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 2-6-4T impending dieselisation, and the last five would have been, too, had they not been at an advanced stage of construction when the order came to cancel them. No significant modifications were made to the design. The tank vent was found to restrict the driver's vision and was moved further forward from 80059 onwards. Initially built with fluted coupling rods, these caused problems on other classes and, from 80079, plain section coupling rods were substituted. The BR standard class 4 4-6-0 was essentially a tender engine derivative of the Standard Class 4 tank. The Standard 4 tanks were originally allocated to