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 locomotives, an equivalent 2-cylinder "Pacific" was produced by mounting a smaller and lighter boiler on the standard 7MT chassis. The arrangement consisted of a modified Standard Class 7 boiler, with smaller steel cylinders and other modifications to save weight and hence increase route availability. The wider firebox, designed for use with cheaper imported coal, was also utilised to spread its weight evenly over the axles, whilst the standard smokebox completed the boiler, which at 225 lbf/in² was rated at a lower working pressure than that of the "Britannias". A single chimney was incorporated into the design, although this was to