V.Practical Aspects

If the observer is at a distance of one metre from the subject the Observer Factor is +1.0 Dioptre. (+1 Dioptre is the power of the lens which can focus parallel rays at 1 metre.) Then...............

V.a) In emmetropia the parallel rays coming out of the subject's eye can be brought to a focus at 1 metre by using converging (convex) lens of 1 Dioptre kept close to subject's eye. In other words, if the power of the lens used to bring the FP to 1 metre is +1 D then we know that the rays coming out are parallel (which normally meet at infinity) and the eye we are examining is emmetropic.



Total Power = +1 D,ObserverFactor +1 D.

So Patient Factor is 0 (zero)

Total Power - Observer Factor = Patient Factor (Ref.Error)

(+1) - (+1) = 0 (zero) Subject is emmetropic

V.b) In hypermetropia the diverging rays coming out of the eye are brought to a focus at 1 metre by using converging lenses of power more than +1 D. (+1 required for parallel rays in emmetropia). The power used in excess of +1 D is the measure of hypermetropia of that eye.



eg. Total Power = +3 D, Observer Factor = +1 D.
So Patient Factor is +2 D
Total Power - Observer Factor = Patient Factor.
(+ 3) - (+1) = +2. This means the patient is 2 Dioptres Hypermetropic

V.c) In myopia of less than 1 Dioptre the converging rays coming out are focused at 1 metre by using converging lens of power less than +1 D .This difference in power compared with the emmetropic eye will give the exact measure of myopia.



eg. Total Power= +0.50 D, Observer Factor= +1 D.
So Patient Factor (ref.error) is -0.50D
Total Power - Observer Factor = Patient Factor (Ref. error)
(+0.50) - (+1.0) = - 0.50 (Patient is 0.50 Dioptre myopic).

V.d) In myopia of 1 Dioptre the rays coming out of the eye are convergent and meet at 1 metre with out using any lens. In other words if we are not using any lens to bring the FP to 1 metre we know that we are dealing with an eye which is 1 D myopic.

Total Power = 0 (zero), Observer Factor = +1 D,
so Patient Factor (error) is - 1.0 D.
Total Power - Observer Factor = Patient factor (ref.error)
( 0 ) - ( + 1.0 ) = - 1.0 ( Patient is 1 Dioptre myopic)

V.e) In myopia of more than 1 Dioptre the converging rays coming out of the eye will focus at the FP which is less than 1 metre from the eye. (between the patient and the observer). So this focus (FP) can be brought to 1 metre by using diverging (concave) lens. So this much of diverging (minus) power is in excess when compared with an eye with FP at 1 metre (ie.eye with 1 D myopia).

eg. Total Power = - 2.0 D, Observer Factor = +1D,
so Patient Factor (error) is - 3.0D.
Total Power - Observer Factor = Patient Factor (ref.error)

( - 2.0 ) - ( + 1.0 ) = - 3.0 ( Patient is 3 Dioptre myopic )