FAR POINT.pptx

Understanding the far point is essential in diagnosing and managing refractive errors of the eye. It is the distance at which objects appear in sharp focus when the eye is relaxed, and it plays a crucial role in determining the appropriate prescription for eyeglasses or contact lenses.

1. FAR POINT Presentation by: QURAT-UL-AIN

2. Far Point It is defined as the point in space that is conjugate with the fovea when accommodation is relaxed. Means, light coming from the far point forms a clear and focused image on the fovea when accommodation is relaxed. So, the far point has same optical characteristics as the fovea when eye is relaxed that’s why we can say that the far point is the point in space that optically resembles with the point of fovea when accommodation is relaxed. If far point does not conjugate with fovea it gives rise to refractive errors. So, the state of eye is depends on the location of the far point of the eye.

3. Far point in Emmetrope Parallel rays focus on fovea Retina conjugate with infinity Far point is at infinity Far point in myope Parallel rays focus in front of the retina Far point is between infinity and eye Minus lens diverges the rays on to the retina and conjugate fovea with infinity. Far point in hypermetrope Parallel rays focus behind the retina. Far point is beyond infinity. Plus lens converges the rays on to the retina and conjugate fovea with infinity.

4. A B A and B are conjugate points Parallel rays from infinity (vergence = 0) In the emmetropic eye, the parallel rays from a point at infinity are focused to a point located precisely on the retina. So, the far point of emmetropic eye is located at the infinity. EMMETROPIC EYE

5. Rays from infinity meet in the vitreous. By the time they reach the retina, the rays have diverged to form a blur circle – not a focal point. Myopic eye is too long for its converging power or you can say myopic eye has too much converging power for its length. A Parallel rays from infinity (vergence = 0) To be conjugate with the retina, the far point of myopic eye will have to offset its excess convergence with an equivalent amount of divergence. MYOPIC EYE

6. A B Far point Diverging Light Distance << Infinity (< 20 feet) A and B are conjugate points To accomplish this, the far point of myopic eye is just anterior to the corneal plane. Rays from this plane are still quite divergent when they reach the eye. This divergence offsets the excess convergence that is built into the myopic eye.

7. A B Far point Little Divergence A and B are conjugate points LOW MYOPIA A B Far point Divergence a lot A and B are conjugate points HIGH MYOPIA So, this is the reason that why near-sighted person see things clear at near (specially at their far point) but not at distance.

8. A A and B are conjugate points Parallel rays from infinity (vergence = 0) In hyperopic eye, rays from infinity never meet- they run out of the eye ball first. Similar to myopic eyes, rays form a blur circle at the retina. We can say the hyperopic eye is too short for its converging power or has a little converging power for its length. To be conjugate with the retina, the far point of hyperopic eye must contribute convergence to compensate for this lack of converging power. HYPEROPIC EYE

9. A B Far point A and B are conjugate points Converging Light To accomplish this, the far point of hyperopic eye is behind the corneal plane. It contributes convergence to make up for the inadequate convergence for the hyperopic eye. Absence of correction or accommodation make farsighted person out of focus at every distance.