LV: Magnification

Magnification is "after" retinal image size/"before" retinal image size. It is needed in LV when the rx doesn't create enough improvement for the patient to do visual tasks. It can be achieved by

1. Increasing the size of the object M=new obj size/old obj size
   
2. Decreasing viewing distance M=old viewing distance/new viewing distance
   
3. Real image/transverse magnification (eg CCTV) M=size of image/size of orig.obj
   
4. Telescopic magnification = angle subtended by telescopic image (theta')/angle subtended by original object (theta)

Different methods of magnification can be combined - in this case the total magnification is found by multiplication. For example - "What is the mag obtained by changing print size from 6mm to 15mm, and visual distance from 30cm to 10cm?"

M1 = new/old = 15/6 = 2.5x mag
M2 = old dist/new dist = 30/10 = 3x mag
Total = 2.5x3 = 7.5x mag

Increasing Size Of Object

• Large screen TV
• Large print stuff - books, typewriter, telephone, bank statements
• Photographic enlargement/photocopying
• Computers - large monitors/keyboards/enlarged screen display

Changing Viewing Distance

• Simple and effective for distance or near vision - sitting closer to TV
• eg moving from 12ft to 6ft away from TV = 2x magnification
• Bringing print from 30cm away to 5cm away = 6x magnification

Real Img/Transverse Magnification

• Mainly electronic magnification systems
• Mainly NV but also systems for distance and intermediate tasks (eg video systems with autofocus)

Telescopic Magnification

• Versatile optical method for D, I & N