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 2-6-4T BR Standard Class 4 2-6-4T The British Railways Standard Class 4 tank is a class of steam locomotive, one of the BR standard classes built during the 1950s. They were used primarily on commuter and outer suburban services. On the nationalisation of British Railways (BR) in 1948, the London Midland Region had a number of ex-London, Midland and Scottish Railway 2-6-4T and the Western Region a number of GWR Large Prairie 2-6-2T types. These tank engines were particularly suited to commuter and secondary services. However, particularly in Scotland and the Southern Region, the situation was not so good, with large