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 4 2-6-0 BR Standard Class 4 2-6-0 The BR Standard Class 4 2-6-0 is a class of steam locomotive designed by Robert Riddles for British Railways (BR). 115 locomotives were built to this standard. The class was designed at the ex-LNER works at Doncaster which was also responsible for building 25 of the 115-strong class. The remaining 90 were split between Horwich and Derby Works. The last in the series, No.76114, was also the final steam engine to be constructed at the 'Plant' (as Doncaster works was known). The Standard Four Mogul was essentially a standardised version of the LMS Ivatt Class