2.01 Spectacle magnification

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Spectacle magnification definition

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The ratio of image size in an eye corrected by a spectacle lens to the image size in an uncorrected eye

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Why is spectacle magnification important

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18 Terms

1
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Spectacle magnification definition

The ratio of image size in an eye corrected by a spectacle lens to the image size in an uncorrected eye

2
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Why is spectacle magnification important

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3
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How to calculate spectacle magnification

Spectacle magnification = shape factor x power factor

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How to calculate shape factor

T = centre thickness (metres)

F1 = front surface power

<p>T = centre thickness (metres)</p><p>F1 = front surface power</p>
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How to calculate power factor

D = back vertex distance (metres)

BVP = back vertex power

<p>D = back vertex distance (metres)</p><p>BVP = back vertex power</p>
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What does a shape factor less than 1 tell you about the specs

F1 (front surface power) is negative

Will minify the image

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What does a shape factor more than 1 tell you about the specs

F1 (front surface power) is positive

Will magnify the image

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What does a power factor more than 1 tell you about the specs

Image will be magnified

9
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What does a power factor of less than 1 tell you about the specs

Image will be minified

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What is the problem with specs that minify the image

Dont get a good VA as the letters are all appearing much smaller

Usually get to 6/6

11
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Effect of increasing front surface power (F1) on shape factor, power factor and spec magnification - on hyperopes

Shape factor increases

Power factor remains constant

Spec mag increases

12
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Effect of increasing front surface power (F1) on shape factor, power factor and spec magnification - on myopes

Shape factor increases

Power factor remains constant

Spec mag increases but is still less than 1 due to the influence of the power factor

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Effect of increasing cantre thickness on shape factor, power factor and spec magnification - on hyperopes

Shape factor increases

Power factor is constant

Spec mag increases

14
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Effect of increasing centre thickness on shape factor, power factor and spec magnification - on myopes

Shape factor increases

Power factor remains constant

Spec mag increases but still less than 1

15
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What do we need to do to power when increasing BVD for myopes

Increasing BVD = shorter focal length = more minus power needed

16
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How to calculate BVP (both ways)

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17
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Increasing the BVD in a positive prescription will require a _ in BVP and will cause an _ in spectacle magnification

Decrease in power

Increase in spec mag

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Increasing the BVD in a negative prescription will require a _ in BVP and will cause an _ in spectacle magnification

Increase in BVP

Decrease in spec mag