user the necessary information regarding imaging properties in re- 
lation to object structures. 
Most general objects are points - as every object may be regarded 
as being composed of points. Since Fourier Theory has been intro- 
duced into opties it is generally known that arbitrary objects may 
also be composed of periodic sinusoidal intensity structures. Thus, 
the imaging quality may be described by the point spread function as 
well as by the behaviour of the contrast as a function of the spatial 
frequency, the so-called optical transfer function (OTF). Of these 
two forms of representing the imaging quality, the OTF is said to 
have some important advantages compared with the point spread functio: 
One of these advantages is the easy combination of succeeding imaging 
steps by multiplying their OTF-values, thus deducing the overall 
imaging quality caused by image motion, graininess and scattering in 
the film, diffraction and aberrations of the lens, etc. 
The OTF does not only give the user the simplest means for describing 
the imaging quality, also the designer can see to what extent his 
tools of correction influence the desired imaging quality by calcu- 
lating the OTF from constructional data or from calculated or measure: 
aberrations. Thus one may say that the OTF is the simplest means to 
see quantitatively the effect of changing design parameters on the 
imaging quality. Y 
There are, of course, simpler tests than OTF measurements for control: 
ling during fabrication whether lenses of a series stay within certai: 
tolerances, for example, the measurement of focal distances or a 
test exposure of a three bar pattern. But OTF-values are necessary 
when deficiencies found in such tests shall be applied to generally 
shaped objects. | | 
Problems of OTF-measurements 
Often it is said that the OTF may principally be the ideal procedure 
to describe the imaging quality, however, it is practically not 
applicable, as: