The lenses in a portrait combination are occasionally removed from their cells for the purpose of cleaning. When the lenses are taken out of their cells, they may be variously transposed, and thus rendered incapable of producing good pictures. In a portrait combination there are four lenses in all,-the so-called front and back lenses being really each formed of a pair. The front ones are always cemented together, and may thus be easily taken for one lens; the back pair are distinct, and are usually separated from each other by a narrow ring. Take the front lens - the pair cemented together - and observe that one surface is considerably curved, and the other almost flat; place the lens in its cell, so that when screwed into the tube the curved side will be to the sitter. The two glasses forming the back lens are very unlike each other; one is thick at the centre and thin at the edge, the other thick at the edge and thin at the centre put the thin-edged one first into the cell, resting on the least curved side; next put in the ring, and then the thick-edged glass, concave side towards the other lens; fix them in their places with the part provided, and screw the cell in its place.

With many portrait lenses there is an arrangement whereby the front lens may be used as a landscape lens; to use it for this purpose proceed as follows Unscrew the back lens and lay it aside altogether, as it is only required in the double combination; then remove the brass hood before the front lens; next unscrew the front lens, and rescrew it in the place where the back lens was. In doing this the flat surface will be presented to the object. The lens tube may now be put on the camera, and the central stops will be in their proper place for use. As the focus of the front lens, when thus used singly, is much longer than when used in combination with the back lens, the picture it will yield is proportionately larger, but a much smaller stop must be employed than when the lens is used for portraiture. The exposure must be considerably longer than when the double combination lens is used.