As stated earlier, exit pupil can be determined by the size of the objective divided by the magnification level. This is a basic concept but is not the complete story. With optics it is all about light transmission, not gathering. As light passes through a scope, some is filtered out due to poor glass and coatings. This is where cheap scopes fail in low light. Better scopes transmit a much higher percentage of the available light to the exit pupil. Much better to have a quality 40 mm objective than low quality 50 + mm one.
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