Three factors determine the brightness of the image in dim light: The quality of optical coatings, a factor determined by the size of the exit pupil and finally a factor determined by the objective lens size and magnification.
Lens coatings: Not all the light arriving at the objective lenses at the front actually passes through – some is reflected. This happens with the light finally arriving at the viewer’s eyes at the other end of the binoculars as well – not all the light travels through the final two lenses either. On top of this the other 10+ glass surfaces inside the instrument contributes to scattering of light as well. This is where optical coatings come into their own: They reduce internal light loss and glare and ensure even light transmission, resulting in greater image sharpness and contrast. In this way lens coatings make a huge contribution towards a better image, in particular in dim light.
Size of exit pupil: The size of the final lens through which the image has to go is crucial, in particular as compared to the size of the human pupil. The smaller the exit pupil, the less light passes through. In bright daylight this is no problem at all, but in dim light, when the human eye needs as much light as possible to see an image clearly, the situation changes altogether. The ability of a binocular to transmit light in terms of the exit pupil is expressed as its performance on the relative brightness index (RBI). It is computed by squaring the exit pupil. For example, 7×35 binoculars have a 5mm exit pupil (35/7=5). So their RBI is 25 (5×5=25). A RBI of 25 or greater is considered good for use in dim light.
Objective lens size and magnification: Some manufacturers claim that a combination of the magnification and objective lens size makes a greater contribution towards image brightness in dim light. A factor, known as the twilight factor, is calculated by taking the square root of the product of the aperture and magnification (aperture x magnification). According to this approach a binocular with 10×40 specifications (twilight factor: 20) will render a clearer image than a 7×35 (twilight factor: 15.4), even though it has a smaller exit pupil (4 mm) than the 7×35 (5 mm).
A final remark on brightness of image in dim light: The bigger the objective lenses, the more light is gathered and the better the chances of getting a brighter image. For this reason binoculars used for star gazing have really big objective lenses. For the same reason pocket binoculars with their small apertures will not be as effective in dim light as normal size binoculars.