[rollei_list] Re: Contrast and Resolution
- From: aghalide@xxxxxxx
- To: rollei_list@xxxxxxxxxxxxx, rollei_list@xxxxxxxxxxxxx
- Date: Mon, 12 Nov 2007 16:42:00 +0000
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 discussion 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 contrast.
> 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
> (where there 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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