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As other poster mentioned the amount of light captured is determined by the clear aperture of the front element.
Faster lenses require less exposure time because they let in light from a wider field of view that the front element allows in.
So 85mm F5.3 scope is around a 120mm lens F1.4 (120/1.4 = 86mm).
Fast lenses with longer focal lengths have large front elements.
Not al the light a scope or lens collects winds up in the image. Some falls outside the sensor and when you have a faster lens the light cone is steeper so more light lands on the sensor and gives a wider field of view.
Greg.
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