[rollei_list] Re: Contrast and Resolution
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Thu, 8 Nov 2007 19:37:51 -0800
----- Original Message -----
From: <Choiliefan@xxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Thursday, November 08, 2007 6:56 PM
Subject: [rollei_list] Re: Contrast and Resolution
In what ways, if any does refraction affect this as the
lens is stopped down?
This distribution of energy must give individual lenses
their signature bokeh
then, no?
I'm trying to get a grasp on this...
Health, Peace
Lance
Selma, NC 27576
I think you mean diffraction. The usual limit given for
the resolution of a lens is the "diffraction limit", that
is, what an aperture the same size as the entrance pupil
with no aberrations would deliver. There are exceptions to
this rule but it is close enough for our purposes. The
larger the aperture the higher the resolution, however, in
practical lenses the aperture size also affects several of
the aberrations, notably spherical and coma, which are
proportional to the stop and have an indirect effect on
others such as astigmatism and field curvature. The smaller
the stop or aperture size the more coma and spherical will
be reduced. The other aberrations remain constant but the
increase in depth of focus aleviates them to some extent.
The problem is that there is a point of diminishing
returns, that is, making the stop smaller will improve lens
performance to the point where the increasing blur from
diffraction becomes larger than the blur from the
aberrations. This is often called the optimum stop. Since
coma varies with the image angle the optimum stop varies
with the angle of coverage required of the lens, the wider
the angle the smaller the stop required. This is why wide
angle lenses, especially older ones, need to be worked at
very small stops.
There are two kinds of chromatic aberration:
longitudinal chromatic is the difference in focus with
variation in color. Most lenses are corrected for this for
two colors, typically blue-green and orange, these are
called acromatic lenses. Some lenses are corrected for three
colors and are called apochromatic. Note that many current
lenses with APO in their names are actually acromats. This
kind of chromatic is constant with aperture, but like
curvature of field the increasing depth of focus with
stopping down reduces the effect somewhat.
The other kind of chromatic aberarration is called
lateral chromatic or chromatic variation of magnification.
In lenses with this fault the colors _focus_ at the same
point but the images are of different sizes. Stopping down
here does not help.
Lateral chromatic can be corrected by symmetry which
automatically cancels it where the overall optical path is
symmetrical, that is, 1:1 object and image distance, but
nearly symmetrical lenses have a great reduction in the
aberration. Lateral chromatic can also be corrected in
completely non-symmetrical lenses, like the Tessar, by
careful choice of glass and design, I don't know the exact
technique. The Kodak Commercial Ektar is an example of a
Tessar type lens with excellent correction for lateral
chromatic.
Bokeh is a description of the character of the out of
focus parts of the image. Its seems to be an artifact of the
balance of higher order aberrations in the lens. Probably
color also affects bokeh in both color and B&W images. There
are many scientific and technical papers written about the
"unsharp" image, mostly beyond me. But, Bokeh is certainly
affected by color correction.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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