Bi-Convex Lenses are popular for many finite imaging applications. They are best suited for use in situations where the object and image are on opposite sides of the lens and the ratio of the image and object distances (conjugate ratio) is between 0.2 and 5. 

The focal length of each lens can be calculated using the following equation:
f= (R1*R2)/((n-1)*(R2-R1)).
where n is the index of refraction and R1,R2 is the radius of curvature for each surface of the lens.
They can be also coated with MgF2 to protect the surface or AR coated to increase the transmission.