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Old June 28th 04, 11:08 AM
Richard Knoppow
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Default Image circle versus stopping down?

Nick Zentena wrote in message ...
The way I understood it stopping down increased the image circle
because it improved the quality of the image. But then I found this quote

"4. Why does the size of the image circle in a Large Format lens change as
the lens is stopped down?

Well . . . it doesn't actually. What actually happens is that when the
lens is wide open, the center of the image is very bright and the corners of
the image are VERY dark. So dark in fact that they do not expose the film
(When the film is properly exposed for the center of the image). As the lens
is stopped down, uniformity of illumination improves and therefore the
circle of usable illumination increases."

That's from here

http://www.schneideroptics.com/info/...mat_lenses/#q3

So if I understand this. You could use a lens wide open and get the same
image circle if you could use something like a centre filter? In other words
the edges shouldn't be soft. Does this make sense to anybody?

Nick




A center filter does not perform the same function as the stop.
The explanation on the Schndeider site is incomplete and
oversimplified. The main aberrations which vary with the stop are
spherical aberration and coma. Spherical exists all over the image,
coma is not present at the center but increases with the image angle.
Spherical aberration causes a hazy surround to highlights. Enough
spherical causes an overall haze and a blury image, it is the cause of
the softness of soft focus lenses. Coma is related to spherical but is
asymmetrical. It causes smeared, somewhat tear-drop-shaped blur spots
along radiuses toward the center. They can point toward or away from
the center of the image depending on where they mainly occur in the
lens. Coma varies rapidly with the stop.
Both of these aberrations vary with the angle the light rays take
through the lens. Other factors being equal they increase with the
speed of the lens, and coma especially increases with the angle of
coverage, i.e., its harder to correct for wide angle lenses.
These aberrations become smaller as the light going through the
lens is concentrated toward the center of the lens. This is what
stopping down accomplishes.
Uniformity of illumination is not affected by the stop except
where there is some mechanical vignetting from the lens mount. For
most lenses this is gone when stopping down about two stops. Some wide
angle lenses are designed to reduce this mechanical vignetting (the
Angulon is an example).
One of the inherent properties of lenses which produce
orthographic images is that the illumination falls off with image
angle. For a "normal" design lens this fall off is approximately
proportional to cos^4 theta where theta is the "half angle" of the
image point. There are designs of lenses which have improved
illumination but these are not better than about cos^3 theta, better,
but there is still fall off. A non-orthograpic lens, like a fish-eye
lens, can have better illumination because the light is more
concentrated near the margins.
A center filter acts to compensate, at least in part, for the fall
off by letting more light in at the margins of the image. The filter
is not exactly a stop but rather a tapered obstruction. The lens
projects a very blury, out of focus, image of the center filter onto
the film. One reason a center filter must be used with the lens
stopped down is that the range of angles the light going through the
filter must be limited for it work.
It is possible that a center filter _may_ act, to some extent, as
a obstructive stop. An obstructive stop limits the rays of light to
the margins of the lens rather than the center, as does a normal stop.
An obstructive filter changes the balance of all the aberrations and
has an effect on the MTF curves. In comparison to a standard stop the
obstrucive stop can raise the edge contrast at the price of lowering
the resolution.
The Schneider explanation about the balance of aberrations to
diffraction is correct. As the stop size is reduced the aberrations
are reduced but the diffraction is increased. At some point the two
curves cross. That is the "optimum stop". Actually, there isn't a
single optimum stop. Since coma and sphrical abberation play a part in
the image quality, and, since both vary with image angle, the optmum
stop will vary with the image angle wanted from the lens. For
instance, a Dagor, which has relatively low coma (as do all
symmetrical lenses) has a circle of illumination of nearly 90 degrees.
At about f/11 it has a coverage with good image quality of around 60
degrees. To get the maximum coverage angle it must be stopped down to
f/45. The image quality at the margins of the image at 87 degrees will
be best at f/45 but not at the center, there it will be best at around
f/11 or f/16.


Richard Knoppow
Los Angeles, CA, USA