1. An imaging system for detecting objects exterior of a vehicle, said imaging system comprising: an imaging device comprising a CMOS array of photo-sensing pixels, wherein said imaging device is part of an exterior rearview mirror assembly that attaches at a side of a vehicle equipped with said imaging system, and wherein said imaging device captures image data; wherein, with said exterior rearview mirror assembly attached at the side of the equipped vehicle, said imaging device has a field of view at least one of rearward of said exterior rearview mirror assembly in a direction towards the rear of the equipped vehicle and sideward of said exterior rearview mirror assembly in a direction away from the side of the equipped vehicle at which said exterior rearview mirror assembly is attached; a control for processing image data captured by said imaging device; wherein said control utilizes edge detection in processing captured image data; wherein, responsive at least in part to said processing of captured image data by said control, said control determines objects of interest present in the field of view of said imaging device; wherein objects of interest determined to be present in the field of view of said imaging device comprise at least one of (i) a vehicle that is at least one of rearward of the equipped vehicle and sideward of the equipped vehicle, (ii) a headlight of a vehicle that is at least one of rearward of the equipped vehicle and sideward of the equipped vehicle, (iii) a bicycle that is at least one of rearward of the equipped vehicle and sideward of the equipped vehicle, and (iv) a bicycle rider that is at least one of rearward of the equipped vehicle and sideward of the equipped vehicle; wherein, responsive at least in part to said processing of captured image data by said control, a driver of the equipped vehicle is alerted to a hazardous condition; and wherein, in determining objects of interest present in the field of view of said imaging device, edges that are not indicative of objects of interest are substantially ignored in order to at least one of (a) reduce processing requirements and (b) reduce false signals.