Ross’s homocentric lens is one of the most useful lenses. It has a flat field, is free from what is called “coma” and astigmatism, and is so perfectly corrected as to fit it for interior and exterior work alike. It is also a rapid hand-camera lens. The homocentric are made in series to suit all cameras. Other lenses may also be noted, viz.: _c._ Dallmeyer’s rapid rectilinear, including about 37°. _d._ Zeiss’s anastigmat, made by Ross, consists of a double front lens and a triple back lens. It is intended for portraits, groups, copying, and general outdoor work. The combinations being brought closely together, gives them great illuminating power. They have an angular aperture of from 858 to 908, and can therefore be used as wide-angle lenses when desired. In consequence of the peculiar system of correction for oblique pencils adopted in these lenses, they behave somewhat differently from the usual types with regard to the mode of compensating the effect of the resulting aberrations between centre and margin of the field. This is, of course, only possible in the case of perfectly plane objects. In all other cases—landscape, instantaneous work, or interiors—the centre should be focussed rather than objects at a distance or foreground. _Focus._—It will be sufficient to say that focal length means the distance between the diaphragm of a lens, and the ground-glass screen when the image of a distant object is seen most distinctly on the screen. _Exposure Tables._—Exposure tables are based on the focal length of a lens, in relation to the diameter of the diaphragm of a lens. Thus, if the focus is eight inches and diameter of diaphragm one inch, the relationship will be expressed by _f_/8 or by the uniform standard number 4, and so on, as in table. The diaphragms are so arranged for size of opening that each succeeding number requires double the exposure necessary with the one preceding it. _f_/11.3, for example, requires double the exposure wanted with _f_/8. U. S. Nos. 4 8 16 32 64 128 256 _f_ _f_ _f_ _f_ _f_ _f_ _f_ Ratio of Stops. --- ---- --- ---- --- --- --- 8 11.3 16 22.6 32 45.2 64