These movements are spoken of as heliotropic and apheliotropic curvatures.
These movements are spoken of as different kinds of geotropic curvatures.
In like manner may be treated other cases, such as that of a primary wave-front of unequal principal curvatures.
The extension of the image away from the axis or size of field available for covering a photographic plate with fair definition is a function in the first place of the ratio between focal length and aperture, the longer focus having the greater relative or angular covering power, and in the second a function of the curvatures of the lenses, in the sense that the objective must be free from coma at the foci of oblique pencils or must fulfil the sine condition (see Aberration).
In the case of a single lens of glass with the most favourable curvatures, Sf is about equal to a 2 f, so that a 4 must not exceed off.