![]() There are rare cases where I've seen maximum detail at exit pupils as big as 4 mm. I usually see a smidge more galaxy detail at 1.5 mm, but the gain often isn't worth the loss in field of view. I generally prefer an exit pupil around 1 mm for globular clusters and small planetary nebulae, a bit wider for open clusters (assuming that the cluster fits in the FOV), and 1.5 to 2 mm for galaxies. Different exit pupils for different kinds of objects, and also for different observers. The maximum pupil size of a human eye is typically 5 – 9 mm for individuals below 25 years old, and decreases slowly with age after that.įor deepsky observing ('faint fuzzies') is there a ideal exit pupil?Ībsolutely not. That is sufficient to fill a typical daytime eye pupil, making these binoculars better suited to daytime than night-time use. ![]() By contrast, 8×32 binoculars, often sold with emphasis on their compactness, have an exit pupil of only 4 mm. However, the loss of light in the daytime is generally not significant since there is so much light to start with. In daylight, when the eye's pupil is only 4 mm in diameter, over half the light will be blocked by the iris and will not reach the retina. The emergent light at the eyepiece then fills the eye's pupil, meaning no loss of brightness at night due to using such binoculars (assuming perfect transmission). A set of 7×50 binoculars has an exit pupil just over 7 mm, which corresponds to the average pupil size of a youthful dark-adapted human eye in circumstances with no extraneous light. An astronomical telescope requires a large pupil because it is designed to be used for looking at dim objects at night, while a microscope will require a much smaller pupil since the object will be brightly illuminated. Since the eye's pupil varies in diameter with viewing conditions, the ideal exit pupil diameter depends on the application.
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