field of view and near vision effectivity error

Question 1

what is field of view ?

Answer 1

total area patient can see when eye is in primary position- i.e. patient is looking ahead

Question 2

what is apparent field of view ?

Answer 2

• angle subtended by the empty frame aperture at the eye’s centre of rotation
• the rim of the frame is important
• if rim of frame results in space within the frame that is less than the patient would see if the glasses weren’t there than patient field of view would be reduced

Question 3

what is real field of view ?

Answer 3

it is obtained when a spectacle lens is glazed into the frame

Question 4

what is field of view influenced by ?

Answer 4

• frame and therefore lens shape
• vertex distance
• power of the lens - only in real field of view

Question 5

explain the field of view diagram ?

Answer 5

R - centre of rotation of eye - as eye moves to see all around it always moves from this point

v- distance from back of lens to R

R’- image of centre of rotation which is produced by lens
- same value as v

y - diameter of half the frame
overall diameter is 2y

Question 6

what happens when you add plus lens ?

Answer 6

the image is magnified , and field of view is reduced

Question 7

what does loss of visual area lead to ?

Answer 7

ring scotoma - tells us that the visual area is in a ring around patients vision - this means that patient can’t see in this area

Question 8

what is jack in the box effect ?

Answer 8

objects appear to be in the vision then they disappear and come into vision again - objects jump into field of view

Question 9

what happens when you add a minus lens ?

Answer 9

objects are minified - more of object will fit into viewing area so field of view increases

Question 10

how to calculate apparent field of view ?

Answer 10

tan β = ( half the frame depth ) y / ( position of R from lens ) v - y/v
v= 25-27mm
B= tan-1 (y/v)

• only takes into account half frame depth so to find full field of view you must multiply by 2

Question 11

what is the equation of real field of view ?

Answer 11

tan θ = y (S-F) / 1000

S(D) = 1000/s
y= half diameter 
F = power - prescription of lens taken into acount

• make sure you multiply by 2 at the end

Question 12

what is back vertex power of lens ?

Answer 12

vergence which leaves the lens when light originates from a distant object ( infinity )

Question 13

what is near vision ?

Answer 13

light not arriving from a distant object but at a known distance from the lens
e.g. book patient is reading to lens

Question 14

what is vergence leaving the lens at near dependent on ?

Answer 14

vergence leaving the lens at near depends on thickness form of lens.

Question 15

why are lenses with the same back vertex power but of different forms not interchangeable?

Answer 15

due to change in form between trial lens and patients actual lens and the change in power from label in trial lens to what we calculate what patient is actually looking through means that lenses with the same back vertex power but of different forms are not interchangeable

Question 16

what is near vision effectivity error ?

Answer 16

• the difference between the image vergence produced by a thick lens at near( what patient actually been dispensed ) and the expected vergence from a thin lens ( trial lens we used )
• difference between what patient actually been dispensed and what we expect the vergence to be from the trial lens we used

Question 17

what is equation for NVEE ?

Answer 17

(t/n)L1(L1+2F1)

Question 18

what is the box system of measurement ?

Answer 18

• describes the vertical and horizontal measurements of lens if it was surrounded by a box
• always in mm
• always horizontal by vertical

Question 19

what is horizontal centre line ( HCL ) ?

Answer 19

( HCL ) midway point horizontally through the box

Question 20

what are frame markings found in spectacle lenses ?

Answer 20

• horizontal lens size x distance between right and left lens ( mm )
  e. g. 50x18 50*18 50 □ 18
• total length of side ( mm ) from screw to the tip taking out the bend
  e. g. 135 mm
• manufacturer / design / suppliers name or trademark
• frame model
• frame colour

Question 21

what box centre distance ?

Answer 21

• measuring from the centre of one box to the centre of another box
• add horizontal lens size and distance between lenses
• this measurement that can be used to work out any decentration that we are going to use to place the optical centres of the lenses in front of the patient’s pupil.
• The patients pupillary distance would match the box centre distance.

Question 22

what happens in perfect dispense ?

Answer 22

• PD should match box centre distance and therefore no decentration
• if PD doesn’t match box centre distance then move optical centre away from box centre so they sit in front patient’s pupil

Question 23

what is effective lens diameter ?

Answer 23

• measure from box centre to furthest point on lens and by doubling measurement

Question 24

what are the three groups for lens mounting ?

Answer 24

1. full rim - rim surrounds whole of lens
2. rimless - no rim surrounding lens
3. supra - frame covers part of lens
   when covers bottom and not top - this can be good for reading

Question 25

what does mounting type determine ?

Answer 25

determines the edge of lens which we need to glaze to allow lens to remain securely in frame and not fall out

Question 26

when do we use BEVEL and MINI BEVEL lens edge ?

Answer 26

used for full frames

Question 27

when do we use flat lens edge ?

Answer 27

rimless style frame

Question 28

when do we use supra lens edge ?

Answer 28

edge not covered must be held in place this can be done by a carving a groove into lens edge and thin clear wire is placed into groove and secured to frame

Question 29

what are ways rimless lens mounting can be produced ?

Answer 29

rimless edge can be 
flat 
scalloped 
facetted 
inlaid

Question 30

what are frame components ?

Answer 30

• pads
• bridge
• rim - incases lens
• side - extension from front of frame that fits around the ear
• joint - where side of frame and front of frame meet

Question 31

what is bridge ?

Answer 31

the bridge forms the main connection between the right and left rims . it may or may not have pads
different designs for plastics and metal frames

Question 32

what is pads ?

Answer 32

extension or attachments to the bridge which form the bearing surface of the frame for the nose
larger pad = greater weight distribution

Question 33

what are the bridge types ?

Answer 33

. fixed pad bridge - plastic frames only , pads moulded into rim and same material as frame

. regular bridge - plastic frames only
no moulded pad shapes

. keyhole bridge - plastic frames only
only pads resting against patient’s nose

. pads on arms - metal frame only

Question 34

what are pad on arms ?

Answer 34

. metal frame only
. pads can be screw on or push in
. can be saddle / comfort / strap bridge with same material attachment
. greater surface area touching nose = more comfortable

Question 35

what are pads on arms made from ?

Answer 35

. silicone - flexible , light weight , soft

. vinyl/acetate - light weight , rigid , cheaper

Question 36

what are some examples of sides ?

Answer 36

. drop end
. loop end
. curl sides