TDDZ> mirror lens just projects a darker image on the focusing screen compared
to
TDDZ> the "rocket launcher".
Is the exposure between them really the same? Due to the central
mirror obstruction, many catadioptric lenses have reduced true speed
compared to a refractive lens, unlike the f/number would suggest.
T/number (transmittance "f/stop" as used in movie industry) would give
a better representation. For example, many of the small 500/8 mirrors
are in fact 500/11 or so. I think the Sigma would be more true to the
transmittance of the lens, unlike the cheap 30$ cats, but still there
might be a small difference. Also, the slightly out of focus image
from the refractor might be more easily identified by eye than a slightly out
of focus image from the catadioptric lens, based on their vastly
different characteristics. Although this is speculation. To put it in
other words, perhaps the refractor's image snaps into focus more
easily. Contrastier, maybe. Cats usually have lower contrast.
Good light!
fra
