TEST 2 Flashcards
How to derive and plot the optical cross
you can derive a prescription directly from an optical cross, you can also create an optical cross based on a prescription. optical is a notation for refractive error and has a meridian and curvature.
What does the optical cross represents
is showing the power of a the sphere and the power and axis of the cylinder. its showing what those two powers are
How to transpose plus and minus cylinder prescriptions
what changes and what does not change?
Transposition refers to the process of changing the prescription from a plus cylinder form to a minus cylinder form, or from a minus cylinder to a plus, without changing refractive value
What does the spherical equivalent represent,
The spherical equivalent is used in contact lenses, certain tests such as the visual field, and is also important for surgical patients. So, how does it work?
Name a situation where spherical equivalent is used?
The spherical equivalent is used in contact lenses, certain tests such as the visual field, and is also important for surgical patients. So, how does it work?
What is the circle of least confusion
Half way in between the 2 focal lines a blurred circle is formed called the “circle of least confusion” and is represented by the spherical equivalent of the spherocylindrical lens
It is the least blurry image possible through a spherical lens.
What is the conoid of sturm
Sturm’s interval (or interval of Sturm) is the distance between the 2 focal lines. Ellipses are formed in the space between
Wh is Vision not completely sharp with the spherical equivalent?
If we provide correction using the spherical equivalent, the circle of least confusion will be focused on the retina. This provides a slightly blurred image but eliminates the distortion from the ellipses
For minor amounts of astigmatism, a spherical SCL may be considered instead of a toric SCL because spherical lenses may provide acceptably good vision and their fitting does not have the potential for as many complications as toric SCL.
How to calculate the sperical equivalent
1) Take ½ of the cylinder power and add it to the sphere
2) Drop the cylinder power
3) Drop the cylinder axis
Optical cross example
Optical cross example
Optical cross example
Optical cross example
Optical cross example
Optical cross example
Optical cross example
Plotting optical cross
Plotting optical cross
see pic
see pic
Optometrists use
Optometrists use minus cylinder
ophthalmologists typically use
ophthalmologists typically use plus cylinder
CONTACT LENS PRESCRIPTIONS ARE ALWAYS IN
MINUS CYLINDER
More transpose examples
sperical Equivalent
practice work sheet for test
transposition work sheet
plus
Hyperopic
plus
shorter eye
astigmatism compund hyperopic
real focal point
magnify
convex
converge
far sighted
too little power
Minus
Near sighted
diverge
emmetropic eye
with the rule
Compound myopic
too much power
Spherical lenses create a
why?
focal point
because they have the same refractive power in all meridians
spherocylindrical lens
creates
2 focal lines perpendicular
to one another.
