To obtain true conjugate, the following factor should be substracted from the diagonal conjugate :
**Core Concept**
True conjugate is a measure of the distance between the center of the lens and the point on the retina where the image forms, in a plane perpendicular to the lens. It is an essential factor in calculating the power of the lens and determining the position of the image formed.
**Why the Correct Answer is Right**
To obtain true conjugate, the diagonal conjugate should be corrected for the effect of the lens's curvature, specifically the sagittal depth of the lens. This is achieved by subtracting the sagittal depth from the diagonal conjugate. The sagittal depth is the distance between the anterior and posterior lens surfaces. By subtracting the sagittal depth, the true conjugate is obtained, which is necessary for accurate calculations of lens power and image formation.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the sagittal depth of the lens is not directly related to the calculation of the true conjugate. While the sagittal depth is an important factor in lens design, it is not the correct factor to subtract from the diagonal conjugate.
* **Option B:** This option is incorrect because the thickness of the lens is not the correct factor to subtract from the diagonal conjugate. The thickness of the lens is an important factor in lens design, but it is not directly related to the calculation of the true conjugate.
* **Option D:** This option is incorrect because the power of the lens is not the correct factor to subtract from the diagonal conjugate. While the power of the lens is an important factor in determining the position of the image formed, it is not the correct factor to subtract from the diagonal conjugate.
**Clinical Pearl / High-Yield Fact**
When calculating the true conjugate, remember that the sagittal depth of the lens is the critical factor to subtract from the diagonal conjugate. This is because the sagittal depth affects the curvature of the lens, which in turn affects the position of the image formed.
**Correct Answer:** C. Sagittal depth