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 put down to their very similar appearance to the BR Standard Class 7. Some of the "Clan" Class locomotives took their names from the Highland Railway Clan Class which was being withdrawn from service at the time, indicating further their intended area of operations. The class was ultimately deemed a failure by British Railways, and the last was withdrawn in 1966. None survived into preservation, although a project to build the next locomotive in line, number 72010 "Hengist", is progressing. Assembly of the frames has commenced at CTL Seal, Sheffield. Under the initial scheme for the creation of a series