Don, You have been told wrong. If what you say were true, the amount of
"vigneting" would vary between f/16 and f/8 with a focal plane shutter! A
different condition you might say? Then consider this tripod experiment:
Experiment: Assume a correct exposure is f/5.6 @ 1/100th sec., but instead
you shoot three times as a multiple (triple) exposure. All exposures are at
1/100th sec., but the first is at f/16 (1/4 the required light), the second
at f/8 (1/2 the required light) the third at f/16 again (1/4 the required
light again) for a total correct exposure. We have now simulated the effects
of opening and closing of a leaf shutter, we just broke it into 3 segments.
Do you expect that "vigneting" due to this type of exposure will be
observed? Of course not!
---------------------------------------------------------------
"I don't mind if you don't like my manners.
I don't like them myself. They're pretty bad.
I grieve over them long winter evenings."
-- Philip Marlowe (Humphrey Bogart)
----- Original Message -----
From: "Don Roberts" <droberts@xxxxxxxxxx>
I hate to disagree with so eminent and learned personage, Andy, but it was
always my understanding that the design of a leaf shutter guaranteed that
the center would receive more light than the edges. Just picture the way
it works. It opens from the center to the edges and then reverses the
process. The center has to receive more light. The difference in the
amount is insignificant though for a properly functioning shutter. There
is less difference at slow shutter speeds than high speeds. The blades
moved at a constant rate; if they were able to move faster from the center
and slow down toward the edges, the disparity could be resolved. It was
virtually impossible to make a leaf shutter that would function at high
speeds which was a reason for the development of the focal plane shutter.
Or so I have been told many times in my early days in photography.
Somebody correct me if I am wrong. Politely, please.
ADavidhazy wrote:
Stephen,
The center of a photograph is not any more exposed than the edges due to
the left shutter.
Lens design has a lot to do with the degree to which the non uniformity
of exposure
from edge to edge or corner to corner of a photograph is apparent.
Something called
the "cosine to the 4th power" is a factor at work. It has to do with the
angle at
which light rays arrive at the lens and the image surface ... and the
distance they have
to travel to get to the image plane.
Some really wide angle cameras had a whirling fan installed in front of
the lens to
physically dodge the image forming rays allowing more to go to the edges
than the center
of the image. Others used a greyscale mask to try to accomplish the same
thing. Some
new lenses have included in their design a function that makes the
aperture appear to get
larger to light rays arriving at an angle than those that arrive more
from directly
in front of the lens.
Stephen Ylvisaker wrote:
When shooting using a camera with a leaf shutter, do any of you have a
way to counteract the leaf shutter effect? I mean, it's certainly not
what I would call vignetting, but it is very apparent the center of the
picture is the brightest point. And, I know it could be taken care of in
the "printing" process by burning the center down a little bit. But, is
there an "in-camera" method to counteract the effect?
greyfell@xxxxxxxxxxx
"...decide...whether or not the goal is worth the risks involved. If it
is, stop worrying....",
and "Worry retards reaction and makes clear-cut decisions impossible." -
Amelia Earhart, aviatrix.
