Ion, Jun 1, 2006
    #5
  6. Quite so.
     
    Måns Rullgård, Jun 1, 2006
    #6
  7. Gerrit 't Hart

    John Bean Guest

    Depth of field is a judgement, not an absolute, and a
    smaller sensor will use a smaller CoC in the assessment.
    Other than that a lens is a lens, and has a focal length. It
    doesn't know what size of sensor is behind it, so how could
    the sensor possibly have any effect on it?

    Note I said it has a focal length, not a "equivalent" focal
    length, which is meaningless if you need to assess DoF.
     
    John Bean, Jun 1, 2006
    #7
  8. Gerrit 't Hart

    Don Stauffer Guest

    Yes, it would be more sensible, but there is a tradition here. It is
    like switching to metric. Most people are comfortable the way it is
    done now, and don't want to change.

    It is one of those things like relative aperture (f/#). There were
    attempts through the years to define a new exposure-related parameter
    that would be the reciprocal of the f/#. Larger aperture, bigger
    number. That never flew. It would be easier in many ways, and there is
    no good technical reason why we use FL/D instead of D/FL
     
    Don Stauffer, Jun 1, 2006
    #8
  9. This is true, but when computed for equal enlarged circles of
    confusion, the smaller-sensor system shows more depth of field.
     
    David Dyer-Bennet, Jun 1, 2006
    #9
  10. Well, wouldn't the FOV be different with different sensor sizes?

    I'm comfortable with focal length and I when I think of focal length
    I'm NOT thinking of the FOV that a 35mm film camera would achieve...I
    never was involved with 35mm SLRs and had a point and shoot before
    moving to digital so no baggage there!
     
    Kulvinder Singh Matharu, Jun 1, 2006
    #10
  11. Gerrit 't Hart

    Wayne Guest


    True only if you assume using a very short focal length lens to compensate the
    view size for the small sensor. Meaning, the DOF comes from the short lens,
    NOT from the sensor size.

    If you use the same one lens on both cameras (speaking of DSLR), the smaller
    sensor will obviously have less DOF when enlarged to equal CoC.

    APC sensors have less DOF (very roughly one stop less DOF, but maybe even
    less) than full size, when using the SAME lens and same aperture and same
    focus distance, etc. The image projected on the sensor is exactly the same
    of course, except for cropping, but the smaller sensor must be enlarged more.
     
    Wayne, Jun 1, 2006
    #11
  12. Yes, of course.
    Yes, of course.

    But in actual use, one selects lenses for angle of view.
     
    David Dyer-Bennet, Jun 1, 2006
    #12
  13. Yes, it does. The simple rule is that if the compact camera's sensor is
    1/N the size of another camera's sensor, it gives N times as much depth
    of field *provided you choose lenses for equal field of view, use the
    same f/number, and print the images to the same size*.

    The more complex explanation: if you reduce the sensor size, you reduce
    the lens focal length by a proportional amount to keep the FOV the same.
    Depth of field is proportional to the inverse *square* of the focal
    length with a constant circle of confusion. But the smaller image has
    to be enlarged more for an equal-sized print, so the circle of confusion
    *on the sensor* also needs to be reduced in size to keep the circle of
    confusion *on the print* the same size. And that reduces the depth of
    field. The overall effect is still an increase in DOF, but by a factor
    of N rather than N^2.

    Other people sometimes quote other results, but they are assuming
    different conditions (like keeping the focal length the same, and
    letting FOV change, or keeping the absolute aperture diameter the same
    and letting f/number change).

    Dave
     
    Dave Martindale, Jun 3, 2006
    #13
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