1. An observing apparatus comprising: an illuminating unit that illuminates a specimen with a surface illuminant in which bright areas and dark areas are arranged alternately; an image-forming optical system that forms an image of light that passes through the specimen being illuminated by the illuminating unit; an imaging unit that picks up the image formed by the image-forming optical system; and an image of the surface illuminant that is formed on an imaging plane of the imaging unit by the image-forming optical system is blurred to an extent that a presence of the bright areas and the dark areas is recognizable, wherein a distance d between the surface illuminant and the specimen and a pitch P of contrasting of the surface illuminant satisfy an inequality P d ≤ 2 ⁢ ( 1 - 0.02 / NA 2 ⁢ ) where NA is an object-side numerical aperture of the image-forming optical system.