In retinoscopy for refractive error at 1m, we add 1D ; if done at a distance of 66 cm, the additional factor will be –
## **Core Concept**
In retinoscopy, also known as skiascopy, the examiner shines a light into the patient's eye and observes the reflection. The technique helps determine the refractive error of the eye. When performing retinoscopy at a distance other than the standard 1 meter, an adjustment needs to be made to accurately determine the refractive error.
## **Why the Correct Answer is Right**
The working distance in retinoscopy needs to be accounted for in the final prescription. At 1 meter (100 cm), the standard addition is +1 diopter (D). When the retinoscopy is done at a distance of 66 cm, the additional factor or the adjustment can be calculated based on the working distance. The lens power needed to focus light from the retinoscope at 1 meter is +1 D. For a 66 cm working distance, we calculate the additional factor as follows:
- At 1 meter (100 cm), the addition is +1 D.
- At 66 cm, using similar triangles or the inverse relationship, the addition would be ( frac{100}{66} times 1D approx 1.5D ).
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests an addition that does not match the calculated adjustment for a 66 cm working distance.
- **Option B:** This option does not accurately reflect the adjustment needed for the specified working distance.
- **Option D:** This option provides an incorrect adjustment factor.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that when performing retinoscopy at distances other than 1 meter, the examiner must adjust the final prescription accordingly. The standard addition at 1 meter is +1 D, and adjustments for other distances can be calculated based on the inverse of the distance ratio.
## **Correct Answer:** .