in case of a spherical mirror , f=r/2 what about a lens where to position the focus i have the following notion in mind can you help me understand whether it is right or wrong...

First of all you need to review the derivation of obtaining focal length of lense. Its not merely r/2, because it depends on the refractive index of lense material and also the surrounding medium.(But in case of mirror its always r/2).After that post your question clearly.