A device and a method for observing an object by imaging, or by lensless imaging. The object is retained by a holder defining an object plane inserted between a light source and an image sensor, with no enlargement optics being placed between the object and the image sensor. An optical system is arranged between the light source and the holder and is configured to form a convergent incident wave from a light wave emitted by the light source, and for forming a secondary light source, conjugated with the light source, positioned in a half-space defined by the object plane and including the image sensor, such that the secondary source is closer to the image sensor than to the holder. This results in an image with a transversal enlargement factor having an absolute value of less than 1.