GB1517648A - Process and apparatus for automatically focussing an optical system - Google Patents

Abstract

1517648 Automatic focusing ERNST LEITZ WETZLAR GmbH 15 June 1976 [26 June 1975 (2)] 24719/76 Heading H4D In an automatic focusing system, e.g. for a microscope or a photographic camera, a grating 20 is movable in an image plane 19 by a distance exceeding a multiple of the grating constant, photo-electric receivers 23, 24, generating electric signals which are modulated in response to the movement of the grating 20 relative to the image for adjusting the focus of the image in plane 19. As shown, the signals are applied via a setting member 25 and amplifier 26 to a drive motor 27, which displaces the lens 13 or the table for automatic focusing in the direction of double arrow 28. The large movement of grating 20 compared with that in known systems compensates for variations in signal strength over the image plane. A blanking of the signals at the end points of the grating's movements prevents interference effects at the edge regions. The grating is periodic or aperiodic, in which case different spatial frequencies of the image can be evaluated. Amplitude or slit gratings or prismatic arrays may be used. A further embodiment utilizes a rotating disc on which several grating elements are arranged, Fig. 10 (not shown), and another embodiment uses an oscillating scanning disc with two rows of slits with different grating constants, Fig. 11 (not shown). A further arrangement, Fig. 9 (not shown), ascertains the phase difference from the signals obtained from two receivers allocated to diametral spaced pupil parts and the sign and magnitude of this difference are utilized to determine the direction and magnitude of defocusing. A light balance arrangement indicates to the user in which direction he needs to adjust the lens of the apparatus.