Varifocals Flashcards
What do our single vision glasses allow ?
for the px to see the prescription through the whole lens
- concentrating on a task fro a long period of time
- no good for multitaskers
Why do px with reading glasses pull their glasses down ?
- to give themseleves a multifocal lens
- look over top of lens for distance and bottom for reading
Why do px go for bifocals?
-focuses at distance and near
What are varifocals?
- focuses at distance , intermediate and near
- provide gradual change in prescription
- think about trifocal lens but with lines rubbed out
What are some advantages of varifocals?
- no jump- smoother transition- no visible lione
- good cosmetics- no lines- not indicative of older age
- look like a SVD
What are some disadvantages of varifocals?
- peripheral distortion - to the sides of the lenses
- lot of horizontal head movements are therefore required for px to get on with these lenses- particularly with older types of varifocals
What are enchanted readers/ occupational lenses ?
are a form of varifocal where the emphasis on the prescription - largest portion of the lenses is given to the intermediate or near vision unlike the varifocal that is given its largest portion to the distance.
-they allow limited distance vision
What are occupational lenses good for ?
- good for viewing presentation on screen
- larger zone for near/intermediate work
-
What are occupational lenses not good for ?
- not suitable for driving in
What does the varifocal progressive surface vary in ?
it varies in curvature to provide a gradual increase in positive powered down along a central corridor
What does the change in radius of curvature of varifcoal lens do ?
cause change in prescription
-prescription is based on r.i of lens
What is the material in the varifocal ?
lens is all one material
What happens to the radius of curvature as you go down the varifocal lens ?
gets steeper
- which is increasing this + power
- elephant trunk design
- r.o.c getting smaller as we get to bottom of lens
Where is varifocal most clear at?
only clear in central viewing corridor
So what happens to varifocal due to changes in curvature at the edges ?
-the change of curvature that produces more positive prescription towards bottom of lenses also introduces surface astigmatism
Where does surface astigmatism occur ?
- at the nasal and temporal sides of the lens-
What does surface astigmatism create ?
creates unwanted cyl power- resulting in areas of blur and distortion
What is the amount of surface astigmatism experienced on lens proportional to?
- to addition power
- the higher the add the px has - the more distortion the lens has
What is the distribution of surface astigmatism dependant on ?
-on design of lens- some have more than other- generally newer / expensive lenses are better
Who are the best candidates for varifocals?
- Previous progressive lens wearers
- Emerging presbyopes with a low Add power
- Individuals who are highly motivated
Who are the
candidates that require consideration include:?
- Presbyopes with a relatively high Add power (>+2.25)
- Previous wide-segment bifocal wearers - area you get Is less in varifocals
- Individuals sensitive to vertigo or motion sickness - due to head movement requirements
- Individuals particularly sensitive to changes in vision
- Those with a vertical imbalance or anisometropia
- Eye movers/ people who require wide visual areas
What is the fitting procedure for varifocals ?
- make sure px sits comfortably
-select the frame
-pre adjust the frame- before you begin- so make sure px area of vision will fit into the frame they have chosen
-amount of measurement depends on lens you choose
-MSU
-Measure the fitting heights
§Measure the distance PDs
§Verify blank size
§Take free-form measurements (if necessary)
How do we dispense varifocals ?
- Take PD
- THINK horizontally- px eye is lined up centrally in the corridor vision- to give them as much vision as possible
- take monocular PD
- cant assume px is exact same distance to nose
- THINk position of pupil vertically- make sure px is starting the progression zone at the right point (From distance to intermediate to reading)
- take heights
What is monocular PD?
distance from centre of pupil to the centre of bridge of nose
How do you take the vertical measurements when dispensing varifocals? (HEIGHTSP)
- using horizontal centre line
- measure from centre of pupil to HCL
- whilst you measure this point- look at centre of pupil to bottom of rim- manufacturers will specify how long that zone needs to be make sure the lens fits in all the vision
- face wrap/face tilt
Why are heights also taken?
to make sure the lens are positioned properly
- also to make sure the lens will fit into the chosen frame
- manufacturers will specify from the centre of pupil and top of lens- to make sure the amount of distance vision can fit in
What is the required frame depth\?
pupil centre to bottom of lens - is 14mm (result in areas of Intermediate and near ebbing smaller and transition being quicker- not all px tolerate this
-some px prefer a longer corridor- that gives more gradual change in prescription - make sure that shorter corridors are wanted or not.
What happens if a frame is too small for required depth?
- px misses out on some d vision and n vision
- make sure frame choice is suitable for the Lens given
What happens if frame is big but sits quite high on px bridge?
they still might not have enough area for near vision
- in this case you can adjust the frame so its in the correct space- where pupil would sit centrally- s
- or might have to change the frame- if cant get too sit in the right place
What are some other measurements to how px sees through their varifocals?
pantoscoptic tilt
Why is pantoscoptic tilt important in varifocal lenses?
brings near vision closer to the eye which increase the field of view
- area for reading little larger
- optical and visual axis of lens are aligned when px is looking down to read
How can pantoscoptic tilt help?
just by increasing this can make a large difference to how much they are seeing
What happens if px has a orthoscopic angle or retroscopic angle ?
- the lens is almost bending outwards from the bottom- going to need to alter that sitting on px head- look at frame from side
- or alter on the frame
What is face wrap / face tilt ?
- isn’t required for all varifocal lenses but required for many of the free form Lens designs
- inclination of the temporal edge of each lens towards the face- makes sure the front of the frame is following naturally the shape of the skull
- peripheral distance zone closer to the eye- increasing the field of view
- also moves the peripheral areas of distortion out of the wearers immediate field of view
Why might px not tolerate varifocal lenses?
- incorrect prescription or measurements - technical
-misunderstanding of measurements
-px not sat correctly or frame not sat correctly
-have to make sure px has a natural head position when taking measurments
-not been educated on what to expect
-
what is the distance between the 2 engraving dots?
always 34mm
-alignment distance markings
What is the engraved temporally ?
will be the reading addition
e.g 15- means 1.50
What is engraved nasally?
manufacturers mark
- brand of lens
- supplier
What is the fitting point?
-where you want the distance prescription to start- line up with px pupil
What is the prism reference point ?
- where we measure any prescription we find in a varifocal lens
- dont measure at fitting point
What is near vision checking point ?
- vision at near portion
- due to design of varifocal more better to use engraving
- but can draw if you feel like px hasn’t got full area of vision in near
Why do we measure the prism from reference point rather than any other point on the lens?
prism thinning
Why do we do prism thinning ?
What does prism thinning do ?
-so therefore prism thinning is used to equalise the thickness of the top and bottom of lens
What does prism thinning do ?
- so therefore prism thinning is used to equalise the thickness of the top and bottom of lens
- making thickness equal throughout lens
How do we do prism thinning ?
remove base up prism from the top of the lens- which makes that part of the lens thinner
- leaves us with base down prism- work on 2/3rds of the addition .
- some manufacturers use intelligent thinning
What is intelligent thinning ?
calculate the amount of base down to put with the lens to produce the best finished result
What happens if px does not have prism in their prescription ?
-looking at prism reference point even if px does not have any prism you will find 2/3rds of the add at that prism point
Why do px get on with this well ?
- same right and left
- no problem when px looks down to read as the amount of prism they experience at near vision point is equal
- however if they have different adds to right and left eye- will be problem -can request lens without thinning
How to choose a progressive lens?
- look at the size and location of the useful areas of vision they require- (e.g large intermediate area, or distance etc)
- think of width and length of corridor vision- (E.g what px needs as part of their visual task analysis but also from the Frame they have chosen and where it sits)
- the rate of power change in the corridor
- the location , magnitude and axis of unwanted spy and cyl refractive powers - if px has suffered with distortion in the past - wants to use a lens where less distortion and place the lens so px doesnt look through it
What to make sure you discuss with px?
- pros and cons with px
-Head movement ØPeripheral distortion ØChange in Rx over lens
§ Demonstrate
§ Adaptation period
§ What to do if there is no progress § Confidence
§ Talk positively!
