Photoreceptors are found in several different parts of the body in addition to the eyes, and as in the eyes, rhodopsin may be one of the photopigments responsible. Visual photoreceptors are photon counters, but at low light levels it is uncertain exactly when and where a photon will arrive. Thus a basic problem of vision is statistical and the maximum number of photons must be sampled to get statistically reliable information about visual contrasts, detail and movement. Sample size can be increased by extending the retinal integration time or the integration area, but this carries the cost of reduced ability to see detail and to resolve moving images. For scotopic vision the spectral absorption of visual pigment may be arranged to maximize sensitivity without greatly increasing physiological noise which reduces the perception of contrast. Photopic vision is usually mediated by from two to four cone types containing different visual pigments which allow the possibility of colour vision. Natural waters differ greatly in colour, and influence both the photopic visual pigments possessed by fishes and their coloration.