magnification and types

1. Magnification
SHOAIB KHATTAK
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2. Key points
 Definition
 Types used in low vision
 Combining magnification
 Prescribing magnification
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3. Definition
 Magnification is increasing apparent size of retinal image so that sufficient impulse is sent
through optic nerve to brain for stimulating enough retina to perceive image easily .
or
 It is the ratio of new retinal image size (when magnified) to retinal image size when not magnified
M = new retinal image size
old retinal image size
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4. Types
 Relative distance magnification
 Relative size magnification
 Angular magnification
 Transverse magnification
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5. Relative distance magnification
 This type of magnification results from reduction in object viewing distance .
 The result of reducing object viewing distance is to increase the angle subtended
by the object at the pupil entrance .
M = old object distance
new object distance
M= d1/d2
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6.  It is the simplest way to magnify the image of an object .
 For example
a patient watching television at 5m and move to 1m distance from the screen
.The magnification will be
M = d1/d2
M = 5/1 = 5x
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7. Relative size magnification
 This type of magnification is achieved by increasing the size of object .
M = h2/h1
M = new object size / old object size
 For example
 large print books
 Large TV screens
 If large print books are N36 and standard prints are N10 .what will be the
magnification
M = 36/10
M = 3.6x
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9. Angular magnification
 It is the ratio of angle subtended at the eye by instrument image and the angle subtended by
object itself .
 Angular magnification is provided by optical instruments . E.g telescopes ,hand and stand
magnifiers .
 It does not involve the change in image or object distance .
M = N/f
where N =25cm is the near point distance and f is the the focal length of the instrument used .
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10.  When more than one lenses are used in the instrument like telescope have eye
piece and objective lens than its total angular magnification is calculated by the
formula
M = -fo/fe
fo is the focal length of objective lens and fe is the focal length of eye piece lens .
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11. Transverse or real image magnification
 It is also called electronic magnification.
 It is used in CCTV and other electronic devices where magnified image of the
object is created on TV screen .
 The real image is created on the same location as the object .
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12. Combining magnification
 Different types of magnifications can be used in combination with one another .
 The total magnification is the product of the two values .
 For example
a patient can see N12 at 40cm and he/she now moves to 30 cm and uses large print books of N24 .
What is the total magnification ?
 relative distance magnification =40/30 = 1.33
 relative size magnification = 24/12 = 2
 Total magnification = 2(1.33) = 2.66x
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13. Prescribing magnification
 Indications
 Magnification is prescribed when the patient present VA is not sufficient to perform daily tasks like
 reading
 Writing
 spoting distance objects like street # sign boards etc .
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14. Howcan we obtain appropriatevalue of magnification
 Identify specific task and predict amount of magnification needed .
 Is monocular or binocular correction preferable .
 Select appropriate low vision aid if needed .
 Perform trial for suitability
 Determine any spectacle correction needed .
 Loan out device after training .
 Follow up visits ,
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15. Requiredmagnification
 The required value of magnification can be calculated by the formula below
M = actual VA/ Desired VA
For example to improve a patient from 6/60 to 6/6 the required value of magnification
will be
M = 60/6 = 10x
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16. Common magnifiers
 Some common magnification devices for
 Near
 Hand magnifiers
 Stand magnifiers
 Microscopes
 And for distance is
 Telescopes
 Gallileon telescope and kaplarian telescope
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17. Thank you
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