Convergence & depth of field Flashcards
RM
What is convergence as related to eye movements?
Both visual axes move nasally to place the image of the near objects on the fovea.
Which of the extraocular muscles are mainly responsible for producing convergence?
Medial recti muscle
What is the effect of convergence on the pupil?
Miosis
Define near point convergence
The nearest point to which the eyes can converge
Describe how the position of the Near Point of Convergence can be found
*Px looks at object on midline with both eyes open
*Object brought closer until target doubles
*The distance from the eyes to this point is the NEAR POINT OF CONVERGENCE
*If practitioner notices eyes stop adducting before an image doubles this will be the NEAR POINT CONVERGENCE
What would be considered a normal range of values for this measurement?
50-100mm
Advantage of a meter angle as a unit of convergence
*Doesn’t require a PD
*It is numerically equivalent to accommodation in dioptres
How is depth of field affected by pupil size
The smaller the pupil, the larger the depth of field. If the pupil is larger, the depth of field will be reduced.
Convergence as related to eye movements
Both visual axes move nasally to place the images of the near objects on the fovea
Which extraocular muscles are mainly responsible for producing convergence?
Medial recti muscle
Effects of convergence on the pupil
Miosis
Near point of convergence
The nearest point to which the eyes can converge
How position of Near Point of Convergence may be found
*Px looks at an object on midline
with both eyes open
*Object is brought closer until
the target doubles
*The distance from the eyes to
this point is the ‘near point of
convergence’
*If practitioner notices the eyes
stop adducting before an image
doubles, this would be the NCP
Range for normal convergence
From 50 - 100 mm
Advantages of meter angles as a unit of convergence
*Doesn’t require a PD
*Numerically equivalent to
accommodation in dioptres
Negative spectacle lens and convergence
Negative spectacle lens will reduce the amount of convergence needed
Convergence
*Describes a movement of the visual axes nasalwards from primary position of gaze.
*In primary position the visual axes are initially parallel.
*A convergence movement causes the visual axes to intersect in front of the eyes (this usually means the two visual axes intersect at the fixation point)
*Assuming R&L symmetry each eye converges by the same angle when the fixation point is on the median line.
Relative convergence
*Is the amount of convergence or (divergence) that a subject can exert for a fixed amount of accommodation.
*Its measured by placing prisms of increasing strength in front of px’s eyes while they view a target at a specified distance.
*Prism base OUT will cuse convergence & as the prism power is increased, at acertain point, accommodation will be stimulated, causing the target to blur.
*The strength of the prism that causes this to occur is known as the blur point.
*If prism power is increased further, eventually the subject will no longer be able to converge to overcome the prism & diplopia (double vision) will occur. This is known as the break point.
*Prism base IN will cause the eyes to diverge
