PORTRAIT LENSES Almost any lens working at an aperture of f/6 or larger is suitable for portraiture provided that its focal length is not less than times that of the longest side of the largest plate which it will have to cover. The name is, however, by common consent confined to the Petzval form, known as the ” ordinary " portrait lens, and also to the Dallmeyer, or " patent," portrait lens. The principal features of this type of lens are great rapidity, some lenses having an intensity of f/2, and extremely good definition over a moderate angle. The field of a portrait lens is usually curved, and this may in some cases be regarded as an advantage, inasmuch as it allows the knees and feet of a sitting figure to be focused equally well with the head without reduction of aperture. The Dallmeyer type of portrait lens has a great advantage over the original Petzval, as it allows the focus to be softened and distributed over several planes by turning the back cell, and thus increasing the difference between the two glasses which compose the back combination. Starting with absolute sharpness, the definition may be softened by degrees until fuzziness ensues by simply rotating the back cell. The latest model of this lens has the back cell fixed, and the softness is obtained by rotating the entire lens tube. The front combination of most portrait lenses may be used alone, and if they are to be employed for portraiture it is advisable that they be left in their normal position—that is to say, with the convex side to the sitter.
By so doing sharper definition over a limited, but sufficient, field is obtained, without serious reduction of aperture. Should it be desired to use the front lens for landscape work, it is desirable that the flat side should be turned to the view, and the diaphragm dosed until the desired sharpness is obtained. Portrait lenses may be used for outdoor work, and latterly have been in demand for reflex cameras. Portrait lenses are liable to give " flare spot " when used out of doors with small apertures, but this tendency is minimised if the diaphragm is placed in front of the lens instead of in the ordinary position. In selecting a portrait lens care should be taken not to choose one of to short a focal length. If possible this should be about twice that of the trimmed print, say 8 in. for cartes, in. for cabinets, and 20 in.
to 24 in. for panels. Besides the recognised portrait type, special portrait lenses are made on the Cooke model, and these have the " diffu sion of focus " adjustment. Most enlarging objectives are of the Petzval portrait lens cone struction, and may be used in the camera with good results. Magic-lantern lenses are also of this type, but are usually corrected for the visual rays only, and will not give a sharp image on the photographic plate.