F-number lengths. Therefore, reduced–depth-of-field effects will require smaller f-numbers when using small-format cameras than when using larger-format cameras. Image sharpness is related to f/number through two different optical effects: aberration, due to imperfect lens design, and diffraction which is due to the wave nature of light. The blur optimal f-stop varies with the lens design. For modern standard lenses having 6 or 7 elements, the sharpest image is often obtained around 5.6–8, while for older standard lenses having only 4 elements (Tessar formula) stopping to 11 will give the sharpest image. The larger number of elements in modern lenses allow the
BR Standard Class 4 2-6-0 4, and was primarily intended for freight use. Although a BR Standard, the 4 2-6-0 class did not have the same design of wheels as the Swindon-built 82XXX and 77XXX Class 3 engines which also had driving wheels, yet all three locomotive classes share the same cylinder casting. The cylinder covers of engines built early in the programme of construction were fitted with "screw-in" type pressure relief valves. From September 1955 revised cylinder covers were introduced for renewals incorporating "bolt-on" type pressure relief valves. With its diameter driving wheels this sixth of the BR standard designs was clearly biased towards