notation in astigmatism Flashcards
how much power is there in the direction of the axis
the axis has o power in the direction of the axis , you can look at the sphere and that is one of the powers off the principle meridians
what direction is it - that is where the axis is
does not matter if it is a positive cylinder or a negative cylinder
the axis is the direction of zero power in the cylinder
the cylinder is the difference between the two principle meridians
where the axis is there is no cylinder power
if a patient is hyperopic along both power meridians what type of astigmatism is that
if they are hypertrophic along both power meridians = compound hyperopic astigmatism
because both power meridians are hyperopic
what is simple hyperopic astigmatism
in 1 meridian the patient has 0 refractive error - the other meridian = hyperopic so it is simple hyperopic astigmatism
simple myopic astigmatism , one of the meridians will be zero power and the other will be myopic
what is with the rule astigmatism
the vertical meridian is the most positive meridian
against the rule is where the most positive meridian is in the horizontal
what is oblique astigmatism
if the meridians are 145 and 135 that is oblique astigmatism
how to calculate ocular refraction
K= K’- f eye
axial length diopters - f eye
how to tranpose axis
the direction of the most positive is the negative cylinder axis
what is best vision sphere used for
if someone has a high level of astigmatism and they want a contact Lense - this would be difficult as a contact Lense moves and stabilising a cylinder would be difficult
so you wouldn’t prescribe a cylinder you would use the best vision sphere
sphere + cylinder / 2
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what is the far point
in the absence of accomodation the far point is the point conjugate with the retina
when a hypermetrope accommodates then the far point won’t be conjugate with the retina it will vary if we are accommodating
what is the astigmatic interval
the gap between the front and back focal lines is called the interval of turn
interval of Sturm = astigmatic interval
i.e. the gap between the front and back focal lines in diopters
the size of the cylinder that will close the astigmatic interval that will correct the astigmatism
the best vision you can get for patients using spheres is the circle of least confusion to the retina
how do you put the circle of least confusion on the retina = sphere + cylinder / 2
how to classify the different types of astigmatism using the diagrams
if their both in front of the retina = myopic
if back focal line is on the retina - then = simple myopic
front focal line on retina = simple hyperopic
both focal lines in front of retina = compound hyperopic
what tells you you are near the end point in ret
if the reflex is really bright this means you are near the end point
what is the definition of an emmetrope
retinal conjugate is at optical infinity
