Focal length of lenses

Discussion in 'Digital Photography' 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

    On Thu, 1 Jun 2006 09:55:25 +0800, "Gerrit 't Hart" <>
    wrote:

    >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
    >



    Familiarity. My 70-300 lens on a Canon 300D, how many steradians,
    off-hand?
    Charles, Jun 1, 2006
    #2
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  3. Gerrit 't Hart

    Bryan Olson Guest

    Gerrit 't Hart wrote:
    > 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?


    Yes, definitely.

    Though I'd add that focal length really does mean something; the
    sad part is that it doesn't mean what most of the time we want
    to say.


    > Or am I missing part of the argument?


    Inertia.


    --
    --Bryan
    Bryan Olson, Jun 1, 2006
    #3
  4. "Gerrit 't Hart" <> writes:

    > 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?


    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, <mailto:>, <http://www.dd-b.net/dd-b/>
    RKBA: <http://www.dd-b.net/carry/>
    Pics: <http://dd-b.lighthunters.net/> <http://www.dd-b.net/dd-b/SnapshotAlbum/>
    Dragaera/Steven Brust: <http://dragaera.info/>
    David Dyer-Bennet, Jun 1, 2006
    #4
  5. Gerrit 't Hart

    RW+/- Guest

    On 01 Jun 2006 00:02:34 -0500, David Dyer-Bennet wrote:

    > "Gerrit 't Hart" <> writes:
    >
    >> 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?

    >
    > 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.


    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
    #5
  6. Gerrit 't Hart

    Matt Ion Guest

    Gerrit 't Hart wrote:
    > 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?


    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
    #6
  7. Gerrit 't Hart

    Bucky Guest

    Matt Ion wrote:
    > 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.


    Does focal length affect depth of field? For example, a 35mm film
    camera and a digital compact camera could have the same angle of view
    (i.e. "equivalent focal length"). Is it correct that the digital
    compact would have a greater depth of field since its true focal length
    is much smaller?
    Bucky, Jun 1, 2006
    #7
  8. "Bucky" <> writes:

    > Matt Ion wrote:
    >> 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.

    >
    > Does focal length affect depth of field? For example, a 35mm film
    > camera and a digital compact camera could have the same angle of view
    > (i.e. "equivalent focal length"). Is it correct that the digital
    > compact would have a greater depth of field since its true focal length
    > is much smaller?


    Quite so.

    --
    Måns Rullgård
    =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?=, Jun 1, 2006
    #8
  9. Gerrit 't Hart

    John Bean Guest

    On 1 Jun 2006 00:43:06 -0700, "Bucky" <>
    wrote:

    >Matt Ion wrote:
    >> 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.

    >
    >Does focal length affect depth of field? For example, a 35mm film
    >camera and a digital compact camera could have the same angle of view
    >(i.e. "equivalent focal length"). Is it correct that the digital
    >compact would have a greater depth of field since its true focal length
    >is much smaller?


    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
    John Bean, Jun 1, 2006
    #9
  10. Gerrit 't Hart

    Don Stauffer Guest

    Gerrit 't Hart wrote:
    > 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
    >
    >

    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
    #10
  11. John Bean <> writes:

    > On 1 Jun 2006 00:43:06 -0700, "Bucky" <>
    > wrote:
    >
    > >Matt Ion wrote:
    > >> 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.

    > >
    > >Does focal length affect depth of field? For example, a 35mm film
    > >camera and a digital compact camera could have the same angle of view
    > >(i.e. "equivalent focal length"). Is it correct that the digital
    > >compact would have a greater depth of field since its true focal length
    > >is much smaller?

    >
    > 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?


    This is true, but when computed for equal enlarged circles of
    confusion, the smaller-sensor system shows more depth of field.
    --
    David Dyer-Bennet, <mailto:>, <http://www.dd-b.net/dd-b/>
    RKBA: <http://www.dd-b.net/carry/>
    Pics: <http://dd-b.lighthunters.net/> <http://www.dd-b.net/dd-b/SnapshotAlbum/>
    Dragaera/Steven Brust: <http://dragaera.info/>
    David Dyer-Bennet, Jun 1, 2006
    #11
  12. On Thu, 1 Jun 2006 09:55:25 +0800, "Gerrit 't Hart" <>
    wrote:

    >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?


    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

    Website : www.metalvortex.com
    Contact : www.metalvortex.com/contact/
    Kulvinder Singh Matharu, Jun 1, 2006
    #12
  13. Gerrit 't Hart

    Wayne Guest

    In article <-b.net>, says...

    >This is true, but when computed for equal enlarged circles of
    >confusion, the smaller-sensor system shows more depth of field.



    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
    #13
  14. Gerrit 't Hart

    Jim Guest

    "Bucky" <> wrote in message
    news:...
    > Matt Ion wrote:
    >> 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.

    >
    > Does focal length affect depth of field? For example, a 35mm film
    > camera and a digital compact camera could have the same angle of view
    > (i.e. "equivalent focal length"). Is it correct that the digital
    > compact would have a greater depth of field since its true focal length
    > is much smaller?
    >

    Its "true focal length" is the same no matter what the sensor size may be.
    What might be different for the various sensor sizes though is the circle of
    confusion.
    Jim
    Jim, Jun 1, 2006
    #14
  15. Gerrit 't Hart

    Bucky Guest

    Jim wrote:
    > Its "true focal length" is the same no matter what the sensor size may be.
    > What might be different for the various sensor sizes though is the circle of
    > confusion.


    I feel like I'm in a circle of confusion. =) This is what I've read,
    and it seems to indicate that the sensor size is going to affect the
    focal length of lens required to produce the same FOV. Therefore sensor
    size also affects DOF (assuming you have the same FOV).

    "Digital compact cameras are fitted with lenses with short focal
    lengths to create 35mm equivalent field of views on their small sensor
    surfaces... Because of the very small focal lengths used, the depth of
    field is much larger than digital SLRs or 35mm film cameras with the
    same field of view."
    http://www.dpreview.com/learn/?/Glossary/Optical/Focal_Length_Multiplier_01.htm
    Bucky, Jun 1, 2006
    #15
  16. Gerrit 't Hart

    Mxsmanic Guest

    Gerrit 't Hart writes:

    > 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?


    It would, but it would require quite a change in mindset for many
    photographers. (Actually, it has always made more sense to speak in
    terms of angle of view, but for whatever reason it has not been done
    much traditionally.)

    --
    Transpose mxsmanic and gmail to reach me by e-mail.
    Mxsmanic, Jun 1, 2006
    #16
  17. Gerrit 't Hart

    Mxsmanic Guest

    Bucky writes:

    > Does focal length affect depth of field?


    Yes. All else being equal, increasing focal length diminishes
    apparent depth of field.

    > For example, a 35mm film
    > camera and a digital compact camera could have the same angle of view
    > (i.e. "equivalent focal length"). Is it correct that the digital
    > compact would have a greater depth of field since its true focal length
    > is much smaller?


    Depth of field is a subjective judgement, so it's hard to say.

    --
    Transpose mxsmanic and gmail to reach me by e-mail.
    Mxsmanic, Jun 1, 2006
    #17
  18. Gerrit 't Hart

    Bucky Guest

    Mxsmanic wrote:
    > Depth of field is a subjective judgement, so it's hard to say.


    I think whether something is in focus or not is subjective, but
    relative sharpness is not subjective. If you take 2 pictures with
    different apertures, is the relative DOF subjective?
    Bucky, Jun 1, 2006
    #18
  19. Wayne <> writes:

    > In article <-b.net>, says...
    >
    > >This is true, but when computed for equal enlarged circles of
    > >confusion, the smaller-sensor system shows more depth of field.

    >
    >
    > 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.


    Yes, of course.

    > 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.


    Yes, of course.

    But in actual use, one selects lenses for angle of view.
    --
    David Dyer-Bennet, <mailto:>, <http://www.dd-b.net/dd-b/>
    RKBA: <http://www.dd-b.net/carry/>
    Pics: <http://dd-b.lighthunters.net/> <http://www.dd-b.net/dd-b/SnapshotAlbum/>
    Dragaera/Steven Brust: <http://dragaera.info/>
    David Dyer-Bennet, Jun 1, 2006
    #19
  20. Gerrit 't Hart

    Mxsmanic Guest

    Bucky writes:

    > I think whether something is in focus or not is subjective, but
    > relative sharpness is not subjective. If you take 2 pictures with
    > different apertures, is the relative DOF subjective?


    No, it can be objectively measured.

    --
    Transpose mxsmanic and gmail to reach me by e-mail.
    Mxsmanic, Jun 2, 2006
    #20
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