What are the keys for image quality?
A binoculars ability to gather and transmit enough of the available light to give a sufficiently bright and sharp image defines its brightness. The brightness of a binocular also enhances colour differentiation in the image observed. R.B.I. (relative Brightness Index, Twilight Factor and R.L.E. Relative Light Efficiency are common indices used in the binocular industry, but are all somewhat flawed in their design and often prove fairly meaningless. Simply square the exit pupil – if your binocular has an exit pupil of 4mm, the relative brightness is 16 ( 4 x 4= 16). CONTRAST: Refers to the degree to which both dim and bright objects in the image can be differentiated from each other and from the background on the image. High contrast helps in observing fainter objects and in discerning subtle visual details. High quality optical coatings provide better contrast in an image. The other factors affecting contrast are: collimation, air turbulence, and objective lens, prism and eyepiece qual
