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 6 of British Railways standard locomotives, larger passenger and mixed traffic types were intended to be of the 4-6-2 "Pacific" wheel arrangement, the main advantage of which was that it could be fitted with a wide firebox capable of burning a range of coal types (and qualities). The "Pacifics" were originally intended to be produced in four power groups: 8, 7, 6, and 5, according to the system of power ratings inherited from the L.M.S. constituent company. Power groups 7, 6 & 5 were to be for mixed traffic (MT) service. The whole standardisation programme was launched with the building of