Since the eye's pupil varies in diameter with viewing conditions, the ideal exit pupil diameter depends on the application. 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. A set of 7×50 binoculars has an exit pupil just over 7.1 mm, which corresponds to the average pupil size of a youthful dark-adapted human eye in circumstances with no extraneous light. 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). 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. However, the loss of light in the daytime is generally not a concern since there is so much light to start with. By contrast, 8×32 binoculars, often sold with emphasis on their compactness, have an exit pupil of only 4 mm. That is sufficient to fill a typical daytime eye pupil, making these binoculars better suited to daytime than night-time use. The maximum pupil size of a human eye is typically 5–9 mm for individuals below 25 years old, and decreases slowly with age as shown as an approximate guide in the table below.