PHOTOGKAPHY. 
57 
image with great depth of focus.) c. Dallmeyer’s rapid rectilinear, of about 
11 in. focus, including about 37°. d. Zeiss’s anastigmat, made by Boss, 
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, instan¬ 
taneous work, or interiors—the centre should be focussed, and rather for 
objects at a distance than for near objects. Considerable economy may 
be effected by purchasing lenses second-hand from respectable dealers, 
such as Messrs. Watson & Son, 313, High Holborn; Mr. Morley, 70, Upper 
Street, Islington, or Messrs. Hunter & Sands, of 20, Cranbourne Street, all 
of whom can be recommended with confidence. 
Focus .—In place of giving a strictly scientific definition of the term 
focus or “ focal length ” applied to a lens, it will be sufficient for the 
scope of this paper to say that focal length means the distance between 
the diaphragm of a rectilinear or symmetrical lens, and the ground-glass 
screen of a camera. That is when the image of an object, say one 
hundred yards in front of the lens, is seen most distinctly on the focussing- 
screen of the camera. 
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 relation- 
f 
ship will be expressed by E or by the uniform standard number 4, and 
8 
so on, as in table. 
U. S. Nos.......’ 
4 
8 
16 
32 
64 
128 
256 
/ 
/ 
/ 
/ 
/ 
/ 
f 
Eatio of Stops. 
8 
H ’3 
16 
22*6 
32 
45-2 
64 
