No lens is 100 0.000000e+00fficient at transmitting light, so one can speak of a
t/ stop, which is the marked value adjusted for light loss. Also, lenses
may or may not give you the maxiumum aperture they claim. The Popular
Photography and Modern Photography tests at:
http://brashear.phys.appstate.edu/lhawkins/photo/mp-cat-tests.txt
http://brashear.phys.appstate.edu/lhawkins/photo/mp-zuiko-tests.txt
http://brashear.phys.appstate.edu/lhawkins/photo/pop-photo-zuiko-tests.txt
give this kind of information. Note that as the number of elements
increase, so does light loss. This is such a significant factor in movie
camera zoom lenses that they are sold based on their maximum t/ stop.
Another kind of light loss is from fall-off, a.k.a. vignetting (although
not the kind you get from too deep a filter or too long a lens hood).
Since a mirror lens only has one aperture, fall-off is a significant
factor in any picture taken with these. A spot meter will tend to
underexpose when you use a mirror lens, since the center is the
brightest area of the lens image. Best to use these with full frame
averaging.
>From the first URL above, it looks to me like the Vivitar 600mm Solid
Cat has pretty significant light loss.
Gary Reese
Las Vegas, NV
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