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grbrown
professor emeritus
Reged: 07/09/05
Posts: 643
Loc: Ampthill, UK, 52:02N 0:30W
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Re: Is aperture king? Does aperture rule?
12/19/06 02:31 PM
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Quite plainly they are linked. In the case of our BTs we have the benefit of 100mm objectives. We shall shortly own EPs that provide exit pupils of 5.8mm, 4mm, 2.8mm, 2mm and 1.4mm, yielding respectively magnifications of 17x, 25x, 35x, 46x, 66x (and in most cases 65+ degrees AFOV).
I can most readily identify with daytime use when comparing these. The 66x EPs are only comfortable to use in very bright sunlight, as the small exit pupil restricts illumination of the image. As light steadily declines, such as on a summer evening, successively larger exit pupils enable illumination to be maintained. So the reason we will shortly have five pairs of EPs is to maximise performance of our BTs during daytime bird-watching, pushing them at both ends of their range of capabilities (it is tempting to go to 1mm exit pupil, but I think that will be a step too far).
Our 25x EPs work remarkably well with their 4mm exit pupil in twilight, better than our 10x42 and 12x50 Trinovids. This is supported by using the Adler Index that EdZ introduced:
10sqrt42 = 65
12sqrt50 = 85
25sqrt100 = 250
When they arrive, our 17x should really open up twilight viewing, with their 5.8mm exit pupil, although the Adler Index works out at a mere:
17sqrt100 = 170
This topic has helped greatly in my modest quest, still ongoing, to quantify binocular performance in layman's terms that even I can understand and for comparing models.
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Graham
"one eye good, two eyes better...the more I look, the more I see"
BT100-45 degrees, plus 35,24,17,13,9mm Sieberts
15x80, 7x50 Steiners
12x50, 10x42, 8x20 Leica Trinovids
7x35 Minolta
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