An optical proximity sensor (1) generates information indicative of a distance (D1) to an object (14A) in a field and in some embodiments also generates information indicative of a spectral reflectance characteristics of the object (14A). The information indicative of the spectral reflectance characteristic can be used to determine whether the object (14A) in the field is a living plant (14A) or another object such as soil (10). Light emitted from the optical sensor (1) for reflection off the object (14A) is modulated so that reflected light from the optical sensor can be discriminated from reflected ambient sunlight. The optical sensor (1) is scanned over the field to map objects in the field and/or to determine the location of rows of crop plants. A sensor in accordance with the present invention has many uses in agriculture including spraying, cultivation and vehicle guidance.