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 2 2-6-0 for those originally allocated to 27A and 27D). These included 78002 (in green livery), 78007, 78027 and 78057. Some of these were used on shunting duties from Bolton and Lostock Hall sheds. Number 78022 preserved on the Keighley & Worth Valley Railway had a spell as Preston station passenger pilot whilst allocated to Lostock Hall. Four members of the class survived into preservation, however one member of the class (78059) is currently in the process of being converted to the tank version of the class. BR Standard Class 2 2-6-0 The BR Standard Class 2 2-6-0 is a class of