Hi List,
There has been several discussions about the qualities of lenses and,
notably the 55 f1.8. It is reported that some lenses are soft wide open and
improve when shut down. Here is my "simple explanation" that comes with years
of usage.
Diffraction
In SLRs, there are several sources of diffraction that can soften the
images. First is the diaphragm. As we stop a lens down, the hole becomes
smaller and, as it does, the greater percentage of the image is formed by the
diffracted light. A perfect lens would have its highest resolution wide open
and would be diffraction limited as it is stopped down. The APO-Ronar is one
such lens with a maximum aperture of f9.0.
Design diffraction. These are diffraction sources not related to the
diaphragm. Taking any typical lens and mounting it on a body, opening the back
and the shutter, these sources can be observed. Sighting from a frame corner
and looking through the lens (the film will see what you see) you will note
that the image may be blocked by the edge of the rear element and/or the front
rim of the lens (filters). This is a source of diffraction - out of focus
diffraction. Also, you will note, on most lenses, that the inner barrel of the
lens is visible. Even though it is blackened, even with a good shade, the
level of illumination is significant to project an "out of focus" image at the
film plane. This is loss of contrast in the corners or softness.
Lastly is the shutter its self. As the shutter curtain or blades travel
along the film plane, a certain percentage of the image will be impacted by
light being diffracted by edges. The narrow the slit width, the greater the
percentage of the image comes from diffracted light. Those that regulararly
use their cameras at speeds less 1/2000 (1/4000 and 1/8000) will notice this
effect, unless these are the only speeds they use. The only value of
1/4000-14000 shutter speeds is a higher flash sync speed usable for fill
flash.
In the case of the 55 f1.8, the secondary diffraction does not go away
until stopped below f5.6. At f1.8, the interior of the lens barrel is highly
visible, at f8, it is one remarkable lens. Our beloved 135 f2.5 SMC Pentax and
SMC Takumar does not fully come good until f11, although the level of impact
at f5.6 to f11 is low.
For me, one habit I developed early, I like to acquaint myself with any
new lens I purchase and do the simple test outlined above. At that point, I
determine "my" maximum optimum shooting aperture. For normals through wides,
it is f8 to f11 and for longer lenses it is f11 to f22.
I have seen test reports of some fast telephoto lenses (Canon in
particular) for sports usage that are very sharp wide to one stop down.
However at normal aperatures, their performance is well down from quality
normal slower lenses of the same focal length. These are special built lenses
for a special purpose. The ultra-fast (I believe f0.79) from Canon years ago
for their range finder was very ordinary at any aperature.
Of note, there are some fast lenses that I use regularly at wider
apertures. They, of course are the screw Takumars and SMC Ks, and are the 35
f2.0, 50 f1.4 and the 85 f1.8. I use them at f2.8 and click between f2.8 and
f4 and they provided sufficient sharpness for my use. However, for normal
work, the 35 f3.5 and 55 f1.8 and 85 f1.8 are my preferred optics.
For me, shooting a fast lens wide open is a loss of a frame. At times it
is the only way to get the image without a tripod - next time, take a tripod
or look for a suitable support!
Bob Rapp
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