Aperture stop

An imaging device, comprising an aperture stop for capturing both an observed image formed by fluorescent light from an observed area of a subject by irradiation of excitation light and a subject image formed by an illumination light, wherein:

• the aperture stop comprises a plurality of stop blade members each having a filter portion formed on a part of or an entire surface of the stop blade member, thereby defining a filter area formed by the filter portions of the plurality of stop blade members, and an aperture area positioned inside the filter area,
• the filter area transmits light in a wavelength range corresponding to fluorescent light from an observed area of a subject, and reduces or blocks transmission of light in a wavelength range corresponding to illumination light to the subject;
• the aperture area transmits the light in the wavelength range corresponding to the fluorescent light from the observed area of the subject, and the light in the wavelength range corresponding to the illumination light to the subject;
• and
• in a maximum area of an optical path at an attachment position of the aperture stop in the imaging device, the light in the wavelength range corresponding to the fluorescent light from the observed area of the subject is not reduced, and an area of the aperture area is variable.