2.01 Spectacle magnification

Spectacle magnification definition

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

Why is spectacle magnification important

How to calculate spectacle magnification

Spectacle magnification = shape factor x power factor

How to calculate shape factor

T = centre thickness (metres)

F1 = front surface power

How to calculate power factor

D = back vertex distance (metres)

BVP = back vertex power

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

F1 (front surface power) is negative

Will minify the image

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

F1 (front surface power) is positive

Will magnify the image

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

Image will be magnified

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

Image will be minified

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

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

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

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

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

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

Increasing BVD = shorter focal length = more minus power needed

How to calculate BVP (both ways)

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

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