Spectacle prescriptions and determining type of refractive error
Note: Anytime a negative or minus sign is denoted, it is denoted with parentheses
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Emmetropia- The normal state of the eye when parallel rays of light converge directly
on retina. No refractive error, no prescription. Everything is clear without lenses.
Ametropia- there is a prescription and refractive error
We use diopters (D) as a unit of measurement for refractive error
o D= 1/focal length in meters
 Find focal length when given diopters
 Ex: (-)4.50D.
o Answer: focal point = (-).22m
 Ex: (+)1.00D
o Answer: focal point= +1.00m
 Find the power of the lens when given focal length
 Ex: 50cm
o Answer: D=1/.50m=+2D
 Ex: 16 inches
o Answer: D=1/.4m= +2.5D
 Understand conversions of inches, centimeters, etc
 1 inch = 2.54 cm
Distance refractive error
o Spherical refractive error- all the light coming in is the same in all meridians.
No matter where light comes in it’s exactly the same. We’re only looking for
one number for the prescription.
 Hyperopes- Parallel rays are focusing behind retina. The patient needs
a convex lens, also called a plus lens (+). It’s measured in diopters, and
abbreviated as D (diopter), DS (diopter sphere), or SPH (sphere).
 Ex- +5D hyperope
 Myopia- Parallel rays are focusing in front of the retina. The patient
needs a concave lens, also called a minus lens (-) to diverge light to
back of retina.
 Ex- -5D hyperope
o Astigmatic Refractive errors- light focuses on back of eye in two separate
meridians at two distinct focal points. At least one of the meridians does not
focus on the retina.
 Tests for astigmatism
 Keratometry- corneal astigmatism
 In reality, astigmatism occurs at cornea AND lens, so what
she’s referring to today is BOTH, not just corneal astigmatism
as measured by keratometry
 The two principal meridians are 90 degrees away from each other. The
patient will need two lenses to focus both of these meridians on back
of retina.
 It’s no longer just a spherical lens, it’s a sphere-o-cylinder lens
(compound lens or sphere-cylinder lens)
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Now there are 3 different numbers, rather than just 1 of the spherical
lens
 3 components:
 1st- power of the spherical component. It’s either (+) or (-)
 2nd- power of the cylindrical portion of eye- always in minus
cylinder form (ophthalmologists use plus, we can convert them,
but optometrists always use negative)
 3rd- axis (orientation) of the cylinder. Denoted as x then the
number that always has three digits.
 To determine power of the two principal meridians:
 1st number is the principle meridian 1
 2nd number is the sum of the sphere and cylinder components
o The second meridian is NOT just the second number in
the prescription!! **
 Examples:
o (+)1.00 -2.00 X 110
 Principal meridian 1: (+) 1.00D
 Principal meridian 2: (+)1.00 -2.00= (-)1.00 D
o (-)3.00 – 4.50X010
 Principal meridian 1: (-)3.00D
 Principal meridian 2: (-)3.00D-4.50D= (-)7.50D
o (+)4.00- 6.50x175
 Principal meridian 1: (+)4.00D
 Principal meridian 2: (+)4.00D-6.50D= (-)2.5D
 To determine the axis:
 You are only given one of the numbers in the prescription, it
relates to the cylindrical component. It is not necessarily
always the vertical or horizontal axis. You can find the other
one by just subtracting or adding 90 degrees to the axis given
in the prescription.
 Examples:
o If one meridian is at 60 degrees, where is the other one?
 Answer: C- 150 degrees
o If one meridian is at 90 degrees, where is the other one?
 Answer: Tricky. It could (by math) either be 0
degrees or 180 degrees, which are
mathematically the same. In optometry we
always use 180, never zero.
o Question asked- “What is the physical difference between spherical and
cylindrical. What does that mean?”
o Her answer: Refers to difference in curvature of astigmatism of the patient.
Spherical is always more plus, cylindrical is always more minus. It helps us
understand how light is refracted in the eye.
o Measurement of axis:
 Phoropter- looks like a protractor.
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Zero is on the patient’s left (doctor’s right), goes
in sequential order to 180. This is the location of
the axis.
 We can separate types of astigmatism
 With rule- 160-20. Anywhere near
horizontal line
 Against rule- closer to vertical number
(90)
 Oblique axis- closer to 45 or 145. Most
rare
Types of astigmatism:
o Note: if the principal meridian is (+) it’s behind the
retina, it its (-) it’s in front of the retina. If it’s zero, it’s
on the retina and it’s called plano (PL).
Simple astigmatismo One meridian is on the retina, the other is either in front
of or behind the retina
o Simple hyperopic (SHA)- 1 principal meridian on
retina, one that’s behind the retina
 Ex- (+)1.50 -1.50 x 180:
 Principal meridian 1= (+)1.50 (behind
the retina)
 Principal meridian 2= (+)1.50 – 1.50= 0
(or PL)
 Ex- (+)4.00 -4.00 x 90:
 Principal meridian 1= (+)4.00 (behind
the retina)
 Principal meridian 2= (+)4.00 – 4.00= 0
(or PL)
o Simple myopic (SMA)- one principal meridian is in
front of retina, one is on retina.
 Ex: Pl- 1.50 x 180
 Principal Meridian 1= 0 or plano or PL
 Principal Meridian 2= 0-1.50= (-)1.50D
Compound astigmatismo Both points are in front or both behind retina depending
on if it’s hyperopic (behind) or myopic (in front)
o Compound hyperopic (CHA)- Both focal points are
behind the retina. Both principal meridians are positive.
 Ex: (+) 1.50 – .50 x 160.
 Principal meridian 1: (+)1.50D
 Principal meridian 2: (+)1.50-.50=
(+)1.00D
o Compound myopic (CMA) Both focal points are in
front of the retina. Both principal meridians are
negative.
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Ex:(-)1.50 – 2.00 x 150.
 Principal meridian 1: (-)1.50D
 Principal meridian 2: (-)1.50-2.00= ().50D
o Mixed astigmatism (MA)- one is in front, one is
behind. One is positive, one is negative
 Ex:(+)2.00 -2.50 x 120.
 Principal meridian 1: (+)2.00D
 Principal meridian 2: (+)2.00-2.50= ().50D
 Ex: (+)2.00-4.00x90
 Principal meridian 1: (+)2.00D
 Principal meridian 2: (+)2.00-4.00= ()2.00D
 So these patients need two different prescriptions
o Presbyopia- Age related condition that occurs around age of 40. The patient
has problems accommodating or focusing eyes up close. Need extra plus
power to make things clear up close. Need ADD lens
 A patient with the prescription (+)1.00– 2.00x 180 comes to see you
and you determine with refraction and near testing that they have
presbyopia. The ADD is a spherical addition power over their distance
prescription, so in this case, we add a (+)2.00D ADD. So their new
prescription would be (+)3.00 -2.00x180.
 Principal meridians for their original prescription (+)1.00–
2.00x 180
o Principal meridian 1: (+)1.00
o Principal meridian 2: (+)1.00-2.00= (-)1.00D
 Principal meridians for their ADD prescription (+)3.00 2.00x180.
o Principal meridian 1: (+)3.00D
o Principal meridian 2: (+)3.00-2.00= (+)1.00D
 Presbyopia is due to accommodation- increase in plus power that we
naturally do with our lens to make things clear up close (whatever is
not at optical infinity or 20ft). We naturally have the ability to
accommodate but we lose this ability over time.
 When the eye is looking at infinity, the ciliary body sphincter
muscle contracts and pulls on zonules of zinn. This makes it
flatter and it doesn’t have as much (+) power.
 When we want to accommodate, tension on zonules are
released. The lens is no longer pulled flat, so it becomes more
fat and more convex. Accommodation adds more plus power.
 Lose this ability over time, and this causes presbyopia.
o Figure out refractive diagnosis (Dx) given prescriptions by calculating the
principal meridians
 -1.00 DS
 Answer: Simple myope (SM)
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(+)5.25- 5.25x095
 Answer: One principal meridian on retina, and one is plus. It’s
a simple hyperopic astigmat (SHA)
 -2.50-0.50x145
 Answer: They are both in front of the retina. It’s a compoint
myopic astigmat (CMA)
 +3.00-3.50x170
 Answer: We have a (+) and a (-) it is a mixed astigmat (MA)
 pl-4.50x035
 Answer: One is on the retina, one is (-) so it’s a simple myopic
astigmat (SMA)
o Figure out which Rx is written incorrectly
 Rules: always put proper symbol in front (+) or (-). Dr. To usually puts
this in brackets so that it’s clear. SPH, D, and DS denote a spherical
prescription. Always write sphere and cylindrical components to 2
decimal points. Go in quarter diopter increments, althought sometimes
you can do less than that but typically it’s quarter diopter increments.
If is is a spherical prescription make sure two write all 3 numbers
(spherical component, cylindrical component, axis). The second
number is always negative.
 **She corrected a problem in notes on pg. 25- it says “-3.25-2.25” in
the second prescription, change it to “-3.25+2.25” she was trying to
demonstrate ophthalmology convention. These numbers are the same
and we will learn how to convert them in other courses.
 Examples: find out what’s wrong with these presciptions
 -1.000 DCyl
o Should be written DS, D, or SPH. We don’t write
cylindrical component by itself EVER.
 -1.50–150x01
o Forgot a decimal, axis should be 3 numbers
 .75-3.00x030
o Needs a sign on the spherical component, should be a
zero before .75.
 +3.00
o Needs a D or a DS
 -1.75+.075 x 175
o move decimal place over in second number, second
number should be (–) not (+)
Vocabulary (latin)- not tested on but may be helpful in our courses or career
o Prefix- status, direction, time, when
o Root- body party
o Suffix- condition, procedure or disease
** The list of vocabulary words, prefixes, suffixes, and roots that she went over in
class is on pages 26 and 27. I did not include it here because it was exactly the same
as the list in the notes and I didn’t want to take up 2 more pages of space! 