A method of forming a combined image based on a plurality of image frames includes obtaining a sequence (22, −33) of arrays of intensity values for representing respective ones of a sequence of image frames depicting substantially the same scene. The combined image is represented by a final array (27, −28) of intensity values. At least some of the intensity values in the final array (27, −28) are each obtained by summing an intensity value from each of a sequence of arrays of intensity values based on respective ones of the arrays in the obtained sequence (22, −33). If, for any of at least one region in the final array (27;28), any of a sequence of sets of at least one intensity value, wherein each set represents the region and forms part of one of a selected plurality of the obtained arrays of intensity values, contains intensity values deviating by more than a certain measure from those in the other sets of the sequence, the intensity values representing that region in the final array (27; 28) are each derived from (A) a respective intensity value from each of one or more arrays based on respective ones of only those of the selected plurality of arrays containing a set with deviating intensity values, or (B) a respective intensity value from each of one or more arrays based on respective ones of only those of the selected plurality of arrays containing one of the other sets of intensity values.