# The dioptre

The dioptre determines the ability of  optical collecting  system ( lens ), which is the power or ability of refracting( refraction) .

The dioptre  is the  focal reciprocal . The lens has the power of 1 D  when  the focal length   (the distance of focus  from   the  lens optical center) is 1 m. When the lens has the  power of 2 D ,  the focal length will be  0.5 m , which is due to the formula given below :

Where f-  is the focal length in meters .

Positive values (+) -are collecting abilities in  focusing lens .
Negative values (-) are collecting  abilities  in  distracting lens .