Antispectroscopic. As applied to a lens. Synonymous with Achromatic.
Aperture, Focal, of a Lens, or Working Aperture. By
this term is meant such clear way of the lens as is actually utilised in impressing the image of the plate. Many persons suppose that the working aperture of the lens is the diameter of the diaphragm, others that the whole surface of the lens is always utilised ; but this is not so. The following method will prove the working aperture of any doublet lens, which varies with each separate diaphragm: - Rack the camera out to the true equivalent focus of the lens; replace the focussing screen by a sheet of cardboard, in the exact centre of which is a minute hole (a pinhole will do); behind this, exactly level, place a strong light, such as a paraffin lamp, and it will be found on looking at the lens that when a diaphragm is inserted in the slot a central portion only of the lens is illuminated. This can be easily seen by breathing upon the lens surface. It should be accurately measured, and this area will be the true working aperture of the lens with the diaphragm used. This area of illumination will be found to differ in geometrical proportion with each separate diaphragm. The working or effective aperture of the lens divided by the focal length is called focal aperture. Thus, if the effective aperture of a lens is I in. (not necessarily quite an exact inch diaphragm measure), and the focal length is 8 ins., the
focal aperture will besometimes for shortness it is expressed
simply as 8. (See Lens and Exposure, and more particularly the fuller account of the Focal Aperture system given under Diaphragm.)
ApheilgOSCOpeI make bright). A device by which
opaque objects are exhibited by means of the optical lantern.
Aplanatic (a, negating particle, and I lead astray).
A term applied to a lens to denote that spherical and chromatic aberration have been eliminated so far as is practicable; it is impossible to do it theoretically. Practically it means that a lens will give reasonably sharp definition with its full aperture. (See Lens.)
Apochromatic. See Achromatic.