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 6 create problems later on, due to its small diameter, which reduced the choke area that allowed the fierce exhaust blast to escape, reducing the overall efficiency of the locomotive. Similarities with the "Britannias" rested with the frames, tenders and running gear, allowing easy standardisation of parts common with other classes. The design was fitted with a standard set of two Walschaerts valve gear systems, and all members of the class were equipped with 4,200 gallon BR 1 tenders. Following experience of occasional cracks appearing in the frame plates near the spring brackets, had the second batch of Class 6 Standard