LV: Equivalent Power Of Hand Held Magnifiers

Feq = F1 + F2 - dF1F2

If a hand held magnifier is used in combination with a near Rx or bifocal it's essentially a lens system, so you can calculate equivalent power.

Example

Px has +2.50add and 10DS magnifier 25cm from their bifocal

Feq = 10 + 2.5 - (0.25 x 10 x 2.5) = 6.25

so Meq = 6.25/4 = 1.56x

Rools

• If magnifier used w/near add and magnifier to eye dist = less than magnifiers focal length the equivalent mag will be higher
• if greater than focal length then mag will be lower
• if equal to the focal length then the mag will be equal to the magnifier's nominal mag