accomodation - specular vs ocular

Question 1

what is the accommodative demand for emmtropes and cl wearers

Answer 1

emmtropes and contact lenses - 1/ distance= accommodative demand

normal distance = 33 cm = 1/33= +3.0d

myopes and spectacle wearers have reduced accomodation

Question 2

how does refractive error effect accommodative demand

Answer 2

the wearing of spectacles , myopic or hypermnetopic changes the accommodative demand of an individual

• if we are thinking about the accommodative demand of someone wearing spectacles then that is spectacle accomodation

Question 3

what is ocular accomodatiion

Answer 3

an emmetrope or a hyperope/myope with contact lenses then we are measuring ocular accommodation

Question 4

what is the ocular accomodation of emmetropes and contact Lens wearers

Answer 4

1/ distance

distance = 33cm = 0.33m

1/0.33 = 3 diopters

distance = 25cm

1/ 0.25= 4 diopters

Question 5

how is myopia and accommodative demand related

Answer 5

myopes in spectacles have reduced accommodative demand compared to the emmetrope/ hyperope in contact lenses

with myopes we are putting the retinal conjugate and putting it at optical infinity

when myopes are corrected we have 0d of accomodation for distance viewing

we use the correct amount of - so that the retinal conjugate is at optical infinity

Question 6

when myope is looking in the distance with spectacles - what is happening to accomodatio

Answer 6

• when myopes looks through spectacles at distance - no accomodation is being done

myopes have less accommodative demand

when you look at a distance object and then a near object your visual system perceives blur -(that you don’t notice) this drives accomodation but to get from blurry to clear myopes have to do less work

Question 7

how do we correct hypermetropes

Answer 7

in hypermetropes we add + so that the retinal conjugate is back at optical infinity - in hypermetropes the optical infinity has passed and therefore we use + Lenses to bring them back to optical infintity

Question 8

how to work out ocular accomodarion

Answer 8

accommodative demand =

L dist - Lnear

i.e. vergence of cornea when looking at a distance object -vergence of cornea when looking at a near object

Question 9

what is the curvture of a light wavefront. for a an emmetrope looking at optical infinity

Answer 9

l corn is the distance from the eye to the object

l dist (curvture of light from a distance) for an emmetrope looking at optical infinity = 0d because 1/ optical infinity = 0d

and for near l corn will be 1/ -0.33 which will be -3

0- –3 = + 3 = accommodative demand

because accomodative demand = list- l near

Question 10

a myope wearing contact lenses r and left are looking at an object 20cm away calculate the accomodative demand compared to when they Are wearing glasses vs contact lenses

Answer 10

step 1

for contact lenses 1/ distance = accommodative demand

and that will be 0 because distance = optical infinity and 1/ optical infinity = 0

and then for contact lenses part it will be

1/-0.2 because It is 20cm away which will be -5

and then you do 0 - - 5 which will be -5

Question 11

how are myopes corrected

Answer 11

a myopes eye has too much + for distance therefore we have to correct it with a negative lens

the correcting lens has take the retinal conjugate and put it at optical infinity (shown by the parallel lines)

when we are wearing spectacles and we are looking at a distance object - the light hitting the cornea appears to be coming from somewhere that isn’t optical infinity

the light hitting the cornea is diverged

has divergence added to it

for a myope ion spectacles corn dosnt come from optical infinity and therefore the ver gence dosnt equal 0 because the lens has added divergence

this is why specular and ocular accommodation = offset
1
because the lens is altering the mergence

Question 12

where is vergence at 0

Answer 12

vergence = 0 at the spectacle lens

Question 13

what is image Vergence equal to

Answer 13

if you have got the object vergence you want to know what the image veregcne is

image vergence = L’SP= Lsp + Fsp

FSP= strength of the spectacle lens

LSP= objecte vergence

Question 14

how to work out image distance when object vergence =0

Answer 14

l’sp =1/L’SP

which is basically the second focal length f’sp

1 divided by image mergence when object vergence =0

Question 15

what is f’sp

Answer 15

f’sp is a specila case of image distance when the object is at optical infinity so the object mergence = 0

but it is an image distance

Question 16

what are the priniciples of correction

Answer 16

principles of correction - take light that comes from optical infinity and make it look like it is coming from the far point because light that comes from the far point images on the retina

Question 17

what is distance to the far point

Answer 17

distance to the cornea = image distance - back vortex distance

and cornea vergence is equal to 1/ distance to the cornea

Question 18

how to work out the effect on the accomodative demand if ametropes are corrected by spectacles

Answer 18

when they are looking at a far point object then object veregence is equal to 0

so object vergence = LSP
LSP=0
L’SP = IMAGE VERGENCE
IMAGE VERGENCE = OBJECT VERGENCE + SRENGTH OF LENS

IMAGE DISTANCE = l’sp - WHICH IS EQUAL TO 1 DIVIDED BY IMAGE VERGENCE L’SP

distance to the cornea lcorn - which is equal to l’sp (image distance) - d which is back vortex distance

and vergence of cornea- L corn- is equal to 1 divided by the distance to the cornea which is corn

Question 19

how to work out l corn

Answer 19

distance to the cornea =

l’SP-D

where l’sp = image distance and d = back vortex distance

Question 20

what does light from real object do to light

Answer 20

light from real objects add divergence so diverging lens will diverge even more

Question 21

how to work out object distance lsp and object verging when it isn’t coming from optical infintity

Answer 21

object distance = distance from lens to object L + back vortex distance

Bvd is always negative

object vergegence = 1 over object distance

Question 22

a myope is wearing glasses right and left , they are looking at an object 20cm away , calculate the accommodative demand compared to when they are wearing contact lenses- spectacle lens power = -11.50

Answer 22

sign convention = -

near lcorn = 1/ l corn

= 1/ -0.2 = (20 cm = 0.2mm)

which = -0.5

distance l corn is equal to 0 because light is coming from the cornea

accommodative demand for contact lenses = l dist - l near

= 0 – 5 = 5 (ocular accomodation = +5 diopters)

i.e. mergence of cornea when looking at a distance object - veergence of cornea when looking at a near object

image vergence L’SP when object Vergence LSP = 0
is equal to 1/ L’SP

image vergence = L’SP= LSP + FSP (object vergence + strength of the spectacle Lens power)

0 + (-11.5)= -11.5 d

and then special case of image distance (l’sp) = 1/L’SP

L’SP= image vergence

1/ -11.5 = -0.0870m

Question 23

what is the object distance for the cornea

Answer 23

the object distance for the cornea when looking at an object at optical infinity is the distance to the far point - bevcaude light that comes from the far point images on the retina

so the far point is equal to lower case k

which is equal to image distance - back vortex distance (which he says in the question weather it is positive or negative)

so if the image distance is equal to -.0870m then you subtract that from the back vortex distance which is 13mm

-0.0870 - ( +0.013) = -0.1

so L distance = 1/ -0.1 = -10.0

(the object is 20cm away from eye not spectacle lens we are trying to work out how far it is from the glasses to work out how much divergence or convergence it has added thus working out the far point)

so the far point is the distance that the object appears to be from the spectacles because the spectacle is adding vergence to the light not where the actual object is

Question 24

how to work out object distance from the lens

Answer 24

object distance from the Lense is lower case lsp

l + d

(l= distance from lens to the object)

d is back vortex distance

therefore object vergence is equal to 1 /lsp

1/ lsp(object distance)

Question 25

image vergnece L’SP- is equal to

Answer 25

L’SP (image vergence) = LSP + FSP

object vergence + strength of the spectacle lens

Question 26

how to work out the object distance from the cornea

Answer 26

object distance from the cornea = lcorn

lcorn is equal to l’sp - d

where l’sp is equal to image distance

you then convert that into diopters by dividing it by one

you then subtract the distance from the near

and that tells you the accomodative demand

Question 27

a myope is wearing glasses right and left they are looking at an object 20cm away

calculate the accomodaitive demand compared to when they are wearing glasses vs contact lenses

back vortex distance = +13mm

strength of the lens = -11.5

Answer 27

answer =

accomodative demand in contact lenses was +5

accommodative demand in glasses = +3.83d

+5 - +3.83= 1.17 difference in accommodative demand

Question 28

a myope is wearing glasses right and left they are looking at an object 20cm away

calculate the accomodaitive demand compared to when they are wearing glasses vs contact lenses

back vortex distance = +13mm

strength of the lens = -11.5

what are the steps for working out this question

Answer 28

step 1 - accomodative demand = l dist - l near

vergence of cornea when looking at a distance object - vergence of cornea when looking at a near object

so start with working out accomodative demand for the contact Lense wearer

so l dist will be 0 because light is coming from optical infinity

l near for the contact lens wearer will be 1/-0.2= -5d

0- - 5= +5 (accomodative demand for the contact lens wearer= +5 diopters

step 2 - image vergence that the spectacle lens has done

L’SP= LSP + FSP (image vergence + strength of the spectacle lens)

0 + -11.50 = -11.5
then work out image distance l’sp 1/ l’sp - because object vergence is equal to 0

1/ -11.5 = -0.8970m

then work out the far point (lower case k) = l’sp - d

-0.0870 - ( +0.013) = -0.1

l distance = 1/-0.1

-10.0

L NEAR
then work out the the distance from the lens to the object -0.2 + (+0.013) = -0.187m

then work out object vergence = 1/-0.187m = -5.35d

then work out the curvature that is leaving the lens (image vergence) L’SP

L’SP= LSP + FSP

-5.35 + - 11.5= 16.85D

L’SP= 1/ -16.85= -0.0593m

now work out the object distance from the cornea lcorn = l’sp- d

= 0.0593- 0.013
convert that answer into diopters by dividing by 1

so l dist - l near

= -10 - (-13.83)= +3.83

+5.00 - +3.83 = 1.17

Question 29

steps for calculating accomodative demand for someone wearing contact lenses vs glasses when x cm away could be myope or a hype rope

Answer 29

accomodative demand = l dist - l near

step 1 - accommodative demand of contact Lense wearer

when looking at distance light is coming from optical infinity therefore l distance = 0 because 1 divided by optical infinity = 0 diopters

l near = 1/ the distance (remember to convert cm into m)

you then subtract l distance - l near and that is the accomodative demand of for a contact Lense wearer

now start by working out image vergence - L’SP

L’SP = LSP + FSP

where L’SP= image vergence

LSP= object vergence
FSP= spectacle lens power

then work out the image distance l’sp

so this will 1/ L’SP (1 divided by the image vergence).

then work out the far point = k = l’sp - d

then you want to convert this into diopters by dividing l diopters by 1

this will give you l dist

then to work out l near you want to work out object distance which is equal to the distance you were given in the question and then you add that to back vortex distance

then you 1/ the value you just worked out will give you the object vergence

then work out image vergence = L’SP = LSP + FSP

LSP= object vergence

FSP= strength of the Lense

to work out l near - l’sp - d

l’sp (image distance)
d = back vortex distance

to work out image distance

1/ L’SP (Image vergence)

them convert the value that you got from l’sp - d into diopters by dividing by 1

that is l near

then subtract l distance from l near to give you the accomodative demand for someone who is wearing spectacles

Question 30

how to work out accomodative demand of someone wearing spectacles

Answer 30

1/ fsp

k = fsp- d

object distance for specs lsp= k + d

LSP = 1/ lsp (object vergence for cornea)

image vergence for spectacles L’SP= LSP + FSP

l’sp = 1/L’SP

l corn = l’sp - d

L corn = 1/ lcorn

Question 31

A patient is wearing spectacles r and left +13.75ds

dvd = 13mm

calculate the difference in accomodative demand between specs and cl’s when viewing an object 16.7cm from the eye

Answer 31

contact lenses = 1 / distance

1 / -0.167 = 5.98

0 – 5.98

+ 5.98 for contact lenses