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 2-6-0 Class 7 engines) the exhaust steam manifold was a steel casting welded into the saddle during manufacture. Only 20 were built, numbers 77000–77019, all at Swindon Works. Their operations were mainly restricted to the North Eastern and Scottish Regions, of British Railways, although 77014 ended its days on the Southern Region. Due to their small number, the class were the antithesis of 'standard' engines and it would probably have been better to build more 2-6-0s to the Standard Class 4 and Standard Class 2 designs instead. None has survived to preservation, though some components were saved from scrapyards for use