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 3 2-6-2T introduced for renewals incorporating "bolt-on" type pressure relief valves. Although the chassis had many almost identical parts to the LMS Ivatt Class 4 the motion brackets were derived from the design of those fitted to the LMS Ivatt Class 2 2-6-0 and LMS Ivatt Class 2 2-6-2T. Unlike a number of the larger BR Standards the exhaust steam manifold within the smokebox saddle was a steel fabrication that was part of the welded saddle. In a number of the large BR standards (BR Standard Class 6 and Class 7 engines) the exhaust steam manifold was a steel casting welded into