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Question on wording

 
 
Kayla
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      07-07-2005
Could someone please explain what vignetting and chromatic abberation
is.

Thanks
Lori
 
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paul.busse@gmail.com
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      07-07-2005
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.

 
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Paul Heslop
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      07-07-2005
http://www.velocityreviews.com/forums/(E-Mail Removed) 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/
 
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doug
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      07-07-2005
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
 
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David Littlewood
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      07-07-2005
In article <(E-Mail Removed)>, Kayla
<(E-Mail Removed)> 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
 
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Don Stauffer
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      07-07-2005
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.
 
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David Littlewood
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      07-07-2005
In article <NDaze.8$(E-Mail Removed)>, Don Stauffer
<(E-Mail Removed)> 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
 
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eatmorepies
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      07-07-2005
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


 
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David Littlewood
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      07-07-2005
In article <(E-Mail Removed)>, eatmorepies
<(E-Mail Removed)> 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
 
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Kayla
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      07-10-2005
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
 
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