Much more marked spreading of the image is caused by such defects as traces of aberration, always present even in the best lenses, and from slight faults in focussing.
The limit of resolution of a photographic image is the smallest distance between the images of two points, or two parallel lines, which can be seen separately under a magnification such that the individual grains of the image are not more apparent than the image itself.' The limit of resolution diminishes in the case of very short or very long exposures ; it varies quite apprec iably (within a ratio of 1.5 to I) according to the thin layer, the use of radiations which are strongly absorbed by the film (the extreme violet or ultra-violet), dyeing the emulsion with some colouring matter which strongly absorbs the active radiations (yellow in the case of ordinary emulsions), and, finally, physical devel opment or partial chemical development.
The table given below indicates the number of lines per millimetre which can generally be developer and, to a lesser degree, with the contrast of the model ; it varies with the nature of the emulsion but without any direct relation with the grain size. An image localized in the surface of the emulsion gives greater resolution than one formed throughout the whole thickness of the emulsion coating. Thus all factors which tend to limit the utilizable thickness of emulsion film lower the limit of resolution. Notable
examples of this are coating the emulsion in a separated by different types of photographic materials 206. In addition to the contractions and distortions which occur in photographic images on film or on paper because of the nature of the support, errors of position may also arise in photographic images on rigid supports on account of contractions and distortions of the gelatine itself during drying, due to the action of the oxidation products of the developer. Further errors of position may arise due to the unsymmetrical intensification of the borders of images close together, causing a mutual attrac tion between neighbouring images.
These different sources of error need only be considered in scientific photography and photo cartography The mutual influence of adjacent regions of an image is generally lessened by using developers free from solvents of silver salts (e.g. ferrous oxalate). The effect of drying is reduced when this takes place slowly and uniformly in a moist atmosphere, or when it is accelerated by the use of alcohol. The effects of irregular drying can be overcome by washing the plate afresh and drying it under suitable conditions. Accurate measurements should never be made on the marginal regions of a photographic plate, or anywhere within about half an inch of the edge.