Abstract: A method and an apparatus for measuring the toner concentration and the amount of electric charge, capable of simultaneously measuring the toner concentration and the amount of electric charge easily and maintaining high precision using a simple device.

Abstract: An optical system of variable magnification comprising one or more multilens units, each of which has a plurality of multilens-arrays positioned at certain spaces in the vertical direction, said multilens units being disposed in such a manner that the optical unit plane, containing optical axes of the lenses in each of the multilens arrays in one multilens unit, intersects other optical unit planes at the center line of said optical system and that each of the optical axes of the lenses in the multilens-array of each of the multilens units gradually deviates one after another from each of those of the corresponding lenses in the next positioned multilens-array within the optical unit plane.Magnifications of the optical system are changed by rotating said optical system around the axis of the intersection line of the optical unit planes to a predetermined position.