A light quantity measuring system with a function of controlling an exposure comprises a capacitor an anode potential of which drops from a predetermined level to a set-up reference level with a rate of change responding to a quantity of light incident on a photodiode through which a discharging current of the capacitor flows. A counter counts a pulse signal with a set-up repetitive frequency, the counted number of which represents a quantity of the light. A counting condition setting means sets the reference level and the repetitive frequency in response to a counted number in the counter so as to prevent an overflow of the counter. A calculating means calculates a quantity of light based on the counted number, the reference level and the repetitive frequency.
An output current of a photodiode is charged in a capacitor, and the voltage change of the capacitor is monitored by a plurality of comparators. The time required for the capacitor voltage to reach a predetermined value is counted by using a clock signal input unit for inputting a clock signal of a predetermined frequency, and a counter for counting an upper part of a number corresponding to a logarithm of a number of clocks of the clock signal generated after the integration start by a photoelectric conversion unit. The outputs of the comparators when the counter counts the upper part are latched, and the lower part of the number is supplied.
A method for measuring a rate of change of frequency includes estimating at least one phasor value, estimating a frequency based on the estimated at least one phasor value, compensating the estimated at least one phasor value based on the estimated frequency, calculating a frequency based on the compensated at least one phasor value, and calculating a rate of change of the calculated frequency based on the calculated frequency.
