definitions

Question 1

with the rule astig

Answer 1

steepest curvature in vertical meriidan

negative cyl axis 180

Question 2

against the rule astig

Answer 2

steepest curvature horizontal meridian

negative cyl axis 90

Question 3

5 classifications of asitgmatism

Answer 3

CHA = astigmatic image behind retina
SHA = front focal line on retina but back focal line / DLC behind retina

mixed = one focal line in front one behind

CMA = astigmatic image in front of retina
SMA = front focal line/ DLC in front of retina and back focal line behind

Question 4

disc of least confusion definition

Answer 4

circular blur patch located dioptrically midway between astigmatic foci

Question 5

BVS

Answer 5

spherical correction placing DLC on retina for distance vision

Question 6

power vector analysis

Answer 6

converts sphere/cyl with oblique axis (cant be added) into power vector components M, J0, J45 which can be added

Question 6

why does astigmatism have half the effect of spherical ametropia

Answer 6

z = p x ast / 2K’
blur (diameter of DLC)

b = p x K / K’
blur

blur produced by 1D of astig is half of the equivalent maount of spherical ametropia

Question 7

ive ignored week 9/11 in this

Question 8

how does a stenopaeic slit reduce blur

Answer 8

reduces pupillary aperture along one meridian only

minimises astigmatic blur when slit axis is parallel to correcting cyl axis (acts 90)

horizontal slit reducs vertical blurring so lines appear sharp

Question 9

accomodation

Answer 9

increase in dioptric power of the eye with an effort to focus at near

Question 10

presbyopia

Answer 10

reduction in accomodation with age that is sufficient to affect an individual and benefit from intervention eg: reading specs

due to increased lens mass

Question 11

near point Mp

Answer 11

point conjugate with the retina whn eye is in its fully accomodated state

(closest distance eye focuses on to produce clear image on retina)

Question 12

range of accomodation racc

Answer 12

linear distance from far point to near point

Question 13

virtual vs real part of range of accomodation

Answer 13

virtual = first D of accomodation reduces blur bringing distant object into sharp focus

real = remaining D (from B) accomodation helps focus on near objects sharply

Question 14

range of distinct vision

Answer 14

real part of the range of accomodation

if amp > K then near point real and farthest point of distinct vision = infinity

if amp < K then near point virtual and range of distinct vision zero

Question 15

facultative vs absolute hypermetropia

Answer 15

facultative = amount of hypermetropia that can be compensated by accomodation to produce clear distance vision

absolute = remaining hypermetropia that cant be neutralised by accomodation

Question 16

amplitude of accomodation

Answer 16

maximum increase in power of the eye with an effort to focus at near

(positive)

Question 17

ocular accomodation (Ao)

Answer 17

accomodation measured at the first principal plane of the eye

Question 18

spectacle accomodation (As)

Answer 18

accomodation measured at the spectacle plane

Question 19

what does the num on each line of snellen represent

Answer 19

distance at which a letter height subtends 5’ arc or limb width subtends 1’ arc

Question 20

emmetropia

Answer 20

second focal point coincides with the retina for an unaccomodated eye

Question 21

far point

Answer 21

object point conjugate with the retina in the unaccomodated eye

(farthest disyance object is clearly focuses when eye accom is relaxed)

Question 22

myopia

Answer 22

second focal point falls in front of retina for an unaccomodated eye

Question 23

ocular refraction

Answer 23

incident vergence required for a sharp optical image to be formed on the retina

Question 24

hypermetropia

Answer 24

second focalpoint falls behind retina for unaccomodated eye

Question 25

how does ametropia occur

Answer 25

failure in coordination during growth of the optical components of the eye

Question 26

spectacle correction of hypermetropia/myopia needs

Answer 26

second focal point must coincide with far point

Question 27

transverse magnification

Answer 27

ratio of image height to object height

Question 28

basic blurred image height

Answer 28

height of chief ray at retina chief ray = ray passing through centre of entrance pupil

Question 29

vision vs visual acuity

Answer 29

vision - Perception of differences in the external world resulting from stimulation of the retina by light Vision Smallest line of letters read with the unaided eye Visual Acuity Smallest line of letters read with spectacles or contact lenses worn (provided they correct all defects of focus

Question 30

second focal point

Answer 30

the image point where parallel light rays, entering a converging lens (convex) from infinity, actually converge after refraction, forming a real image for a diverging lens (concave), it's the point from which parallel rays appear to diverge, forming a virtual image

Question 31

minimum angle of resolution

Answer 31

Angular separation of two points or lines at the resolution acuity limit The minimum angle of resolution (MAR) is the smallest angular separation between two points that the eye can distinguish as separat

Question 32

astigmatism

Answer 32

variation in focusing distance with meridian

Question 32

far point

Answer 32

The far point is the farthest point at which an object can be placed along the optical axis of the eye and still be focused clearly on the retina without any accommodation (muscle effort) by the eye's lens