NS9874 : Looking north over Forth Valley from Cockleroy, LinlithgowThe clue is in the darkness! Under these conditions, the camera aperture will be at its widest to let in the maximum light. The downside of this is that it results in a shallow depth of focus. In other words, although the trig point is in focus, the further away an object is behind this point, the more blurred it will become. The circle of light referred to in the text looks like a flare stack at Grangemouth, behind Cockleroi from the direction given, which has been rendered well out of focus for the reason given above.