1.11 Near vision effectivity

What is near vision effectivity error and its effect on pts

The prescription you get in the consulting room may not have the same effective power at near when made up in a different lens form

Pt may have near vis difficulties/have to accommodate more in the final spectacles compared to your trial case lenses eg high hypermetropes

How is back vertex power if a lens determind (ray diagram)

When might the back vertex power not give a true reflection of the lens performance

When the source of light is coming from a near object ie. not infinity

What does lens performance depend on when the object is at near

Incident vergence

Lens form (front surface power)

Lens thickness

Refractive index

What is incident vergence (L2) altered/modulated by

Lens form

Lens thickness

Refractive index

How can we quantify the nvee (2 ways)

Quantifying the actual error in relation to the trial lenses we have used to refract the pt

Comparing the emergent vergence obtained with a given thick lens to the theoretical emergent vergence that we would obtain with a thin lens of the same back vertex power

Back vertex power formula

When do we use the back vertex formula

To calculate vergence leaving the back surface of a thick lens when light is coming from infinity

Variation of BVP formula when light is coming from near

How to calculate theoretical thin lens power

Lens:

F1 = +17.00 DS

F2 = -3.18 DS

t = 10mm

Calculate the vergence of light leaving the back surface of the lens when light is coming from

• Infinity

• Near task of 33cms

Comment on NVEE

Comment in the nvee

Does nvee affect myopes

No

The lens performs almost the same whether its a low or high powered lens

The nvee is not significant

Equation which gives an approximation of the NVEE

Steps to ray trace through a thick lens - how do we calculate

• Incident vergence

• Vergence after F1

• Incident vergence at F2

• Vergence after F2

• Incident vergence on F1= 1/distance (eg 1/33cms = -3.00)

• Vergence after F1 = incident vergence on F1+ surface power of F1

• Incident vergence at F2 = n / ( (n/vergence after F1) - centre thickness in metres)

• Vergence after F2 = incident vergence at F2 + surface power of F2