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
International Standard Authority Data Number a good portion of time on conceptualizing an international standard number "that will facilitate the linkage of variant authorities for the same identity." Their discussion was very complex in its discussion of which agencies would actually assign such numbers. For example, a national library might be tasked with assigning identifiers to authors within its country, but this would lead to duplicate identifiers for authority data that describe transnational people. The project was ultimately determined to be unfeasible. Tillett suggested that the cluster identifiers used by the Virtual International Authority File might meet the needs expressed in the proposal. The concept