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
W.G Bagnall 0-6-0ST "New Standard 18" sold in 1986 and is currently at the Stephenson Railway Museum under overhaul, now named Thomas Burt MP. As train lengths grew on the WSR and more suitable motive power became available 2996 was sold in 1989 moving initially to the Strathspey Railway. After subsequent spells on both the Battlefield Line and the Great Central Railway (Nottingham) it has been under overhaul as well at the Lakeside and Haverthwaite Railway, and has since been put back into working service. W.G Bagnall 0-6-0ST "New Standard 18" The W. G. Bagnall New Standard 18 0-6-0ST is a type of industrial steam locomotive