Question

focal length and power
of optical lens that we use

Answer 1

   We generally use spherical lenses or cylindrical lenses.  Cylindrical lenses have some advantages.

focal length = f  in cm or in meters

1/f  = 1/u + 1/v

1/f =  (μ - 1) [ 1/R₁ + 1/R₂ ]            --- lens makers formula.
   here  μ  is the refractive index of the glass used to make the lens.
       R1 and R2 are the radii of curvature of the lens. 

Power = P = 1/f  in Diopters,  when f is in meters.

generally people use optical lenses for short sight or long sight.

   For short sight,  we use a diverging lens (concave lens.).  Usually the focal length is the distance to which the person is above to see.  If the person is not able to see distinctly, farther than 5 meters,  then the focal length could be - 5 meters.

   Then power is  -1/5 = - 0.2 Diopters.

  For long sight,  we use a converging lens.  convex lens.  The focal length is the shortest distance that the person could see clearly and distinctly.
If the person is not able to see clearly at less than 50 cm,  then the focal length is  +50 cm.  Then power of the lens = 100/50 =  + 2 Diopters.

usually the power of lens we use is  between  - 0.1 D to  +4 D.