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 .