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 2 2-6-2T BR Standard Class 2 2-6-2T The British Railways standard class 2 2-6-2T was a class of steam locomotive, one of the standard classes of the 1950s. The class was designed at Derby Works and introduced in 1953. The design derived from the LMS Ivatt Class 2 2-6-2T which BR had built after nationalisation. Modifications were made to the Ivatt design including a reduced cab to reduce the loading gauge and some standard fittings. BR classified them 2MT, emphasising a mixed-traffic role. As most services which required 2MT 2-6-2Ts were already served by the 130 Ivatt engines, B.R. only ordered 30