Image sharpness/brightness - Binoculars
Binocular manufacturers are always doing their utmost best to produce binoculars that yield the best possible images.
Image sharpness/brightness must be as high as possible so that even the minutest details of the object are clearly discernible in the image that the viewer observes through the binoculars. All the optical and mechanical defects that are in binoculars reduce the clarity or brightness and/or the contrast of the image and consequently the details you observe as a viewer.