Easy way to find the sagitta depth.
To locate a lens properly in a mechanical mount, one has to know the axial separation between the vertex and the ring of contact between the curved surface of the lens and a lo­ cating shoulder. The location of the vertex of one of the surfaces of the lens is determined as sagitta or sag. Figure 1 explains the variables involved in the calculation of sagitta depth. The depth of sagitta may be calculated according to the following formula: S = R √R (d/2) . Although this formula is not complicated, its solution is nevertheless timeconsuming even with a pocket calculator. Therefore a no­ mograph has been designed (Fig. 2) that allows one to solve the depth of sagitta when the other data (R and d) are known within 1 sec. The use of this nomograph is extremely simple: one connects with a ruler the known values of R and d on both side scales and reads the results, S, at the intersection point of the ruler with the S scale (central). The nomograph may be used with any units.