Re: Why are lenses unsharp wide open?

Discussion in 'Digital Photography' started by Martin Brown, Jul 31, 2011.

  1. Martin Brown

    Martin Brown Guest

    On 30/07/2011 19:33, Alfred Molon wrote:
    > Just wondering - does it come from physics or is it pure economics, i.e.
    > too expensive to make a lens with corner to corner sharpness at F1.4?


    There is nothing in the physics stopping you making a fast lens or more
    easily a catadioptric mirror lens with any arbitrary focal ratio if cost
    is no object. However, the costs get extremely high for fast optics and
    the circular field of view that is truly diffraction limited is not all
    that large.

    But it will cost a lot and to get the absolute optimum focal plane
    diffraction limited image in something that can be realistically
    manufactured you often have to live with a curved focal plane.

    The problem is that for slow focal ratio lenses f8 and higher the small
    angle approximation sin(x) ~= x approximation is a good one. Faster
    lenses have to correct for ever higher terms and something has to give.

    What kills it is that the surfaces become incredibly hard to make and
    the position of every element absolutely critical. So you can design
    something that will raytrace OK in theory but is unmanufacturable!

    The mirror based telescopes suffer only from the geometric aberrations -
    any lens based system has chromatic aberration to consider as well.
    Digital cameras that can now tolerate lateral chromatic aberration and
    correct for it in software post processing have given the lens designers
    an extra degree of freedom to play with.

    A paper describing a state of the art fast survey telescopes design
    which are limited only by geometrical aberrations is online at:

    http://adsabs.harvard.edu/abs/1984MNRAS.210..597W

    And a much more recent review of f1 optics that are being designed for
    finding Near Earth Objects in a survey instrument.

    http://www.amostech.com/TechnicalPapers/2007/Telescopes_Instrumentation/Ackermann.pdf

    These are *very* expensive designs to implement.

    Regards,
    Martin Brown
     
    Martin Brown, Jul 31, 2011
    #1
    1. Advertising

  2. Not all lenses are unsharp wide open.

    Both Canon and Nikon make tele lenses (300mm and up) that
    are sharpest wide open. The field of view of a 300m lens
    is so narrow that the off axis aberrations are not
    a terrible problem. The main problem is chromatic aberration
    and spherochromatism. Spherical aberration (at one wavelength)
    is no longer a problem at all, due to replicated aspheric optics.
    Of course, if they are sharpest wide open in means 1/4 wave
    accuracy summed over all elements ... and for say a 300mm
    f/2.8 lens this is not easy!

    Look at the MTF curves for Canon's 400mm f/2.8L IS II USM,
    they are superb. But also $11,000. Or the 500mm f/4L IS II USM.




    But the f/1.4 50mm lens (on a full frame 35mm camera) is
    a terrible design problem. First, you have to stuff all that
    light through the standard lens mount. Except for Canon
    this is a really big problem, and even they are not immune.
    Of course, some designers intentionally limit off-axis
    illumination to avoid the difficulties of correcting
    at f/1.4 across the field.

    With aspheric optics of course nowadays it is taken for granted
    that at one wavelength on axis there will be negligible spherical
    aberration. But the off axis aberrations are exceedingly difficult
    to control, even with multiple aspheric surfaces, as is spherochromatism.
    While the long teles can get most of their chromatic correction
    across the whole spectrum OK by putting most of the positive
    and negative powers in glass types that "naturally" match
    each other and cancel, that is impossible in fast lenses, since
    the shape necessary to control astigmatism makes using such (low index)
    glasses as the main power elements infeasible. So the chromatic
    errors of several elements have to have "delicate" cancellations.


    Doug McDonald
     
    Doug McDonald, Jul 31, 2011
    #2
    1. Advertising

  3. Martin Brown

    PeterN Guest

    On 8/3/2011 10:48 AM, George Kerby wrote:
    >
    >
    >
    > On 8/2/11 11:16 PM, in article
    > , "Rich"<>
    > wrote:
    >
    >> Doug McDonald<> wrote in news:j14as2$q3u$1@dont-
    >> email.me:
    >>
    >>>
    >>>
    >>> Not all lenses are unsharp wide open.
    >>>
    >>> Both Canon and Nikon make tele lenses (300mm and up) that
    >>> are sharpest wide open. The field of view of a 300m lens
    >>> is so narrow that the off axis aberrations are not
    >>> a terrible problem. The main problem is chromatic aberration
    >>> and spherochromatism. Spherical aberration (at one wavelength)
    >>> is no longer a problem at all, due to replicated aspheric optics.
    >>> Of course, if they are sharpest wide open in means 1/4 wave
    >>> accuracy summed over all elements ... and for say a 300mm
    >>> f/2.8 lens this is not easy!
    >>>
    >>> Look at the MTF curves for Canon's 400mm f/2.8L IS II USM,
    >>> they are superb. But also $11,000. Or the 500mm f/4L IS II USM.
    >>>
    >>>
    >>>
    >>>
    >>> But the f/1.4 50mm lens (on a full frame 35mm camera) is
    >>> a terrible design problem. First, you have to stuff all that
    >>> light through the standard lens mount. Except for Canon
    >>> this is a really big problem, and even they are not immune.
    >>> Of course, some designers intentionally limit off-axis
    >>> illumination to avoid the difficulties of correcting
    >>> at f/1.4 across the field.
    >>>
    >>> With aspheric optics of course nowadays it is taken for granted
    >>> that at one wavelength on axis there will be negligible spherical
    >>> aberration. But the off axis aberrations are exceedingly difficult
    >>> to control, even with multiple aspheric surfaces, as is

    >> spherochromatism.
    >>
    >>
    >>> While the long teles can get most of their chromatic correction
    >>> across the whole spectrum OK by putting most of the positive
    >>> and negative powers in glass types that "naturally" match
    >>> each other and cancel, that is impossible in fast lenses, since
    >>> the shape necessary to control astigmatism makes using such (low index)
    >>> glasses as the main power elements infeasible. So the chromatic
    >>> errors of several elements have to have "delicate" cancellations.
    >>>
    >>>
    >>> Doug McDonald
    >>>

    >>

    >
    > Forget something, Rich, you senile old bastard?
    >

    Why complain. He used fewer electrons to say hat he usually says.

    --
    Peter
     
    PeterN, Aug 3, 2011
    #3
  4. Doug McDonald <> wrote:

    > But the f/1.4 50mm lens (on a full frame 35mm camera) is
    > a terrible design problem. First, you have to stuff all that
    > light through the standard lens mount. Except for Canon
    > this is a really big problem, and even they are not immune.


    Canon made the 50mm f/1.0 to ... underline that point for
    their then-new EF mount.

    -Wolfgang
     
    Wolfgang Weisselberg, Aug 8, 2011
    #4
    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. Rich

    Lenses that function best wide open

    Rich, Nov 26, 2006, in forum: Digital Photography
    Replies:
    14
    Views:
    565
    Ilya Zakharevich
    Dec 1, 2006
  2. Dauphin de Viennois

    Re: DSLR lenses not good wide open at wide angle?

    Dauphin de Viennois, Jul 16, 2008, in forum: Digital Photography
    Replies:
    2
    Views:
    552
    Roy Smith
    Jul 16, 2008
  3. Bruce

    Re: Why are lenses unsharp wide open?

    Bruce, Jul 31, 2011, in forum: Digital Photography
    Replies:
    4
    Views:
    471
    Wolfgang Weisselberg
    Aug 8, 2011
  4. Anthony Polson
    Replies:
    9
    Views:
    295
    Martin Brown
    Nov 9, 2012
  5. Martin Brown
    Replies:
    3
    Views:
    288
    Martin Brown
    Nov 10, 2012
Loading...

Share This Page