1. Method of forming a combined image based on a plurality of image frames, including: obtaining a first set ( 18 ; 30 ; 47 ; 64 ) of at least one array of intensity values for representing an image frame at a resolution corresponding to the number of intensity values in the array, and obtaining a second set ( 19 ; 31 ; 46 ; 61 ) of at least one array of intensity values for representing an image frame at a resolution corresponding to the number of intensity values in the array, wherein the combined image is represented by a final array ( 20 ; 42 ; 54 ; 71 ) of intensity values, wherein at least some of the intensity values in the final array ( 20 ; 42 ; 54 ; 71 ) are each obtained by executing a step ( 29 ; 41 ; 57 ; 70 ) of summing an intensity value from each of at least one array ( 27 ; 39 ; 56 ; 69 ) of intensity values based on at least one array of intensity values in only the first set ( 18 ; 30 ; 47 ; 64 ) and an intensity value from each of at least one array ( 19 ; 40 ; 46 ; 61 ) of intensity values based on at least one array of intensity values in only the second set ( 19 ; 31 ; 46 ; 61 ), wherein, prior to executing the summing step ( 29 ; 41 ; 57 ; 70 ), only the intensity values of the arrays in the first set ( 18 ; 30 ; 47 ; 64 ) are mapped from a scale within a first range to a scale within a second range.