Device (1) and method for delimiting a field (3) impinged with light from a light source (2), particularly on the surface of an optical element (4) and/or sample (5), having a slit diaphragm (6) including a slit height (7) and a slit width (8), which includes first and second linearly movable slides (9, 9′), positioned parallel to one another on two separate parallel lines, which are movable at least partially symmetrically to one another in relation to an optical axis (10), each of the two slides (9, 9′) including at least one optical opening (11, 11′), which—for continuous adjustment of at least the slit height (7) or the slit width (8)—may be positioned at least partially on the optical opening (11′, 11) of the neighboring slide (9′, 9) in the region of the optical axis (10). The device (1) according to the present invention and the method according to the present invention are distinguished in that the device (1) includes a motor drive (15) having an axis of rotation (16) for moving the two slides (9, 9′) in a movement direction (19), this axis of rotation (16) of the motor drive (15) being positioned perpendicularly to this movement direction (19).