Depth of field in microscopy refers to the thickness of the specimen that is acceptably sharp at a given focus level. It is the range of distance in which the object can still be clearly observed when the distance between the lens and the object is slightly varied. In microscopy, depth of field is very short and usually measured in units of microns. The term depth of focus, which refers to image space, is often used interchangeably with depth of field, which refers to object space. Depth of field varies with numerical aperture and magnification of the objective, and under some conditions, high numerical aperture systems (usually with higher magnification power) have deeper focus depths than do those systems of low numerical aperture, even though the depth of field is less.
In microscopy, depth of field is particularly important in photomicrography because the film emulsion or digital camera sensor must be exposed or illuminated in a plane that falls within the focus region. At high numerical apertures of the microscope, depth of field is determined primarily by wave optics, while at lower numerical apertures, the geometrical optical circle of confusion dominates the phenomenon.
In summary, depth of field in microscopy refers to the range of distance in which the object can still be clearly observed when the distance between the lens and the object is slightly varied. It is very short in microscopy and varies with numerical aperture and magnification of the objective.