Patent GB-1465074-A (GB)
Title: Focus device
IPC: G02B7/28
Filed: 1974-03-04
Abstract: 1465074 Determining focal plane photoelectrically ASAHI KOGAKU KOGYO KK 4 March 1974 9681/74 Heading G1A Apparatus responsive to the focal sharpness of a lens comprises a photo-cell in the optical path of a lens for producing an output representing the focal sharpness; means for converting the photocell output into frequency variation of an A.C. signal; and means responsive to the frequency of the A.C. signal. The resistance of a CdS photo-cell attains a maximum P, Fig. 1A, when an image formed thereon by a lens becomes sharply focused. If, alternatively, the photo-cell is spaced from the predetermined focal-plane its resistance takes some other value, e.g. P<SP>1</SP> which may lie in a region where the resistance has a substantial derivative with respect to lens displacement. Photo-cells 203, 204, 205, Fig. 5 responsive to focal sharpness constitute the tuning resistors of a phase-shift oscillator, of which the frequency is converted to an amplitude by a resonant circuit 221 tuned to resonate at that frequency produced by the oscillator when the image is sharply focused in the predetermined focal plane. The output of the resonant circuit 221 is amplified by transistors 224, 227 provided with a threshold-setting potentiometer 228 and applied to an indicating meter 230, and to a servo-mechanism (not shown) for shifting the lens.  Other forms of oscillator include a Wien bridge wherein a pair of photo-cells constitute the tuning resistors Fig. 6, (not shown), and an L-C oscillator having tuning elements mechanically coupled to a current meter controlled by a photo-cell Fig. 8, (not shown). The variation of oscillator frequency in response to photo-cell resistance may be enhanced by tuning the oscillator with resistive elements, e.g. bi-polar or field-effect transistors, controlled non-linearly by the photo-cell resistance Fig. 17 (not shown). The frequency-sensitive means may comprise a pair of resonant circuits tuned to slightly different frequencies and compared in a differential amplifier Fig. 23 (not shown); this circuit may further include relays respectively to switch out each resonant circuit when the other approaches resonance Fig. 24 (not shown). Other frequencysensitive means include low-pass and high-pass L-C filters Figs. 9, 11, (not shown); an active Wien-bridge filter Fig. 6 and active and passive twin-T filters Figs. 13A, 13B, (not shown).  Compensation for ambient illumination of the object may be provided by a compensating photocell illuminated with diffused light from the object and applied either to restore the oscillator frequency to a reference value or to vary the tuning of the frequency-detecting means in step with the oscillator. In Fig. 5 a compensating photo-cell 244 controls a current meter 243 mechanically coupled to diaphragms 246 in the path of the sensing photocells 203, 204, 205; alternatively the meter 243 may be coupled to the tuning capacitors 209, 210, 211. In Fig. 20 (not shown) a compensating photocell (731) is compared with a detecting photo-cell (708) in a differential amplifier, the output being used to control transistors (736, 737) determining the frequency of a Wien-bridge oscillator. In Fig. 4 (not shown) the compensating photo-cell (135) controls a potential applied to a variablecapacitance diode (139) in a resonant circuit (121) of the frequency-sensing means. In Fig. 6 the detecting photo-cells (366, 367) control a Wienbridge oscillator while a pair compensating photocells constitute the tuning resistors (376, 377) of an active Wien-bridge filter.