A portrait lens is a leus so constructed as to give fair definition with a large aperture. Rapidity of working is the most important quality of a portrait lens, and to obtain this certain sacrifices have to be made. Sharp definition, which is generally undesirable, is obtained only at the expense of modelling. Roundness of field is a defect common to portrait lenses, but the newer and more expensive lenses are a vast improvement on the older and cheaper kinds. A good rectilinear lens is better than a bad portrait lens. A single lens, or one half of a rapid rectilinear lens, makes a good portrait lens, but it is slow.