The photometric fidelity of the optical system of microdensitometers is compared to that of flying - spot scanning devices. Microdensitometers, which were originally developed to study fine structure of silver halide photographic images, move the object plane through a fixed, Koehler-illuminated dual projection optical system that controls scattered light and ensures uniform photo-metric response everywhere on the scanned area. Flying-spot and video-based scanners now being applied to photographic photometry are optical reciprocals of one another. They operate on a critically-illuminated image of the film similar to that produced by a camera, and therefore require precautionary measures to limit errors from flare light and their inherent lack of spatial stationarity. The low inertia of such systems permits high data-sampling speeds with the possibility of preprogrammed or adaptive scans under computer control. Advances in stage design, however, now allow microdensitometer optical systems also to sample at photon noise limited rates. Conditions under which image-plane scanners can be used for quantitative microphotometry of photographically-recorded images are outlined.