Lens Characteristics Flashcards
What is a prism?
a triangular shape with an apex and base
What is the apex?
It is the point at the top of a prism.
What is the base?
It is the wider, flat surface at the bottom of a prism.
What happen to an object when viewed through a prism?
It is moved in a different direction as compared to seeing the object with the naked eye.
What happens when light passes through a prism?
it is redirected toward the base of the prism.
What makes the object seen by the patient through a prism appear to be displaced?
Toward the apex.
What is the ratio used to predict image displacement?
P=C/D
C=displacement of the object in cm.
D=distance from the prism in meters
ex:What is the power of a prism that displaces light 1cm at a distance of 0.5 meter?
P=1cm/0.5m
P=2.00D
Therefore the power of the prism 2(triangle) diopters.
Lenses are a combination of prisms. Plus lenses are prisms joined …?
Base to base.
that is why plus lenses are thicker in the middle and thin on the edges.
Minus lenses are prisms joined…?
apex to apex.
Thin in the middle and thick on the edges.
What is a focal length or point of focus?
every lens has this and is defined as the distance a lens takes to focus parallel beams of light.
Stronger lenses have ….?
short focal lengths
Weaker lenses have…?
longer focal lengths
What is a lensometer?
it measures the focal length of a lens and converts it into diopters, which is the unit of measure used for writing Rx.
What is the formula used to calculate the focal length?
f=1/F
f=focal length
F=lens power in diopters
D=distance from prism in meters
Describe the focal length of a plus lens.
having a “real” focal length because the base to base prisms converge light to a measurable point of focus behind the lens.
Describe the focal length of a minus lens.
has a “virtual” focal length because apex to apex prisms diverge light rays that never come to a point of focus.
What is a virtual focal point?
Their paths must be reconstructed backward to determine focal point..
EX: What is the focal length of a +2.00D lens?
f=1/2 f=0.5m real
What is the focal length of a -4.00D lens?
f=1/4 f=0.25m virtual
What is the power of a lens with a virtual 2m focal length?
F=1/2 F=-0.50
What is the optical center?
The single point on the lens through which light passé without being deviated or refracted.
In the optical center, what happens to the plus lens?
This is where they meet.
In the optical center, what happens to the minus lens?
it is where apex meets apex.
What happens when you view a lens through the lensometer?
It is the point where the target is exactly centered.
What is “against motion”?(only for + lens)
While prisms bend light toward the base they also shift the image viewed toward the apex. Because of this, when looking through a moving plus lens you will notice that the image will move in the opposite direction.
What is “with motion”?(- lens)
when looking through a moving the center of a minus lens, there is no image displacement. You will notice that the image of what you are looking at will move in the same direction.
How many dioptric power values do all lenses have?
2.
One is the actual power, which may be measured by the lensometer.
The other the effective power that only the wearer perceives.
Does the measured power remains the same or changes?
It always stays the same but the effective power can change depending on the vertex or fitting distance of the lens from the eye.
What happens when plus lenses are moved closer to the eye?
their power becomes less effective or weaker.
What happens when a plus lens is moved further from the eye?
the effect of the power becomes stronger.
What happens when a minus lens are moved closer to the eye?
their power becomes stronger .
What happens when a minus lens are moved further from the eye?
their effective power becomes weaker.
How do you ensure that the patient is seeing through the intended power?
The spectacles should be fit to the same vertex distance used during the refraction.
What does opticians use to measure the vertex distance?
vertex distometer.
What does the distometer measure?
The distance from the back of the lens of the eye in mm.
How can the effective power be quickly calculated?
F=D/1-(dD)
F: effective power
D: true power
d: distance in meters the lens is moved
Strong plus or convex lenses that distort a square to a in pincushion shape
.
Storng minus or ______lenses distort a barrel shape.
.
What happens when a plus lens is moved further from the eye?
the effect of the power becomes stronger.
What happens when a minus lens are moved closer to the eye?
their power becomes stronger .
What happens when a minus lens are moved further from the eye?
their effective power becomes weaker.
How do you ensure that the patient is seeing through the intended power?
The spectacles should be fit to the same vertex distance used during the refraction.
What does opticians use to measure the vertex distance?
vertex distometer.
What does the distometer measure?
The distance from the back of the lens of the eye in mm.
How can the effective power be quickly calculated?
F=D/1-(dD)
F: effective power
D: true power
d: distance in meters the lens is moved
Strong plus or convex lenses that distort a square to a in pincushion shape
.
Storng minus or ______lenses distort a barrel shape.
.
