### Working Distance Calculations in Retinoscopy

Static Retinoscopy can be done in two ways; using a working distance (WD) lens in front of the patient’s eye or without it.

A working distance lens is used to compensate for the distance between the examiner and the patient’s eye. Usually, the working distance is taken as arm length i.e 66cm (+1.50DS)

- When a working distance lens is used to neutralize the reflex, the total lens system in front of the patient’s eyes is considered as
*Gross retinoscopic (GR) value*and if the WD lens value is deducted from it, the remaining power is considered as*Net retinoscopic (NR) value;*which is taken as the final value of retinoscopy. - Formula
**GR value = WD lens value + NR value**

Examples:

- If a Working Distance lens of +1.50 DS is used and the reflex is neutralized with -3.00DS then,

NR= -3.00DS; WD= +1.50 DS

GR= (+1.50) + (-3.00) = -1.5DS

So, the power of the lens with which reflex is neutralized (irrespective of the power of WD lens) i.e NR value is taken as the final retinoscopy value. Here, **-3.00DS** is taken as the final power of retinoscopy.

- If WD is +2.00DS and reflex is neutralized with -3.00DS/-1.00DC × 180 then

NR=-3.00DS/ -1.00DC × 180

GR = (+2.00) + (-3.00 DS / -1.00DC × 180)

= +1.00DS/ -1.00DC × 180

The final retinoscopy power is taken as **-3.00DS/-1.00DC × 180**

** Note:** Working Distance lens value is deduced only from the spherical power of NR value. In the above example, +2.00DS is reduced from -3.00DS.

- When retinoscopy is performed without using a WD lens the total power in front of the patient’s eye is equal to GR value.

(GR value) – (the distance from retinoscope head to the patient’s eye) gives the NR value.

Example:

- If the reflex is neutralized with -3.50 DS and the working distance between retinoscope head and patient’s eye is +1.50DS then

GR= -3.50DS; WD= +1.50DS

NR= (- 3.50) – (+1.50)

= -5.00DS.

The value of final retinoscope power is taken as **-5.00DS**

- If the reflex is neutralized with +1.00DS and WD is taken as +2.00 DS then

GR= +1.00DS

NR= (+1.00) – (+2.00)

= -1.00DS

The final retinoscopy power is **-1.00DS**

** Note:** Always subtract the Working distance value from the neutralized lens power to get the final value; while WD lens is not used.

- If the reflex is neutralized with -1.00DS/-1.00DC×180 and WD is +2.00 then

GR = -1.00DS/-1.00DC×180

NR= (-1.00DS/-1.00DC×180) – (+2.00DS)

= -3.00DS/-1.00DC×180 {cylinder remains unchanged}

The final power is taken as **-3.00DS/-1.00DC×180**