The "Cooke" lens is remarkable for the simple means by which the various corrections are made, consisting as it does of only three single lenses separated from each other. Obviously it must be used entire. These lenses do not cover so large a plate in proportion to their focal lengths as most of the other anastigmats, but perform excellently over the plates for which they are constructed.
The "stigmatic" of Dallmeyer is the latest lens of general utility. It gives good definition to the margin of the circle of light that it transmits, reduction of aperture being necessary, when its full field is employed, to get equality of illumination rather than to improve the marginal definition. Its two combinations are different, and either may be used alone as a single lens, giving focal lengths of approximately one-and-a-half and twice the focal length of the whole lens.
The "planar" of Zeiss introduced just as we write, is a symmetrical doublet characterized by a very large aperture, from _f_/3.6 to _f_/4 up to 10 inches in focal length, and a little smaller above that. It is therefore comparable with portrait lenses. Although it is symmetrical, a single combination cannot with advantage be used alone as a single lens. Telephotographic lenses are subsequently referred to.
The one aim of opticians in improving photographic lenses has been to get good definition all over a comparatively large flat surface without having to use small apertures. A defining power on the axis of the lens, that is, at the centre of the field, far exceeding what can be taken practical advantage of in ordinary photography, has long been possible. But until recently, the defining power always rapidly deteriorated as the distance from the centre was increased. But to judge of the quality of a lens, or to compare one lens with another, there are other matters that must be understood, and these we shall proceed to consider. Focal length, aperture and image angle are the chief details concerning lenses, granting that the aberrations referred to above are satisfactorily corrected.
_Focal length._--The focal length or focal distance of a thin lens is the distance between it and the point to which it converges parallel rays. The rays of light are parallel when they issue from an object at an infinite distance. For ordinary practical purposes, any object, that is not nearer than a thousand focal lengths of the lens may be regarded as at an infinite distance, that is the image of an object so far off, and the image of the sun or stars (which are situated at the nearest approach to an infinitely great distance that we know of) would if separately focussed give an inappreciably small difference of position of the focussing screen. But no photographic lens is very thin. The measurement from the back surface of the lens to the screen, when focussed on a distant object, is called the "back focus," but this is of no use whatever except as to the determining of the camera length necessary. The "equivalent focal length" is the focal length (or focal distance) of a thin lens that would give the same effect, so far as focal length is concerned, as the lens in question. When the simple expression "focal length" is used, it always refers to the equivalent focal length. The single word "focus" is sometimes used erroneously instead of "focal length."