[rollei_list] Re: Contrast and Resolution
- From: Peter J Nebergall <iusar4s@xxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Sun, 11 Nov 2007 22:52:28 -0600
I started reading PP and Modern in 1964. I was ten. I remember reading
of "resolution," and how distortions decreased as the lens stopped down,
and I verified this through my old Praktiflex with manual 58mm Biotar. I
could see it improve.
What I didn't quite comprehend was "lines per millimeter." I could get
the concept, and that a Summicron gave you more of them than an old
Auto-Sekor or Xenon, but exactly how were they computed? I know it's
like todays "DPI" for digitals... but I was taught the Summicron and the
old Goerz Dagor were rulers of the roost. I still have them, and using
them makes me wonder, every time I look at Mom's D2 work, why it is
"unsharp." But how was a lens' resolution in "lines per mm" computed?
Peter Nebergall
On Mon, 12 Nov 2007 16:42:00 +0000 aghalide@xxxxxxx writes:
Somewhere in my "stuff" I have a collection of lens-test kits I stuck
away for future viewing. They include one from the NBS (national bureau
of standards) charts of high and low contrast, Modern Photography
magazine Kit with only high contrast targets, Dutch Foto magazine
targets, and others.
I remember the problem we hade at Modern was translating the resolution
to practical info.
Different focal lengths had different translation for excellent, very
good, etc. The first two or three lens openings had tougher designations,
as were the last two or so openings. Longer focal lengths
on larger formats were also more forgiving, I think. We shot tests
without the camera shutter, using only the variation of light to test the
lens openings. So shutter shake was not possible. Also, tests were made
at 20X the lens focal length, when possible. Since our test lab was
limited in size sometimes we had to use 15X focal length.
My recollection goes back to 1966 when I left Modern Photography and was
finished testing lenses for publication. We viewed the results of each
test with a 20x lupe (microscope) of excellent quality.
Them's were the days. Ed Meyers
-------------- Original message from Richard Knoppow
<dickburk@xxxxxxxxxxxxx>: --------------
>
>
> -----Original Message-----
> >From: Neil Gould
> >Sent: Nov 12, 2007 6:32 AM
> >To: rollei_list@xxxxxxxxxxxxx
> >Subject: [rollei_list] Re: Contrast and Resolution
> >
> >Hi Eric,
> >
> >> Date: Sun, 11 Nov 2007 10:46:03 -0500
> >> From: "Eric Goldstein"
> >>
> >> Hi Neil -
> >>
> >> Glad to be of help but now you have me confused... how is it
possible
> >> to remove the variable of human perception from this discussion?
> >> Without it, there would be no discussion!
> >>
> >In other words, there would be no basis for discussi on if resolution
were
> >to be measured objectively, for example via sensors? I hadn't thought
of
> >that possibility... ;-)
> >
> >Neil
> >
> I think this is a bit of an overstatement. Both absolute resolution and
> effective contrast can be determined from an MTF chart, either
calculated from
> the lens prescription or measured from an actual lens. The measurement
from the
> aerial image is not too difficult.
> The difference in the shape of the frequency response curve will be
> evident.
> Now, the effect on the perception of sharpness will have to be
determined
> experimentally because there is not absolute rule for this. In general,
the eye
> will interpret an image of high resolution but low overall contrast as
being
> less sharp than an image of good resolution but high contrast. A very
low
> resolution image will be seen as blurred regardless of the contr ast.
> So called high contrast lenses will have an MTF which stays at a fairly
> high level of contrast out to pretty good resolution, say around 30 or
40 lp/mm
> for a 35mm lens, but falls off rather quickly above that. A high
resolution lens
> may have low contrast resolution out to 100 lp/mm or even more. If the
two
> images are compared under magnification the improved rendition of fine
detail by
> the high resolution lens will be obvious, but for relatively low
magnification
> the high contrast image will look sharper.
> BTW, there are a couple of ways of plotting MTF curves. Most
manufacturers
> choose about three values of resolution and plot each as contrast vs:
image
> angle for several values of f/stop. Another way is to plot the curve as
a
> frequency response curve, that is, frequency or lines vs: contrast
ratio with
> curves or charts for several values of f/stops and image angles .
> Both types of plot will show two curves for image angles greater than
zero
> because the effect of diffraction is different for radial and
tangential lines.
> This is caused by the distortion of the stop when viewed at an angle.
Beyond the
> optical axis the stop becomes football or cat's eye shaped. Since
resolution is
> proportional to the size of the stop it will be greater in the
direction of the
> long dimension of the stop.
> This is not the same as astigmatism, which in a camera lens means the
> difference in focus between light from a point on the image entering
the lens
> though a radial line passing through th center of the lens and light
entering
> along a segment of a circle or tangentially. In general, these to
bundles of
> light rays will focus at different distances from the focal plane. Most
> anastigmat lenses are correceted so that the two fields cross at the
center
> (w here t here is no astigmatism) and at the edge of the image. The
closer these
> two fields are to each other the sharper the image. While diffraction
at the
> stop affects radial and tangential lines differently it is not the same
effect
> as astigmatism because it does not affect the plane of focus of the
two.
>
>
>
> --
> Richard Knoppow
> dickburk@xxxxxxxxxxxxx
> Los Angeles, CA, USA
> ---
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