DISPENSING QUESTIONS Flashcards
Everything we should consider
Prescription
px requirement- occupation, hobbies and age, working distances
lens type- SV, multifocals, PALS
does the CD=PD important as we wont have to decentre the lens so gives minimum decentration whilst ensuring px is looking through oc. Gives minimum blank size reducing centre thickness.
Smaller eye size= smaller blank size to start with as well.
frame size and shape= to suit rx and lens type (plastic= smaller minimum edge thickness which can impact on blank size and centre thickness)
semi finished or stock lenses- stock choose blank size carefully to reduce thickness, semi finished better as back surface can be finished to give minimum edge thickness
facial measurements
lens form- flat, curved, aspherics, lenticular- best form high plus requires steep surface curvatures= thick lenses. Compromise by flatter forms but impacts off axis image quality.
lens material- high index, impact resistance
coatings and tints- scratch resistant, anti reflection, solid tint, photochromic
special requirements- spots, low vision aid, short corridor varifocals, occupational varifocals, safety specs, enhanced single vision lenses/ freeform
likely dispensing outcome- lens thickness and weight
field of view
potential problems- near vision effectivity, swapping lens form, swapping from spherical to aspherics (effect on accom and convergence, FOV) TCA in high index, reflection, anisometropic effects, differential prism, unequal mag.
High myopes don’t change best form may not like it, we can measure the surface curvatures with a lens measure and comparing new to old. Some myopes prefer flat form eventhough more distortion and poorer off axis performance.
Aspherics help for positive lenses thinner lenses and better performance so aspheric to spherical= bad
minus lenses thick edges= reduced neg power= can read better so switching from spherical to aspheric reduces that as it gives good peripheral optics so they wont be able to read as great as it acted as an add before.
with spherical surfaces not possible to find best form solution for any fo the 3 design philosophies for powers greater than +7 so her lenses with spherical wont have off axis image quality-> aspherics.
Monocular or bin pxs, distant and near, BVD
Bifocals for all segs
D28 more noticeable than round segments, for d segs negative rx, base up due to the segment at the NVP is cancelled out by some of the base down prism induced by distance portion of the NVP= less induced prism at the NVP. Better near centration for minus lenses.
Round segs- less noticeable than D segs, generally more reading area but for negative rxs they add to the base down prism induced by the distance portion at the NVP= more jump than D-segs and image quality at NVP is poorer. Better near centration for plus lenses.
E-style- largest reading area, most noticeable dividing line so cosmetically the worst, segment centre can be on the segment divide so base up at NVP cancels out base down due to distance. As the segment centre is on the segment divide there will be no jump.
-1.00 eyes are too powerful so can probably read if she takes her specs off.
Aspherivcs= myopes and convergence
work best for positive lenses, normally on front surface, confirm by moving lens measure over surface from centre to edge and should change powers.
Myopes converge less through their specs tha hyperopes as they have a base in effect of negative lenses at their near point so object A will appear to come from object B further away so don’t have to converge as much.
Myopes specs to cl
amp of accom low then his amp of accom is just sufficient fo comfortably focus at his reading distance when wearing his specs as he doesnt have to accomodate as much when wearing them compared to cls. His accom of amp. So yes minus lenses edges of them become more positive so can read better. So dont have to accom as much through spectacles compared to contact lenses, and also may be some off axis decrease in mean negative power in specs giving effective addition in periphery of lenses. Positive moe edge of lenses.
Spherical to aspheric lenses why myopes find it hard and also what is a better solution
myopes often benefit from the increase in positive MOE at the periphery of their lenses which is a decreases in negative power which means their lenses act a little like a varifocal which is a reason why moderate ot high myopes dont need a reading add as early on in single vision reading specs.
In aspherics this increase in positive moe at edge is less so bc presbyopes have a low accom reserve they may have been using this effect to compensate and now no longer has this advantage. And his accom is not enough.
High index lenses may have been a better solution as they produce better results for minus lenses anyway but these require a flatter surface for the same rx so may reduce the peripheral moe but not as much as aspherics. Also increased tcas= colur fringes/ blurring in peripheral zone.
FOV question aspheric
Positive lenses reduce fov
negative lenses increase fov
aspheric lenses for positive lenses flatten in the periphery which means they have a reduced power and so reduced prismatic effect in the periphery so the FOV is greater for aspheric lenes and they also give thinner lenses so a larger frame can be used without making the lenses too thick
Standard axes and if cyl is horizontal at 180 or 90 or 45 what frame is the best
if cyl is horizontal so at 180= widest edges will be top and bottom if spherical lens so narrow frame would reduce the edge top and bottom. Balance this with minimum depth required for varifocals.
Cyl at 90- frames vertically oval or something round would work better as if it was narrow it would make the edge thickness greater
45 degrees- aviator style frame
3) Mrs h 66, rx +5.00/-0.75 x 90 L +5.00/-0.75x 90 Add=2.50D
you give her a pair of varifocals for everyday use and a separate pair of reading specs bc she is a reader and reads in bed has to tilt head in uncomfortable way in varifocals. She returns a few days later and says varis are fine but has to hold book a little further away when using reading specs. She also finds if she slips the specs a little further down her nose where they are fitted she can read much better at her normal reading distance. What may the problem be how would you check and what can you adjust the rx to
back vertex distance in the trial frame during the refraction could have been larger than the bvd the specs are fitted to. You would need to check if the bvd was recoded for the trial frame and compare it to the fitted frame. If it was bvd it would persist for distance as well and same for her varifocals. So check but probably isnt that.
If it isnt a problem then it is due to near vision effectivity error. More likely explanation since varifocals are okay as the lens manufacturers for varifocals usually include an adjustment to the addition to compensate for the NVEE which would explain why she can read in her varifocals but not in her reading specs. Assume trial lenses have 0 nvee.
When her specs moved further down nose effective positive power increases which is why she can read more easily when her specs slip down her nose. This suggests she needs more positive power in her lenses.
To check what adjustment you need to make to the rx fit rms h’s specs carefully and over refract. So its not enough positive power. For near objects the positive thickness contribtues less positive power for a near object so need to add some extra positive power for near.
NVEE
High plus think ab last question.
Plus lenses become more positive as you move them away
Neg lenses become less positive as move away
Thickness contributes some positive power for distance but less for near so for near may need that extra add
Why cant mutlifcoals and pals be supplied as stock lenses
multifocal and pals cant be supplied as stock as the lenses cannot be rotated to provide the correct cyl axis. Come as semi finished and normally finish rx on back surface.
Neg vs positive lenses
Neg lenses- more edge thickness
pos lenses= centre thickness (plastics can have the least edge thickness can be glazed as knife edge so centre thickness is reduced, need greater edge thickness for supras for the groove to go into so this will require higher centre thickness as well and rimless edge has to be thicker as well to be strong)
Decentration blank size
blank size= (HCD-PD)+LD+2mm
Lens thickness
minimum decentration= best results so pxs pd must closely match the Horizonal centre distance of the frame.
astigmatic rxs the max power and max thickness lies along 90 degrees to cyl axis
smlaler eye sizes reduces edge thickness for minus lense
smaller eye sizes for positive lenses reduces centre thickness
minus lenses have to have a minimum centre thickness and plus lens as well for edge thickness, for plastic it can be knife edge if metal frame bit thicker as needs to be bevelled into the frame)
Aspheric lenses for myopia and hyperopia
minus lenses can start off with flatter form lens rather than best form and then use aspherics to correct any peripheral aberrations as flat form doesnt have good off axis performance if just spherical.
-ve oblique astigmatism so find aspheric surface thar cancels out that so like oblate ellipse so spherical surface will be flatter already and then oblate ellipse reduces edge thickness even more. (aspheric positive surface astig cancels out -ve astig from flatter form)
Aspheric lenses for hyperopia- again go for flatter lens surface and then use aspherics to improve off axis performance. Positive lenses= positive oblique astigmatism with flatter surfaces so choose surface w negative oblique astigmatism so convex hyperbaloid to improve off axis performance of flat form lens and it cancels out astigmatism, it flattens towards the edge. Centre thickness further reduced.
High index better for minus lenses
generally more dense so dont really make a diffence to the weight
aspherics for positive lenses greatest thickness savings
constringence lower tca higher w high index and reflectance is higher so all high index lenes come with anti reflection coating as standard.
Weight
Weight- lens thickness to minimum, small eye size, symmetrical lens shapes, matching pxs pds to box centre distance, weight of frame, weight bearing is mostly on the nose so larger nose pads or plastic frames can be comfortable making sure the bridge of the plastic frame fits well.
