Image scanning apparatus and method

An image scanning apparatus has imaging sensors 15,16,17 and a focusing system 1 defining an optical axis by directing light from a target where the light focussed on each imaging sensor has a different focus level. The focussing system has an optical path modifier 61 where first and second light paths between a first and second image sensor and the optical path modifier respectively are different. A scanning system moves the target relative to the optical axis generating a target image using image data from the image sensors. The apparatus takes into account variations in surface topography. The optical path lengths of the first and second light paths may be different. A combination of the specific position upon the target and the specific light path may result in a focus level different from all other focus levels. A retarding element with a refractive index to modify the optical path length may be placed in the first or second light paths. The optical path modifier may be a first beam splitter 61 and a second beam splitter 62 may be a combiner. The imaging sensors may selectively receive light from each of the first and second light paths by optical shutters.