So, why *do* mirror lenses have such narrow DOF?

Discussion in 'Digital Photography' started by Paul Ciszek, Jul 14, 2012.

  1. Paul Ciszek

    Paul Ciszek Guest

    The two big complaints I hear about mirror lenses are:

    1) The depth-of-field is frustratingly narrow

    2) The f number is too high

    Those two would seem to conflict with each other. Although, since the
    light-accepting orifice of a mirror lens is a ring rather than a circle,
    it has a larger diameter than it should for the amount of light gathering
    area that it has. Is it a matter of "This thing focuses like a f/5 but
    exposes like a f/8"?

    Or is there something funky about the Maksutov-Cassegrain optics that
    makes it follow completely different rules than lens-based optics do?

    --
    Please reply to: | "We establish no religion in this country, we
    pciszek at panix dot com | command no worship, we mandate no belief, nor
    Autoreply is disabled | will we ever. Church and state are, and must
    | remain, separate." --Ronald Reagan, 10/26/1984
     
    Paul Ciszek, Jul 14, 2012
    #1
    1. Advertising

  2. Paul Ciszek

    Wally Guest

    On Sat, 14 Jul 2012 15:35:49 +0000 (UTC), (Paul
    Ciszek) wrote:

    >The two big complaints I hear about mirror lenses are:
    >
    >1) The depth-of-field is frustratingly narrow
    >
    >2) The f number is too high
    >
    >Those two would seem to conflict with each other. Although, since the
    >light-accepting orifice of a mirror lens is a ring rather than a circle,
    >it has a larger diameter than it should for the amount of light gathering
    >area that it has. Is it a matter of "This thing focuses like a f/5 but
    >exposes like a f/8"?
    >
    >Or is there something funky about the Maksutov-Cassegrain optics that
    >makes it follow completely different rules than lens-based optics do?


    Depth of field is due to simple and basic geometry. (That does not
    mean the calculations are simple.)

    Each pixel on the sensor subtends a cone of light that comes from the
    lens. The base of the cone is at the lens and is roughly a circle.
    Actually it's the shape of the iris if the lens is not fully open, for
    a refracting lens. With a mirror lens, the base is a circle with the
    obstructing concentric circle missing.

    Imagine a point of light being focused on the sensor of the camera. It
    is a point because the sensor intersects the cone at the apex. If you
    change the lens-sensor distance, then the light won't be in sharp
    focus any more -- because now the sensor plane intersects the cone a
    bit away from the apex.

    So now, instead of a point, you have a small circle on the sensor. The
    size of that circle depends on simple geometry -- the size of the base
    of the cone (ie the effective aperture of the lens) and the distances
    along the axis from the aperture to the sensor and to the focal point.
    None of this has anything to do with the mirrors in the mirror lens.
    DOF only depends on that cone and the axial distances.

    Calculation of DOF is fairly tricky because most of the time we don't
    know the size of the base of the cone (the exit pupil). The standard
    published DOF formulas usually assume it is equal to the size of the
    aperture. But in modern complex lenses, that's rarely the case.
    Generally in telephoto lenses, the exit pupil is smaller than the
    aperture. That would help to increase DOF. For mirror lenses, as you
    mention, the diameter is greater to allow for the presence of the
    obstruction, and that would decrease the DOF.

    W
     
    Wally, Jul 14, 2012
    #2
    1. Advertising

  3. Paul Ciszek

    RichA Guest

    On Jul 14, 11:35 am, (Paul Ciszek) wrote:
    > The two big complaints I hear about mirror lenses are:
    >
    > 1) The depth-of-field is frustratingly narrow
    >
    > 2) The f number is too high
    >
    > Those two would seem to conflict with each other.  Although, since the
    > light-accepting orifice of a mirror lens is a ring rather than a circle,
    > it has a larger diameter than it should for the amount of light gathering
    > area that it has.  Is it a matter of "This thing focuses like a f/5 but
    > exposes like a f/8"?
    >
    > Or is there something funky about the Maksutov-Cassegrain optics that
    > makes it follow completely different rules than lens-based optics do?
    >
    > --
    > Please reply to:         | "We establish no religion in this country, we
    > pciszek at panix dot com |  command no worship, we mandate no belief, nor
    > Autoreply is disabled    |  will we ever.  Church and state are, and must
    >                          |  remain, separate." --Ronald Reagan, 10/26/1984


    Part of it is also a bit of an illusion. Mirror lenses tend to focus
    a lot closer than refractive lenses of the same focal length, which
    means the DOF is shallower in some mirror lens images leading to the
    "feeling" they have very shallow DOF.
     
    RichA, Jul 15, 2012
    #3
  4. (Paul Ciszek) writes:

    > The two big complaints I hear about mirror lenses are:
    >
    > 1) The depth-of-field is frustratingly narrow


    Never heard that complaint before.

    And so far as I know, it's the same as for a refractive lens of the same
    focal length and aperture.

    > 2) The f number is too high


    > Those two would seem to conflict with each other. Although, since the
    > light-accepting orifice of a mirror lens is a ring rather than a circle,
    > it has a larger diameter than it should for the amount of light gathering
    > area that it has. Is it a matter of "This thing focuses like a f/5 but
    > exposes like a f/8"?


    There *is* some of that. The light transmission of a lens, measured in
    f/stop units, is called the t/stop. Most 500mm f/8 mirror lenses are
    about t/10.

    > Or is there something funky about the Maksutov-Cassegrain optics that
    > makes it follow completely different rules than lens-based optics do?


    I don't believe so.

    The two big complaints I hear about mirror lenses are:

    1) The bokeh is full of donuts

    2) The aperture is fixed.

    --
    David Dyer-Bennet, ; http://dd-b.net/
    Snapshots: http://dd-b.net/dd-b/SnapshotAlbum/data/
    Photos: http://dd-b.net/photography/gallery/
    Dragaera: http://dragaera.info
     
    David Dyer-Bennet, Jul 15, 2012
    #4
  5. David Dyer-Bennet <> wrote:
    > (Paul Ciszek) writes:


    >> The two big complaints I hear about mirror lenses are:
    >>
    >> 1) The depth-of-field is frustratingly narrow


    > Never heard that complaint before.


    > And so far as I know, it's the same as for a refractive lens of the same
    > focal length and aperture.


    They're often the first really long lens bought as an experimental toe
    in the long lens water. Long lenses have a number of oddities and
    problems which make them hard to get good quality images from. Mirror
    lenses have a few extra ones, so I think the "mirror factor" often
    gets blamed for problems that are in fact generic long lens problems.

    But if you were a salesman selling long expensive refractor lenses
    would you say that to a frustrated customer who came in complaining of
    problems with his mirror lens? :)

    >> 2) The f number is too high


    >> Those two would seem to conflict with each other. Although, since the
    >> light-accepting orifice of a mirror lens is a ring rather than a circle,
    >> it has a larger diameter than it should for the amount of light gathering
    >> area that it has. Is it a matter of "This thing focuses like a f/5 but
    >> exposes like a f/8"?


    > There *is* some of that. The light transmission of a lens, measured in
    > f/stop units, is called the t/stop. Most 500mm f/8 mirror lenses are
    > about t/10.


    >> Or is there something funky about the Maksutov-Cassegrain optics that
    >> makes it follow completely different rules than lens-based optics do?


    > I don't believe so.


    > The two big complaints I hear about mirror lenses are:


    > 1) The bokeh is full of donuts


    > 2) The aperture is fixed.


    Even when it's not producing doughnuts the harsh hard edged bokeh
    makes it more obvious when things are slightly out of focus. So the
    depth of sharp focus looks shallower than the same depth of focus on a
    lens with smooth soft edged bokeh.

    --
    Chris Malcolm
     
    Chris Malcolm, Jul 15, 2012
    #5
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. Nottoman

    Help me narrow down my freeze up problem

    Nottoman, Jun 15, 2004, in forum: Computer Support
    Replies:
    7
    Views:
    1,999
    Nottoman
    Jun 17, 2004
  2. Louis
    Replies:
    2
    Views:
    501
    Marggi
    Aug 18, 2003
  3. Tony W.

    Narrow depth of field on Olympus Camedia?

    Tony W., Jan 22, 2004, in forum: Digital Photography
    Replies:
    3
    Views:
    386
    Tony W.
    Jan 22, 2004
  4. Barry Bean

    Please help me narrow my choices

    Barry Bean, Feb 3, 2005, in forum: Digital Photography
    Replies:
    22
    Views:
    616
    Scharf-DCA
    Feb 12, 2005
  5. SS

    Wide or narrow angle?

    SS, Oct 4, 2005, in forum: Digital Photography
    Replies:
    7
    Views:
    2,297
Loading...

Share This Page