Measuring principle
With the standard autocollimation method for testing the adaptation of lenses the lens is adjusted so that the observer sees the image of a finely graduated test figure of an autocollimation telescope with identical sharpness to the eyepiece reticle of the autocollimator telescope. Alternatively, the color change method is used, in which a bright crosshair on a dark background is used instead of the finely divided test figure. The image of the crosshairs illuminated with white light in the eyepiece image plane of the autocollimation telescope shows color fringes if the objective is not correctly adjusted. With the long-established method, the sharpness of the image is not the adjustment criterion, but the symmetrical position of a single line within a double line. Thus, the different and subjective interpretation of the image sharpness has no influence on the measurement. The display of the image on a monitor and the type of symmetrical adjustment allow a largely fatigue-free working in mass production. The same instrument can also be used to adjust the objective in relation to the film plane. If, based on practical experience, adjustment to a finite distance is required, the testing instrument can be equipped with an attachment lens (1 m, 1.5 m, 2 m). With the procedure used, the setting is made at the centre of the image as is also the case with the autocollimation and collimation method. This will not in all cases be an optimum setting over the entire field of view because of off-axis errors (field of curvature, astigmatism, coma). Averaging of the image quality over the field of view will often be favourable of necessary. The deformation of film is additionally to be considered.
