What Roger is talking about is the absolute diameter, not the
f-number. Thus, if I have the following lenses, the absolute aperture
diameters, wide open are:
...
100/2 = 50.0mm
135/2.8 = 48.2mm
200/4 = 50.0mm
300/4.5 = 66.7mm
Hmm makes my 180 f2... very interesting...
Absolute aperture:
-> 90mm
... wow.
Amount of light multiplication factor / ratio:
-> 6.36 x
another wow...
Equivalent exposure:
-> 6.36x ~7 = 44.5 seconds
Better than the 50/1.2 even. Nice exercise in calculation actually. I've
seen a photograph of Orion's belt with a 180/2.8 and even that was
impressive in that a small black area corresponding to the Horsehead nebula
was distinctly visible as was the Orion nebula. I'll have to try the 180/2
out.
Even more promising seems the fact that the 180/2 has absolute chromatic
aberration correction (it has no infrared mark even), so that star images
should be even more pointlike and not affected by a red ring effect.
Thanks for the inspiration John.
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