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Re: [OM] 21/3.5 an interesting lens

Subject: Re: [OM] 21/3.5 an interesting lens
From: dreammoose <dreammoose@xxxxxxxxx>
Date: Thu, 07 Mar 2002 02:18:43 -0800
The problem is that light does funny/interesting things when it passes through small holes, it spreads out, rather than going straight. (watch ocean waves go through the narrow opening of a breakwater from above). When the hole through the aperture blades is big enough, the amount of light affected at the edges is tiny and doesn't noticably effect the picture. When the hole gets small enough, the picture deteoriates. And, of course, the opening of the aperture blades becomes more of a polygon than a circle at small apertures, and sometimes less even in shape - goodness knows what that does. The thing is, this size of hole issue has nothing to do with the wide open aperture of the lens or its focal length, it's just a local physical effect of light going through the hole formed by the aperture blades. In a simple lens, a 1.3mm hole is f16 for a 22mm lens, f18 for a 24mm, f38 for 50mm, etc.

With complex, multi-element retrofocus designs, there are undoubtedly factors that complicate this, probably including physical difficulties getting the diaphram in the optically ideal place, but the basic principle is that longer focal length lenses can use smaller focal rations. As with DOF, degradation is a continuum, not a clear demarcation, so some judgement is used by the designer. In the case of a shift lens, the DOF or other needs of the lens design and wide shifts, and its uses may dictate in favor of going a bit 'closer to the wall' than in a more conventional lens. All the regular 18-28mm lenses only go to f16.

Moose

M. Lloyd wrote:

If Diffraction is bad at f22, why does the 24 shift
have a stop at f22 considering that it is also an f3.5
<snip>



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