a slit exposure type photocopier apparatus includes an original document platform on which an original document is to be located . an electrostatic latent image support member receives a projected image of the original document as a result of a scanning optical system and an exposure light source . the scanning optical system includes a projection light assembly which is movable along the optical axis in order to change image magnification . a light measuring sensor can be positioned adjacent a side of the projection lens assembly and can measure light reflected from the original at a position displaced , in the forward scan direction , from the actual position on the original that is being projected as a recorded image on the electrostatic latent image support member . the actual displaced position for light measurement will be a function of the image magnification and will be capable of compensating for the lag time involved in changing the exposure output of the exposure lamp .