In article ,
Paul van Walree wrote:
>In <5ptq2h$44e@nntpb.cb.lucent.com> rma@clockwise.mh.att.com (Robert M Atkins
) writes:
>
>>You do get more DOF at f22, and the sharpness of the DOF limits are about
>>the same as at f8, but the maximum image sharpness is lower, i.e. the
>>sharpest part of the image won't be as sharp at f22 as it is at f8.
>>Stopping down past about f22 you start to have trouble with the
>>standard DOF formulae because they are based on simple geometric optics
>>which ignore diffraction, and at f stops smaller than about f22 diffraction
>>starts to become the dominant factor in determining DOF and sharpness.
>
>I am inclined to say, first, that this f22 figure should be very much
>dependent on the focal length of the lens. For tele's, f22
>still corresponds to a large physical opening, and hence suffers
>much less from diffraction than f22 at a wide-angle lens.

No, you are getting confused here between angular resolution and
linear resolution (diffraction limited spot size). ANGULAR resolution
depends on the physical size of the aperture, but LINEAR resolution
depends on the ratio of the physical aperture size to the focal length,
i.e. the f-stop. At f22 both a 14mm and an 800mm lens have the same
diffraction limited spot size (resolution) in the film plane. They
give equally sharp (or fuzzy) images. They should both resolve about
70 lp/mm in the aerial image. However the angular resolution of the
14mm lens will be about 180 seconds of arc, while the 800mm lens
should resolve about 3 seconds of arc. This makes sense - what a
telephoto does is show more detail in a distant object.

>Second, I am not sure that it is only the sharpness that is affected
>by stopping down a lens. Diffraction of light at the diaphragm blades
>is gloriously observed in night photography, where small, bright
>light sources give rise to star-shaped images over the dark
>background, with a symmetry that corresponds to the symmetry of
>the diaphragm. Instead of showing up via a circle of confusion,
>diffraction thus gives rise to a `star of confusion' for each
>object point. Thus I would say that the primary degradation
>of the image occurs because the contrast is lowered. Of course,
>images with inferior contrast appear less sharp, so that contrast
>and sharpness cannot be considered as independent quantities in this
>regard.


Yes, in a way. However, circles of confusion are still pretty much circles as
far as resolution determination goes, The diffraction "spikes" are
*very* weak (which is why you only see them on VERY bright point sources.
They may lower contrast slightly, but it's a very secondary effect.
The principle cause of resolution degradation is the increase in the
diffraction limited spot size.


Bob Atkins - Murray Hill, NJ
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