image brightness

Definition: Brightness of the image as seen through an optical system. * Apparent image luminance as viewed in an instrument. * It varies with the object luminance, the optical system's transmittance, diameter of the exit pupil of the system and, indirectly, contrast of the system (as a contrasty image appears brighter than a low contrast one).
Brighter manual focus
Image brightness is not just about comfort i.e. the brighter the image, the easier it is to see and compose it, while colour appreciation is also improved. But it is additionally about focus accuracy. As image brightness increases, so it is easier to see small differences in contrast -- it is this variation in contrast which is crucial to the accuracy of manual focusing.

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