1. IN A SINGLE LENS REFLEX CAMERA SYSTEM HAVING A CAMERA HOUSING, AND A PLURALITY OF EXCHANGEABLE LENSES OF DIFFERING MAXIMUM APERTURE, THE CAMERA HOUSING HAVING A LIGHT METER INCLUDING A LIGHT SENSITIVE ELEMENT POSITIONED IN THE IMAGING RAY PATH OF ANY OF THE LENSES WHEN MOUNTED TO THE HOUSING, EACH LENS HAVING A LENS MOUNT WHICH INCLUDES A DIAPHRAGM AND A DIAPHRAGM SELECTOR RING FOR SELECTING A DIAPHRAGM OPENING OVER A RANGE FROM A LARGEST TO A SMALLEST OPENING, DIVISIBLE IN STEPS, WITH SEQUENTIAL SELECTING STEPS EACH EFFECTIVE FOR SELECTIVELY HALVING OR DOUBLING OF THE LIGHT INTENSITY, THE SELECTED DIAPHRAGM OPENING BECOMING THE ACTUAL APERTURE WHEN THE DIAPHRAGM IS CLOSED DOWN TO THE SELECTED APERTURE VALUE, THE COMBINATION COMPRISING: INDICATING MEANS INCLUDED IN THE LIGHT METER TO PROVIDE AN INDICATION OF THE LIGHT INTENSITY PERMITTED TO REACH SAID ELEMENT THROUGH ONE OF THE LENSES WHEN MOUNTED TO THE CAMERA HOUSING; REFERENCE MEANS FOR THE INDICATING MEANS; AND CONTROL MEANS FOR PROVIDING A VARIATION IN THE RELATION BETWEEN THE REFERENCE MEANS AND THE INDICATING MEANS INCLUDING A CAM FOLLOWER ARRANGEMENT HAVING FEELER MEANS FOR CONTROLLING SAID RELATION, FURTHER INCLUDING A CAM ON THE SELECTOR RING OF THE LENS MOUNTED TO THE CAMERA AND HAVING A CONTOUR SIMILAR FOR THE SEVERAL SELECTOR RINGS, BUT HAVING A DIFFERENT POSITION RELATIVE TO THE DIAPHRAGM APERTURE VALUES AS SELECTABLE BY THE SEVERAL SELECTOR RINGS, THE CONTOUR PROVIDING A PARTICULAR POSITION OF THE FEELER FOR THE LARGEST DIAPHRAGM APERTURE SELECTABLE FOR THE LENS MOUNTED TO THE CAMERA INDEPENDENTLY FROM THE VALUE THEREOF AND PROVIDING POSITIONS FOR THE FEELER VARYING LINEARLY FOR DIAPHRAGM APERTURE SELECTIONS SMALLER THAN THE LARGEST VALUE.