[rollei_list] Re: Contrast and Resolution
- From: Richard Knoppow <dickburk@xxxxxxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Mon, 12 Nov 2007 05:29:15 -0800 (GMT-08:00)
-----Original Message-----
>From: Neil Gould <neil@xxxxxxxxxxx>
>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" <egoldste@xxxxxxxxx>
>>
>> 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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