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 though towards the end of their working lives, some nameplates were painted with a red background. None of the original engines survived into preservation, however progress is being made by a Registered Charity (No. 1062320), the Standard Steam Locomotive Company, on constructing a new locomotive that would have been the first of the uncompleted batch of 15, number 72010 "Hengist". As 999 BR standard steam locomotives were built in the years up to 1960 the builders consider this is the 1000th locomotive build to be commenced to a British Railways standard design. The Standard Steam Locomotive Company believes that significant