The focal distance f of a lens can be calculated using a lensmaker's equation:
1/f = (n - 1)(1/R_1-1/R_2),
where n is a refractive index, R_1 and R_2 are curvature radii. For a biconvex lens the R_2 is taken negative. If |R_1| = |R_2| = R, we have
f = R/(2*(n-1)).
If one cuts lens perpendicularly to the principal axis, the obtained lens will be convex only on the one side and flat on another side: R_1 = R and R_2 = infinity. So,
1/f_1= (n-1)/R =>f_1=R/(n-1)=2f.
infact i want to know is there any relation between diameter and focal length?