Question on wording

Discussion in 'Digital Photography' started by Kayla, Jul 7, 2005.

  1. Kayla

    Kayla Guest

    Could someone please explain what vignetting and chromatic abberation
    is.

    Thanks
    Lori
    Kayla, Jul 7, 2005
    #1
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  2. Kayla

    Guest

    Vignetting, properly, refers to a printing technique in which the
    images fades into the surrounding paper, often done in an oval or
    circular pattern. As a problem, it refers to a dark circular pattern
    seen around a square or rectangular image, usually caused by the
    wrong-sized shade/filter/other lens accessory.

    Chromatic aberration refers to (usually) apparently soft focus through
    an uncorrected lens, caused by the fact that the different wavelengths
    (colors) making up white light are refracted at a different rate by a
    given material.

    Paul B.
    , Jul 7, 2005
    #2
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  3. Kayla

    Paul Heslop Guest

    wrote:

    > Chromatic aberration refers to (usually) apparently soft focus through
    > an uncorrected lens, caused by the fact that the different wavelengths
    > (colors) making up white light are refracted at a different rate by a
    > given material.
    >
    > Paul B.


    or

    "Chromatic aberration of a lens is seen as "fringes" of color around
    the image"

    This seems to be mainly a blue or bluish colour and isn't just around
    the image but can be seen within, on the points of high contrast
    --
    Paul (And I'm, like, "yeah, whatever!")
    -------------------------------------------------------
    Stop and Look
    http://www.geocities.com/dreamst8me/
    Paul Heslop, Jul 7, 2005
    #3
  4. Kayla

    doug Guest

    Kayla wrote:
    > Could someone please explain what vignetting and chromatic abberation
    > is.
    >
    > Thanks
    > Lori


    If you put a small filter on your lens, the shaded area it will produce
    at the extreme corners of the image is vignetting. It was once used in
    darkrooms during printing to produce a "special effect". Vignetting in
    reverse puts a white halo around the image and was very popular with
    Portrait photographers in the 1920s. Some wedding photographers use this
    technique to hide unwanted (unnoticed?) garbage in the background.

    Chromatic aberrations are caused by both Digital sensors and poorly
    designed lenses. Put the two together and you have your average, cheap,
    P&S digital camera selling in Wallmart for $100!

    Some software is available to correct CA but it is only partly
    successful. The only real solution is to buy lenses which are APO lenses
    and never use a cheap digital.

    The most frequent occurrence of CA in digital cameras is when you shoot
    something with strong (sometimes just white) background contrast. This
    produces a green or blue halo on the side of the object lit the most.

    Douglas
    doug, Jul 7, 2005
    #4
  5. In article <>, Kayla
    <> writes
    >Could someone please explain what vignetting and chromatic abberation
    >is.
    >
    >Thanks
    >Lori


    Vignetting refers to the situation where the image is not fully
    illuminated across the image area. It may, in extreme cases, result in a
    complete cut-off of the image in a circular or elliptical pattern, or it
    may in less extreme cases show as a darkening of the image in the
    corners.

    The extreme version used to be popular as an artistic technique,
    especially for portraits and wedding photos, but is considered
    (certainly by most people I have discussed it with) a rather dated
    technique.

    The less extreme version - the corner darkening - is usually a sign of
    some error in technique or defect in design. For example, if you use a
    lens hood which is too intrusive for the angle of view of the lens, or
    stack too many filters (especially on wide-angle lenses) you will see
    vignetting. Also, it is really quite difficult to design lenses which
    give even coverage across the whole image*. Even the best lenses will
    often show a 1/4 or 1/3 stop darkening in the corners when used wide
    open, though this usually disappears when the lens is stopped down. The
    designers of lesser lenses will probably give up sooner and accept a
    higher degree of vignetting. Another cause - probably one that will
    increase in occurrence - is the use of a lens that does not have quite
    enough covering power for the film or sensor size. For example, if you
    use one of the latest crop of "designed for digital" lenses on a 35mm
    film body you will very probably see vignetting.

    Chromatic aberration is a term which covers two quite separate phenomena
    - and a lot of confusion can be caused when people talk of them as one.

    Longitudinal chromatic aberration (CA), or Axial CA, is caused by the
    fact that a simple lens will focus light at a distance which depends on
    wavelength (an effect known as dispersion). Short wavelength (blue)
    light is focussed closer than is longer wavelength (red) light, with all
    the intermediate colours in between. Left uncorrected, this will cause
    general fuzziness, perhaps with some minor colour fringing for subjects
    of unusual colours. Longitudinal CA is improved by stopping down the
    lens. Lens designers correct it by using various kinds of glass with
    different dispersion factors. All photographic lenses are at least
    "achromats", for which the focus is corrected for two wavelengths. The
    best lenses are "apochromats", which are corrected for three
    wavelengths. Fortunately, the corrections available to lens designers
    means that most high-end lenses do not suffer much from this aberration,
    though there are plenty of cheap ones that do.

    Lateral chromatic aberration is sometimes known as Transverse CA or
    "chromatic difference of magnification". The latter term gives the clue:
    light of different colour produces images of different magnification,
    i.e. of different size. Normally this will be that the blue image is
    smaller and the red larger, giving blue fringes near the lens axis and
    red towards the edge. Lateral CA is worse for long focal length lenses
    and, unfortunately, is not improved at all by stopping down the lens. It
    is also one of the more difficult aberrations to cure by lens design.
    Large telephoto lenses in particular require the use of very exotic
    glasses to reduce it to acceptable proportion. This is very expensive,
    and explains why most cheap long lenses are very mediocre performers.

    It is interesting that Longitudinal CA is impossible to cure by
    post-processing, but can be reduced to acceptable limits by good design,
    whereas Lateral CA is impossible to cure by design - it is one of the
    hardest to reduce - but is one of the easiest to improve by post
    processing (there are programs which separate the red, green and blue
    channels, apply slight magnification changes, and re-merge them).

    Be aware that colour fringing on images may not, or not all, be caused
    by lens aberrations. Longitudinal CA is unlikely to cause much fringing
    in normal circumstances. Lateral CA ^will^ cause fringing, and will be
    especially noticeable on cheap long-focus lenses. However, on digital
    cameras there can be artefacts which arise from the design of the sensor
    and its associated micro-lenses, so you should not condemn the lens too
    hastily.

    This may of course be far more than you ever wanted to know. However, a
    lot of newsgroup posts seem to confuse these issues, so I thought it
    would be useful to clarify.

    *For the terminally curious, the illuminance of the image produced by a
    simple lens falls off as cos^4 of the angle of the image forming ray to
    the optic axis. For a wide angle lens with a 60 degree field of view,
    this would mean the corner would only get 6% of the illuminance of the
    centre. Various measures are used to correct this, especially by using
    very large negative (concave) elements at front and rear, but it is
    never entirely removed for rectilinear lenses (though it can be for
    fisheyes).

    David
    --
    David Littlewood
    David Littlewood, Jul 7, 2005
    #5
  6. Kayla

    Don Stauffer Guest

    Kayla wrote:
    > Could someone please explain what vignetting and chromatic abberation
    > is.
    >
    > Thanks
    > Lori



    Vignetting is as evident in black and white photography as in color. It
    is a darkening towards the edges and corners of the image.

    Chromatic aberration is a color fringing thing near sudden changes in
    luminance or color. It is like a little rainbow surrounding an edge.

    In an image of a star, for instance, instead of a single white dot the
    image would appear as a sort of rainbow colored circle. While chromatic
    aberration may be worse towards edges, it can also occur in center of image.

    As an extreme, vignetting can make an image appear only in center
    portion of frame, surrounded by a dark circular area. Vignetting is
    primarily a problem with wide angle, seldom ocurring at longer focal
    lengths. Sometimes in portrait work vignetting is intentional, and
    induced by a mask placed in front of lens.
    Don Stauffer, Jul 7, 2005
    #6
  7. In article <NDaze.8$>, Don Stauffer
    <> writes
    >
    >As an extreme, vignetting can make an image appear only in center
    >portion of frame, surrounded by a dark circular area. Vignetting is
    >primarily a problem with wide angle, seldom ocurring at longer focal
    >lengths.


    Whilst this is broadly true, the only Canon lens (out of quite a lot) I
    had a vignetting problem with was a 100-300mm zoom.

    David
    --
    David Littlewood
    David Littlewood, Jul 7, 2005
    #7
  8. Kayla

    eatmorepies Guest

    snip <lots of explanation>
    >
    > *For the terminally curious, the illuminance of the image produced by a
    > simple lens falls off as cos^4 of the angle of the image forming ray to
    > the optic axis. For a wide angle lens with a 60 degree field of view,
    > this would mean the corner would only get 6% of the illuminance of the
    > centre. Various measures are used to correct this, especially by using
    > very large negative (concave) elements at front and rear, but it is
    > never entirely removed for rectilinear lenses (though it can be for
    > fisheyes).


    Nicely put. Following on from your final comment; how does a fisheye perform
    on a small sensor? In particular, how fisheye would the Canon EF15mm fisheye
    be on the 350D half size sensor?

    John
    eatmorepies, Jul 7, 2005
    #8
  9. In article <>, eatmorepies
    <> writes
    >snip <lots of explanation>
    >>
    >> *For the terminally curious, the illuminance of the image produced by a
    >> simple lens falls off as cos^4 of the angle of the image forming ray to
    >> the optic axis. For a wide angle lens with a 60 degree field of view,
    >> this would mean the corner would only get 6% of the illuminance of the
    >> centre. Various measures are used to correct this, especially by using
    >> very large negative (concave) elements at front and rear, but it is
    >> never entirely removed for rectilinear lenses (though it can be for
    >> fisheyes).

    >
    >Nicely put. Following on from your final comment; how does a fisheye perform
    >on a small sensor? In particular, how fisheye would the Canon EF15mm fisheye
    >be on the 350D half size sensor?
    >
    >John
    >
    >

    Well, I don't have a fisheye (don't really care for the images) but I'm
    sure they would perform exactly as one would expect - a very wide image
    with all the barrel distortion, but missing the outer part of the image.
    IOW, instead of the 180 degree diagonal field of view, it would only
    "see" something like 120 degrees - haven't done the maths to work out a
    precise figure.

    David
    --
    David Littlewood
    David Littlewood, Jul 8, 2005
    #9
  10. Kayla

    Kayla Guest

    Thanks Re: Question on wording

    Thanks for your explanations. It was a big help in understanding it.
    I'm trying to learn how to use my Canon Pro1 and it's a real challenge
    but I'm working on it.

    Lori
    Kayla, Jul 10, 2005
    #10
  11. Kayla

    Alan Meyer Guest

    David Littlewood wrote:
    > ...
    > This may of course be far more than you ever wanted to know. However, a
    > lot of newsgroup posts seem to confuse these issues, so I thought it
    > would be useful to clarify.


    It is useful.

    > *For the terminally curious, the illuminance of the image produced by a
    > simple lens falls off as cos^4 of the angle of the image forming ray to
    > the optic axis. ...


    I fear that it is the lack of curiousity that is more likely to be
    terminal among us than the the reverse.

    Thanks for the explanations.

    Alan
    Alan Meyer, Jul 19, 2005
    #11
  12. Kayla

    Alan Meyer Guest

    Re: Thanks Re: Question on wording

    Kayla wrote:
    > ...
    > I'm trying to learn how to use my Canon Pro1 and it's a real challenge
    > but I'm working on it.

    ....

    Good for you!

    More of us should try to undestand the astonishing technology
    we use.

    Alan
    Alan Meyer, Jul 19, 2005
    #12
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