1. A device comprising: a viewing screen which presents first and second reference icons having an adjustable reference characteristic controlled with substantially only a single adjustable parameter; an image-gathering component for providing the screen with frames of a video image of a scene in which first and second scene locations have a scene characteristic whose value is to be approximately determined; and a control system responsive to external instruction (a) for causing a frame of the image of the scene to be captured while the first reference icon is approximately superimposed on the first scene location or for causing a frame of the image of the scene to be captured and then for causing the first reference icon to be approximately superimposed on the first scene location in the captured frame and (b) for causing the second reference icon to be approximately superimposed on the second scene location in the captured frame as the adjustable parameter is adjusted to enable the reference characteristic to approximately match the scene characteristic, the device determining the value of the adjustable parameter, or the value of a further parameter containing the adjustable parameter, as an approximation to the value of the scene characteristic in the captured frame when the first and second reference icons are respectively approximately superimposed on the first and second scene locations in the captured frame.