What does APO mean optically and practically ?

Discussion in 'Leica and Rangefinders' started by tony_brookes|5, Jul 26, 2001.

  1. Can some kind contributor tell me exactly what APO and ASPH mean both optically and in practice ?
     
  2. APO stands for "apochromatically corrected". In most lenses, optical
    design concentrates the focus of blue light and green light into a
    single plane, but red light falls slightly into another plane of
    focus. Red subjects, therefore, would be ever so slightly out of focus
    compared to blue and green subjects in the same frame. Not sure you'd
    ever notice though, the difference is so slight. This is the same
    basic principle that requires you to shift the focus for infrared
    photography, related to the wave length of red light. In APO lenses,
    the design and expense has been put in to making red light focus on
    the same plane as blue and green. Under a microscope you would see
    that all light subject is now in focus, creating a sharper image
    overall. Many manufacturers offer APO designs, but in most of these
    only the very center of the lens is APO corrected. Leica prides itself
    on making most of the frame APO corrected.

    <p>

    ASPH stands for "ashperic design". Most lenses have a spherical design
    - that is, the radius of curvature is constant. These are easy to
    manufacture by grinding while "spinning" the glass. This design
    however restricts the number of optical corrections that can be made
    to the design to render the most realistic image possible. ASPH
    lenses, however, involve usually 1 element that does *not* have a
    constant radius of curvature. These elements can be made by 1)
    expensive manual grinding, 2) molded plastic, 3) Leica's patented
    "press" process, where the element is pressed into an aspherical
    ("non-spherical") shape. This design allows the manufacturer to
    introduce corrections into compact lens designs that weren't possible
    before. Practically, the lens performs "better" (up to interpretation)
    due to increased correction of the image, in a package not
    significantly bigger than the spherical version.

    <p>

    I'm sure there are many experts on this site that can "enhance" my
    layman's explanation.
     
  3. A minor addendum to the layman's explanation above.

    <p>

    In practice, APO and ASPH mean "more expensive."
     
  4. Thanks a lot Ken. Very clear and very helpful.
     
  5. Some logical consequences of all this: 1) With APO lenses and infrared
    photography, you do *not* have to shift the focus, because the red
    light focus has already been corrected. 2) Only longer lenses can make
    good use of APO correction. Normal and short (wide angle) lenses have
    enough depth of field to make the focus shift moot. 3) Any lens stands
    to gain from ASPH elements as part of the design, but this adds to the
    expense of manufacture (as does APO correction). The manufacturer has
    to strike a balance between the cost increase and image quality to be
    gained.
     
  6. 4) If you put a non-APO teleconverter behind your expensive 100mm
    APO-R or 180mm APO-R lens, you just negated the APO correction you
    spent so much for. If you must teleconvert, an APO lens needs an APO
    teleconverter.
     
  7. These subjects have been explained to me countless times, but I've
    never heard or read a better or more clear explanation than Ken's in
    this thread. Thanks, Ken!
     
  8. So, Mike, let's see if I've got this straight:

    <p>

    APO = Always Priced Outrageously

    <p>

    ASPH = Also Specially Priced High

    <p>

    Right?
     
  9. As far as "in practice", I've noticed that well designed APO and
    aspherical lenses are in general sharper wide open than there more
    traditionally designed counter parts. Don't own any Leica lenses
    labeled as such, but have read the 135mm Tele Elmar is pretty much an
    APO design.
     
  10. I agree with Tony...

    <p>

    ...I knew this Vivitar converter had something to do with the new
    bouque I was getting, what to do I convenced all my clients it was
    great, now they all want the new buquue...
     
  11. Consequence #5: many times there are complaints that the current 90mm
    APO-Summicron-M ASPH is "too sharp" for portraiture. Human skin
    reflects a lot of red light. The APO correction specifically works to
    better focus red light. Thus skin blemishes are in sharper focus with
    any APO lens. Before laying out extra money for this lens, consider
    what you will use it for.
     
  12. Just to add a bit to what's been said; it's not enough to just
    have APO correction in lenses that are of long focal length. Designers
    realized that the uncorrected colors in an APO design using
    conventional glass would still color fringe. So, most APO designs
    today also use low dispersion glass as well. Even superachromatic
    lenses use low dispersion glass to reduce fringing from the
    uncorrected colors. See the Zeiss 250 and 350mm lenses of this type.
    APO lenses that use conventional glass have much more spherochromatism
    (change in spherical aberration with wavelength)than APO low
    dispersion types. With shorter focal length(90,100, 135) lenses, the
    designer can probably get by without color problems when using an APO
    design without LD glass. An exception is the short view camera lens in
    which it will be used far off the optical axis at imes and lateral
    color needs to be controlled.

    <p>

    To have no separate infrared mark on a lens, there needs to be low
    spherochromatism and therefore just having an APO designation is not a
    guarantee of this.

    <p>

    Also, if a lens is APO, that does not mean that the three corrected
    colors are corrected in all zones of the lens. Those colors will focus
    in front or behind the film if they pass through the uncorrected zones
    of the lens. This was the problem with conventional glass. LD glass
    greatly improved this so that the uncorrected zones would control the
    focus deviation from the film plane much better.

    <p>

    Not only do aspheric surfaces correct out spherical aberration, they
    are useful in controlling distortion. Check out the display of this at
    the Solms plant near Wetzlar.
     

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