1. Method of forming a combined image based on a plurality of image frames, including: capturing the plurality of image frames by means of an image capturing device ( 5 ); 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 from the plurality of image frames, 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 from the plurality of image frames, wherein the combined image is represented by a final array ( 20 ; 42 ; 54 ; 71 ) of intensity values; determining a mapping function ( 22 ; 33 ; 49 ; 65 ) for mapping the intensity values of the arrays in the first set ( 18 ; 30 , 47 ; 64 ) from a scale within a first range to a scale within a second range; mapping the intensity values of the arrays in the first set ( 18 ; 30 , 47 ; 64 ) on the basis of the mapping function; obtaining at least some of the intensity values in the final array ( 20 ; 42 ; 54 ; 71 ) 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 ); and arranging the first set ( 18 ; 30 ; 47 ; 64 ) and the second set ( 19 ; 31 ; 46 ; 61 ) such that the image frames represented by the arrays in the first set ( 18 ; 30 ; 47 ; 64 ) are represented at lower resolutions than the image frames represented by the arrays in the second set ( 19 ; 31 ; 46 ; 61 ), wherein at least some of the arrays of intensity values in the first and second set are obtained by reading out measurement values from an image-capturing device comprising an array of light-sensitive cells, wherein each intensity value in the final array ( 20 ; 42 ; 54 ; 71 ) is based on at least one intensity value in an array comprised in the second set.