From: David Ruether on 23 Dec 2009 09:00
"David Ruether" <d_ruether(a)thotmail.com> wrote in message
> "Chris Malcolm" <cam(a)holyrood.ed.ac.uk> wrote in message
>> And of course if you deliberately want to provoke more diffraction at
>> wide stops normally free of diffraction effects, change the shape of
>> the aperture restriction to one with a high edge to area ratio.
>> Chris Malcolm
> Such as a slot - but rarely does a lens designer seek to worsen
> diffraction effects...;-) There are functional reasons for doing so,
> though. If you look at the shape of the iris of a cat's eye, it is
> designed to minimize diffraction effects in one orientation at
> the expense of worsening it in the one at 90 degrees rotation
> from it. This allows the cat better night vision sharpness for
> horizontal lines and vertically moving things compared with the
> alternative and also compared with the acuity of its eye at wide
> stops if the iris were round.
Hmmm.... The above works better with the cat's iris in *bright*
light and *smaller* "stops"...;-) Sorry for the confusion (my cat
corrected me last night...;-).
From: Paul Furman on 28 Dec 2009 11:40
David Ruether wrote:
> "egbert_no_bacon" <egbert_no_bacon(a)hotmail.co.uk> wrote in message
>> what is diffraction, and do you have an example or know where an image
>> of such is posted for me to see please
> If a lens were optically perfect (no manufacturing or design flaws (which
> is impossible, although some process lenses used for printing tiny circuits
> approach this), stopping down the aperture from wide open would
> progressively soften the image due to diffraction. Since lenses are
> compromises and stopping them down can help overcome some image
> flaws seen mostly at their wide stops, there are two conditions that
> therefore intersect - most lenses improve in performance as they are
> stopped down (and their flaws are progressively overcome) until the
> diffraction limit for a given stop is reached (after which diffraction
> progressively worsens the image quality with greater stopping down).
> The stop at which this occurs depends on the quality of the lens at its
> wider stops (stopping a very high quality lens below maybe f4 would
> begin to show diffraction softening with further stopping down, but
> putting an aperture in front of a simple magnifying glass may never
> show serious diffraction effects since orther image faults swamp
> As for what diffraction is, it is the tendency of light to be bent as it
> passes an edge. The ideal purpose of a lens is to focus entering light
> rays coming from points in the subject to corresponding points on
> the sensor. If the aperture is wide, most of the area of the lens passing
> light has proportionally little diffraction effect on the light passed. With
> stopping down, a far higher proportion of the light passing through the
> lens will be affected by the diaphragm edges of the lens, causing
> the image points to become larger, resulting in a less accurate rendering
> of subject points on the sensor (and image resolution loss).
> For samples shot with a range of f-stops that show the difference
> diffraction changing with aperture can make in an image, go here --
Agreed on all that except the link only shows the star patterns and the
images aren't big enough to show softness. Here's a very simple
demonstration of softening, including non-diffraction softness wide,
sharp at f/8 and diffraction softness at f/36, for an old 500mm f/4.5
lens, cropped to full pixels:
It's a test worth doing at each stop to see for yourself how your lens
performs on your camera. I suggest shooting with the camera on a solid
table held down firmly as vibrations can mess up the results, even on a
all google groups messages filtered due to spam
From: David Ruether on 28 Dec 2009 18:32
"Paul Furman" <paul-@-edgehill.net> wrote in message news:hhan2r$d0c$1(a)news.eternal-september.org...
> David Ruether wrote:
>> For samples shot with a range of f-stops that show the difference
>> diffraction changing with aperture can make in an image, go here --
> Agreed on all that except the link only shows the star patterns and the images aren't big enough to show softness. Here's a very
> simple demonstration of softening, including non-diffraction softness wide, sharp at f/8 and diffraction softness at f/36, for an
> old 500mm f/4.5 lens, cropped to full pixels:
> It's a test worth doing at each stop to see for yourself how your lens performs on your camera. I suggest shooting with the camera
> on a solid table held down firmly as vibrations can mess up the results, even on a tripod. --
> Paul Furman
If you have a sharp monitor and assemble several images together
on it at once, you can clearly see the effects of diffraction softening
the image from about f5.6 as you go downward toward smaller
stops using the samples at --
http://www.donferrario.com/ruether/diffraction.htm, which are full
1:1 pixel size. Also, as you go from about f5.6 toward wider stops,
the images also get gradually softer.
From: Paul Ciszek on 29 Dec 2009 11:05
In article <n9kYm.75$mr6.10(a)newsfe08.iad>,
Martin Brown <|||email@example.com> wrote:
>> On Dec 22, 10:27 pm, "newshound" <newsho...(a)fairadsl.co.uk> wrote:
>>> "Martin Brown" <|||newspam...(a)nezumi.demon.co.uk> wrote in message
>>>> Paul Ciszek wrote:
>>>>> I only know undergraduate physics type optics; is it possible to arrange
>>>>> a photograph with an "ordinary" camera that shows diffraction ripples as
>>>>> seen in physics textbooks? Or does diffraction in photography just
>>>>> take the form of a generic limit on sharpness?
>>>> Yes. Just make the aperture mask small enough that diffraction dominates
>>>> the image quality. A pinhole in aluminium foil will do nicely.
>>>> Obviously you need a still life time exposure - preferably something with
>>>> a specular highlight or point source in it.
>>>> Martin Brown
>>> You can see "Newton's rings" looking at sodium lights through spectacles in
>>> the rain
>Or through the thread of umbrella fabric.
>> this thread now looks like war to me, is this usual
>There is no disagreement in the posts above they are examples of where
>you can see obvious diffraction effects with relatively little effort.
All of the examples given are ways to illustrate the phenomenon of difraction,
sure. In the first case you are modifying your camera into a pinhole
camera; in the other two, you are taking a picture of difraction occuring
somewhere else. It would appear that the answer to the question "Does
diffraction in photography just take the form of a generic limit on
sharpness?" is "Yes" for any normal (i.e., non-pinhole) camera.
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