You can use the diameter of the exit pupil calculation to determine how much relative light is available under different conditions. When we are in bright light our own eye pupil constricts to as small as 1mm in diameter. However, in very low light such as in the afternoon or early morning our eye pupil can expand to around 7mm. Our pupils open and close to allow enough light to see details and a sharp image.
The "exit pupil" on a binocular can be used to correspond to our own eye pupil. Generally a 5mm "exit pupil" will provide good brightness over many conditions. But in low light conditions it will not provide enough light to see as your own eye pupil may want. The result is a darker image and you may not see as much detail.
