In retinoscopy for refractive error at lm we add -1 D. If done at a distance of 66 cm, the addition factor will be:
## **Core Concept**
Retinoscopy, also known as skiascopy, is a technique used to objectively determine the refractive error of the eye. The working distance between the retinoscope and the patient's eye affects the measurement, and adjustments are made accordingly.
## **Why the Correct Answer is Right**
In retinoscopy, when measuring at a distance of 1 meter, a -1 diopter (D) lens is typically added to the retinoscope to compensate for the working distance. If the retinoscopy is performed at a distance of 66 cm (or 0.66 meters), the lens power needed to compensate for this distance can be calculated. The lens power (L) needed is given by (L = frac{1}{text{distance in meters}}). So, for 1 meter, (L = 1) diopter, and for 0.66 meters, (L = frac{1}{0.66} approx 1.5) diopters. Since at 1 meter we add -1 D, at 0.66 meters, we would add -1.5 D.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests adding -2 D, which seems excessive for a 66 cm working distance based on the standard calculation.
- **Option B:** This option suggests no change, which is incorrect because the working distance has changed from 1 meter to 0.66 meters.
- **Option D:** This option suggests adding -1 D, which applies to a 1-meter working distance, not 0.66 meters.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that when performing retinoscopy at distances other than 1 meter, you need to adjust the corrective lens power by the working distance in diopters. The formula to find the lens power needed for a specific working distance is ( text{Lens Power} = frac{1}{text{working distance in meters}} ). For a 1-meter distance, it's -1 D; for 0.5 meters, it would be -2 D; and for 0.66 meters, it's approximately -1.5 D.
## **Correct Answer:** .