The photographic plate: Spatial resolution
 

If an exposed plate is examined under a microscope, it will be seen that the images are made up of individual blackened grains. Any seemingly sharp edge in the image is fuzzy on the scale which depends on the grain size. Microscope views of star plates reveal that each image is made up of an agglomeration of grains with a central condensation; the edges of images are not well defined. It is the grain sizes in the emulsion which normally determine the limit of any spatial resolution. The large choice of emulsion types embraces plates covering a wide range of graininess and, hence, resolving power. Quantitatively, the resolving power of any particular type of plate is determined by forming the image of a grid which has numbers of black and white ruled lines on it at different separations. The resolving power, R, is defined as the highest number of ruled lines per mm that can be resolved. The various emulsion types give a range in R from about 50 to 1000. In any photographic observation, it is important that the value of R is chosen so that all the detail which is provided by the telescope and other optics is recorded without any deterioration.
Because of the large plate formats that are available, the photographic process scores well when large amounts of spatial information need to be recorded. Each collection of grains within an area equivalent to ∼1/R2 acts as a resolved picture element or pixel. Thus, for a plate measuring 100 mm × 100 mm with a value of R = 100, the plate contains

an exceedingly good number in comparison with other detectors.