LV: Field of View of + Lens Magnifiers

There is an equation which I can't be arsed to derive. It's for linear field of view.

y = D/aFm

y = linear fov
D = diameter of magnifier
a = lens to magnifier distance
Fm = power of magnifier

NBZ

• the actual field size is less than calculated due to aberrations
• magnifier must be held as close to the eye as possible if you want to maximise the field of view BUT lens to eye distance has no impact on retinal image size
  
• Give the minimum magnification required - don't prescribe for deterioration

Example Calculation

"How does the field of view change when a patient holds a 90mm round +6.00D magnifier 25cm from his eye and then brings it up to a distance of 6cm from his eye?"

D = 90mm, Fm = +6.00D a=0.25m or 0.06m

so from 25cm away y = 0.09/(0.25 x 6) = 60mm
and from 6cm away y = 0.09/(0.06 x 6) = 250mm

Rules to live by

1. Magnifier held at 1 focal length from eye ->> FOV = diameter of the lens
2. Magnifier held at 1/2 focal length from eye ->> FOV doubled
3. Magnifier held at twice focal length from eye ->> FOV 1/2 diameter of lens
4. etc etc etc etc etc etc

NB AGAIN - Remember to take into account acuity reserve when prescribing - px needs to be a little bit above the threshold to read fluently - ie when reading a book. If just using for 'spotting' in supermarket or something this isn't as important.