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 4-6-0 members of the class (75014 and 75078) are presently operational, with 75014 being the most recent engine to return to service. 75027 has been cosmetically restored for display as part of the Bluebell Railway's 'Steamworks!' exhibition. Bachmann and Hornby have both recently released models of these engines in 00 gauge. BR Standard Class 4 4-6-0 The British Railways Standard Class 4 4-6-0 is a class of steam locomotives, 80 of which were built during the 1950s. Six have been preserved. The class was introduced in 1951. They were designed for mixed traffic use on secondary routes where the otherwise ubiquitous