IV.Theoretical Approach

If we can locate the Far Point of subject's eye, then we can calculate the refraction of that eye. But observer moving towards FP is not practical. (In emmetropia FP is at infinity and in hypermetropia FP is virtual point behind patient's eye).

The method used for calculation is keeping the subject and the observer at fixed places, and bringing (shifting) the FP of the subject to the position of the nodal point of observer's eye. This is done by using converging or diverging lenses.
Now we know the exact distance of the FP from patient's eye (ie. exact distance at which we are sitting) and also the power of lens used to bring the FP to this position.
( From the measurement of distance we can calculate the power required to bring the FP to this position in emmetropia - I call it observer factor - see below. The power used other than this power, gives the refractive error - patient factor - see below)

The Total Power of lenses we use contain two parts.
First one is the power used because of the position of the observer (Observer Factor or Induced Factor or False Factor). If the subject is emmetropic this is the only factor that will be there. This is the power used, only because of the position of the observer and not because of the patient's refractive error.
Second part is the power of lenses used because of the patient's refractive error (Patient Factor or True Factor). From the above three values we can calculate the refraction of the subject's eye.

Total Power = Observer Factor + Patient Factor.
Total Power - Observer Factor = Patient Factor (Refraction of Patient's Eye).