DEFINITION (Fr., Definition ; Ger., Definition) The degree of sharpness with which objects are rendered by the lens. As a rough standard for purposes of comparison, it is generally assumed that the allowable " circle of confusion " shall be one-hundredth of an inch in diameter —that is to say, a circle of that size shall be accepted as a satisfactory rendering of what should be a point. This only holds good for contact prints from the negative, as it is obvious that any enlargement would increase the error proportionately and so bring it above the standard limit. Some forms of lenses give a curved or saucer-shaped field, so that when the centre is sharp the definition falls away towards the edges. This defect may be counteracted by using a smaller stop. Variation in definition also arises from the inability of a lens to bring to a focus objects on all planes at the same time. Improvement in this direction also is brought about by the use of a small stop. It is not always necessary or even advisable to have equally sharp definition in all parts of the subject, and judicious selective focusing is frequently of great advantage. Some lenses are specially con structed to enable the operator to introduce at will any required degree of softness or diffusion over the entire area. Such softness of definition
is often most effective in portraiture and in some classes of outdoor work. It is generally out of place in architecture, copying, and the rendering of subjects for scientific purposes. In such cases the standard of critical definition should be one two-hundredth of an inch or less, and this stand ard is easily attained by good-class lenses. This matter is pursued further in the article appearing under the heading "Depth of Definition," which should be read in conjunction with the above.
When spherical aberration is entirely absent the centre of the field will be so sharply defined that the most delicate sensitive film is too coarse to register the smallest details. The structure of the sensitive film varies from that of the daguerreotype, which is practically grainless, through albumen on glass, collodion, and slow gelatine emulsions, until the rapid gelatine emulsions which show a granular structure even when magnified only a few diameters. (For the extent of definition which may be reasonably expected from a given type of lens at full aper ture, see " Field of Lens.")