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 8 1992. BR Standard Class 8 The BR Standard Class 8 was a class of 4-6-2 "Pacific" steam locomotive designed by Robert Riddles for use by British Railways. Only the prototype was constructed, which was named "Duke of Gloucester". Constructed at Crewe Works in 1954, the "Duke", as it is popularly known, was a replacement for the destroyed LMS Princess Royal Class locomotive number 46202 "Princess Anne", which was involved in the Harrow and Wealdstone rail disaster of 1952. The "Duke" was based on the BR Standard Class 7 "Britannia" design. It incorporated three sets of modified Caprotti valve gear, relatively