LV: Stand Magnifiers

The stand magnifier allows the correct lens - object distance hands free. However they are more difficult to illuminate and can restrict access to the working plane (eg when writing). There are both fixed focus and variable focus types.

Optical Sitch

Stand height is usually less than the focal length and the object not at the focal plane but within the focal length. This means

• the light is divergent when it leaves the magnifier
• the image is a finite distance from the eye SO the eye must have appropriate accommodation or near VA to view the image at this apparent distance - add or accomm must neutralise the emergent divergence
• the amount of acc/add needed depends on the mag to eye distance - more accom/add needed as they get closer together
• ALSO assume the vertex dist for the add is zero for stand mag calcs.

Example Calculation

Stand mag of +20DS has height of 4.5cm. What near add would a totally presbyopic px need if the mag to eye distance was 10cm and the eye was very close to the magnifier?

F = 20D
l = 0.045m
z = 0

l' = 1 / l + F
l' = 1/0.045 + 20
l' = -42.22222cm

so...1/-42.222222cm = 2.36D of accommodation req so +2.25 add

if eye 10cm from magnifier then l' = -42.22222 + (-10) = -52cm so add = 1/0.052 = 1.91D so +2.00 add req

Calculating Magnification

+20D magnifier 20cm from reading add, stand height 4cm

L1 = 1/l1 = 1/-0.04 = -25D
L1' = -25D + 20D = -5D
this means light from object is diverging 5D when leaving magnifier

L2 = 1 / (1/L1') - z
L2 = 1/ (1/-5) - 0.2
L2 = -2.50D

Feq = FM + FA -zFMFA
Feq = 20 + 2.50 - (0.2 x 20 x 2.50)
Feq = +12.50D

so M = 12.5/4 = 3.125x

If you hold the magnifier 5cm from the reading add instead

L1 = 25D, L1' = -5D
L2 = 1/ (1/-5) -0.05
L2 = -4D

Feq = FM + FA -zFMFA
Feq = 20 +4 - (0.05x20x4)
Feq = 20D

so M = 20/4 = 5x mag

SOOOOOOO The higher the near add and the closer the eye to mag distance the higher the magnification.

Determining Emergent Vergence in Practice w/out Calculation

• View Object Thru Magnifier and find max plus lens which still gives clear view when held up against it
• Use a telescope to eliminate effect of accommodation
• As the first one there but reverse the light path and focus a distant object on the object plane of the magnifier eg a ceiling light imgd on page and again use plus lenses
• NB Often companies give max allowable near add which = add when eye->mag dist = zero. Add decreases as eye moves away from it

Advanatages of Stands

• Good for px with hand tremors
• Avail w/ or w/out illumination
• Portable
• Relatively inexpensive
• Some patients find it easier to use than hand mag

Disadvantages

• Limited FOV
• Accomm or bif required
• Difficult to determine equiv power
• Can be bulky
• Posture/fatigue probs
• Illumination difficult if it doesn't have it built in