The twilight factor

Some manufacturers claim that a combination of the magnification and objective lens size makes a greater contribution towards image brightness in dim light than the diameter of the exit pupil. This is known as the “twilight factor”, and is calculated by multiplying the magnification with the aperture and then get the square root of the product.
According to this approach a binocular with 10×50 specifications (twilight factor: 22.36) will render a clearer image than a 8×42 (twilight factor: 18.33), even though it has a smaller exit pupil (5 mm) than the 8×42 (5.25 mm).
The more light the instrument can gather, the better – irrespective of the size of the exit pupil.

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