[rollei_list] Re: Contrast and Resolution
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Thu, 8 Nov 2007 14:39:39 -0800
----- Original Message -----
From: "Neil Gould" <neil@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Thursday, November 08, 2007 7:28 AM
Subject: [rollei_list] Re: Contrast and Resolution
Hi all,
The discussion regarding contrast vs. resolution is
interesting and raises
a point that I would like explained. How was resolution
determined
independent of contrast? Today, we see resolution
specifications that
include a contrast figure, and it is "common knowledge"
(a.k.a. layman's
understanding) that lower contrast will accompany lower
resolution
performance. Yet, the comments in this thread state the
opposite! Can one
of you provide a definition and/or a method of determining
resolution that
excludes contrast?
Regards,
Neil
The most common measurement for resolution is a
modulation transfer graph arranged to show resolution vs:
contrast. The contrast being the that between the dark and
light lines. The curve will show the effect being discussed.
It is a misunderstanding to think the "contrast" being
discussed here is overall image contrast, rather it is the
contrast between the light and dark lines. This is
essentially the same as the MTF curves shown for film.
For a "high contrast" lens the MTF chart will show the
contrast staying at a high level into the mid range of lines
per unit measure and then falling off fairly rapidly. For a
high resolution lens the contrast at mid values will usually
be lower but the curve will fall off slowly and extend
further than the high contrast condition. At very high
values of resolution the high resolution lens will have
_higher_ contrast than the "high contrast" lens.
The difference in the two conditions depends on how the
designer has balanced the higher order aberrations and on
what I will call the illumination of the entrance pupil.
If one charts the energy distribution of a beam of
light focused by the lens one finds that the high contrast
lens concentrates the light in a small beam or spot but has
many smaller peaks or beams surrounding the main beam. These
cause a sort of flare around the main beam lowering the
contrast. By suitable adjustment these secondary beams can
be much reduced at the cost of making the main beam broader.
The second condition reduces the resolution but increases
the contrast. The process is known as apodisation or removal
of the feet. For those familiar with radio antennas or
acoustic radiators an exactly analagous condition exists
there, the suppression of minor lobes with the concurrent
broadening of the main lobe.
Since the eye tends to interpret high contrast at the
edges of objects in an image as sharpness a lens with
moderately high resolution but high edge contrast will be
interpreted as being sharper than one with very high
resolution but only moderate edge contrast.
In some applications resolution is more important and
lenses are optimised for maximum resolution rather than edge
contrast.
Resolution of a lens must be measured using the aerial
image rather than on film because the magnitude of
resolution of the two is similar. Modern pictorial films
have resolutions as high as around 100 line pairs per
millimeter from a high contrast target (1000:1). Many lenses
will exceed this but a lens having over 100 lp/mm is
considered outstanding. There are a couple of formulas for
calculating the approximate value of combined resolution but
they do not take into account the fact that there is not
actually a single value. A true combination requires the
convolution of the two curves but the approximate value is
(1/film * 1/lens)/(1/film + 1/lens). Sometimes the root mean
square is given as the right approximate value.
Note that on film the effect of edge contrast is called
"acutance", a term invented by Kodak. On film the value of
contrast of mid resolution values can actually be higher
than the contrast of the original target. This is the effect
of so called acutance developers. I am not sure if a lens
can be made to produce the same effect. On film, the
increase in acutance is often accompanied by a loss of the
highest values of resolution. In that respect the effect is
the same as for lenses but the mechanism is different.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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