LV: Plus Lens Magnifiers 1

M = F/4 as long as the object is at the focal point. So a +8.00D lens has 2xMagnification at 12.5cm.

If the patient's object is at the focal point of the lens then he/she mustn't wear a near add. NB If a patient doesn't seem to be achieving the expected magnification their current specs may not have a +4.00D reading add and the baseline acuity may be incorrect. Always measure vision at 25cm with a +4.00D add to determine a correct baseline acuity.

For younger patients who can accommodate adequately the magnification =
Visual angle subtended by magnified image/angle subtended by object viewed at 0.25m

If the object is at F then the image the patient looks at will be at infinity.

• Object at F, L=F and L'=0
• Image is at infinity so no near add is required
• The Eye to magnifier distance has no effect on magnification

If the object isn't at F there are two possible situations that could arise

1. General Case

M= qL/1-dL'

where q = 0.25m assumed near point
d = lens to eye distance
L = incident vergence
L' = emergent vergence

M depends both on the distance of the lens from the eye and the distance of the object from the lens.

2. Image at near point

M = 1 + F/4

The object is between F and the Lens. The image is produced at 0.25m ie the near point so a +4.00D add or accommodation is required. The magnifier is held close to the Px's eye (d=0). This is known as maximum magnification/manufacturer's magnification