Vignetting

lemeister

Sorry for what's probably a dumb question, but could somebody explain what vignetting is?
I bought a fuji S5500 last year and am delighted with it. One of the only bad points I came across about it on the web nefore buying was vignetting, but I've yet to discover what exactly this is, even though on some of the review sites it points you to the area of the photo were it occurs!
As I said I'm delighted with the camera but would just like to figure this out and see if it does happen on my camera

Karoma

http://www.google.ie/search?hl=en&q=define%3Avignetting&meta=
(Then switch to Images.google.ie and see examples)

Oriel

http://www.dpreview.com/learn/?/Glossary/Optical/Vignetting_01.htm

Karoma

Q18. What are vignetting and light falloff?

A. Vignetting is a reduction in light falling on the film far from
the center of the image that is caused by physical obstructions.
Light falloff is a reduction of light far from the center because of
fundamental optical reasons: First, an off-axis object sees a
foreshortened apparent aperture (entrance pupil) so less light is
collected. This results in a cos(theta) falloff, where theta is the
angle off axis. Second, in a rectilinear lens the solid-angle-to-area
magnification increases with cos^3(theta), spreading the light from a
patch near the edge over more film than if the patch had been near the
center. (The patch is presumed to face the camera at a constant very
large distance.) As a result there is an overall cos^4(theta)
falloff. The optical designer can compensate for these effects by
making the entrance pupil enlarge and tip when viewed from off the
optical axis. An alternative approach is to compensate by using a
filter whose density varies appropriately with distance from the
center.


Q19. How can I tell if a lens has vignetting, or if a filter is
causing vignetting?

A. Open the back and, if necessary, trick the camera into opening the
shutter and stopping down. Imagine putting your eye right in the
corner of the frame and looking at the diaphragm. Or course, you
really can't do this, so you have to move your head and sight through
the corner of the frame, trying to imagine what you would see. If you
"see" the entire opening in the diaphragm and through it to object
space, there is no vignetting. However, at wide apertures in most
lenses the edge of the rear element or the edge of the front element
or filter ring will obstruct your vision. This indicates vignetting.
Try to estimate the fraction of the area of the diaphragm that is
obstructed. Log base two of this fraction is the falloff in f-stops
at the corner.

You can also do this from the front. With SLRs hold the camera a fair
distance away with a fairly bright area behind the viewfinder hole.
With non-SLRs open the back and arrange so a reasonably bright area is
behind the camera. Look through the lens, and rotate the camera until
you are looking right at one corner of the viewing screen or frame.
(If you are using the mirror-down technique with an SLR, choose an
upper corner of the frame, i.e. look from below the axis.) Now for
the hard part. Look at the aperture you see. If there is vignetting
you see something about the shape of an American football. If the
filter is causing the vignetting, one of the edges of the football is
formed by the filter ring.

A third way to detect vignetting is to aim the camera at a small
bright spot surrounded by a fairly dark background. (A distant street
light at night would serve well.) Deliberately defocus the image some
and observe the shape of the spot, particularly in the corners. If it
is round there is no vignetting. If it looks like the intersection of
some arcs (i.e. like an American football), then there is vignetting.
Note that near top of the image the top of the circle may get clipped
a bit. This is because in many cameras some light (from the top part
of the image) misses the bottom of the mirror. This affects only the
viewfinder, not the film. You can use depth of field preview (if your
camera has it) to determine the f-stop at which the spot becomes
round. With wide-angle lenses the circle of confusion may not get
large enough for this technique to be useful.

Source: http://www.photo.net/photo/optics/lensFAQ