The invention relates to a densitometer in which the slope of the density curve is adjustable. The densitometer provides a rotatable grey wedge disc and a rotatable chopper disc disposed so that a measuring light beam can pass through both discs and an object to be subjected to measurement and a reference light beam can pass only through the chopper disc. Both beams then image on a photoelectric detector which feeds corresponding signals to a comparator. When a starting position on the rotatable grey wedge disc passes a predetermined point, a circuit is opened which permits the comparator to connect a counter to a counting pulse transmitter, provided also that a predetermined relationship, for example equality, exists between said signals. The pulse repetition rate of the counting pulse transmitter is preferably controlled in dependence upon the rotational speed of the chopper disc and the slope of the density curve can be varied by adjusting the ratio between the rotational speed of the grey wedge disc and the pulse repetition rate. Where the pulse repetition rate is dependent upon the speed of the chopper disc, this adjustment is effected by a variable speed gear between the discs.