At 03:45 8/12/01, Chris O'Neill wrote:
I haven't a clue what you mean by absolute aperture, but most of my
attempts have been made with either a 50/1.4 or a 24/2.8 lens.
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:
50/1.2 = 41.7mm
50/1.4 = 35.7mm
85/2 = 42.5mm
100/2 = 50.0mm
135/2.8 = 48.2mm
200/4 = 50.0mm
300/4.5 = 66.7mm
Since amount of light is proportional aperture area, not diameter, the
relative increase in light capture is a ratio of their squares. Using the
50/1.4 as a baseline:
50/1.2: 1.36x
50/1.4: 1.00x
85/2: 1.42x
100/2: 1.96x
135/2.8: 1.82x
200/4: 1.96x
300/4.5: 3.49x
With normal terrestrial photography, it's the f-number that counts because
the light is spread out as you increase magnification and reduce the angle
of view. However, with astrophotography of stars an increase of focal
length on a miniscule pinpoint of light still leaves you with a miniscule
pinpoint of light the same size and brightness. Thus, exposure is related
totally to absolute aperture diameter, not the f-number, and I need less
exposure time with the 300/4.5 than with any of the other lenses. Exposure
time with the 50/1.2 and 85/2 wide open is approximately the same. All
focal length does is determing how much of the sky you take in. This
doesn't count for larger objects, such as the sun and moon, which you can
magnify in size. [DON'T photograph the sun unless you know EXACTLY what
you are doing; Galileo permanently blinded one of his eyes looking at it
with a very simple, very low power telescope.]
Now for the rub, which forces a tradeoff for those with fixed
tripods. Earth rotation effect is magnified with focal length. You can
get away with a longer exposure using the 50mm than you can with the 300mm
before you see rotation effects using a fixed camera mount. Using Roger's
25 second rule:
50mm = 25 seconds
85mm = 15 seconds
100mm = 12 seconds
135mm = 9 seconds
200mm = 6 seconds
300mm = 4 seconds
Unless you have a method of very accurately tracking the rotation, the
increased focal length in the series of lenses listed above forces a
reduction in exposure time faster than the increase in absolute aperture
increases the light capture from faint stars. The 50/1.2 wide open is
optimal for capturing the faintest stars before rotation kicks in and
produces noticeable star trails. Using Roger's 25 second limit for the
50mm, the equivalent exposure wide open compared to the 50/1.4 using the
limiting by Earth rotation:
50/1.2 = 25x1.36 = 34 seconds
50/1.4 = 25x1.00 = 25 seconds (doh!)
85/2 = 15x1.42 = 21 seconds
100/2 = 12x1.96 = 25 seconds
135/2.8 = 9x1.82 = 16 seconds
200/4 = 6x1.96 = 12 seconds
300mm = 4x3.49 = 14 seconds (4s with 300/4.5 better than 6s with the
200/4)
You will record fainter stars, given a dark enough sky, with the 50/1.2
lens wide open (equivalent to holding the 50/1.4 open for 34
seconds). Second best is the 50/1.4 and 100/2, followed closely enough by
the 85mm lens to put them in the same class. The 135, 200 and 300 are less
than the 50/1.4, 100/2 and 85/2 but aren't that far apart from each
other. If photographing constellations I would use the 50/1.2 (50/1.4 if
no f/1.2 version) for the larger ones. For smaller constellations the
100/2 or 85/2 should work.
Note that running some lenses wide open can result in stars near frame
edges, particularly bright ones in corners, having an odd shape. Many will
stop down at least one stop.
-- John
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