Visual Acuity

• The focal point can be on, in front of, or behind the retina.
• Refractive error is a mismatch between the power of the visual system and the length of the eye.
• Refractive error changes depending on the curvature of the cornea, the power of the lens, which are both refractive ametropias, and the length of the eyeball, an axial ametropia.
• In an emmetrope the focal point is on the retina and the far point is at infinity (6m), while in a myope the focal point is in front of the retina and the far point is in front of the eye.
• This is because the cornea is too curved, the lens is too powerful and the eyeball is too long in a myope.
• In a hyperope the focal point is behind the retina and the far point is behind the eye, this is because the cornea is too flat, the lens is too weak and the eyeball is too short.
• Astigmatism is a condition where there are two focal points because the shape of the eye is irregular.
• With the rule astigmatism, the cornea is more curved on the vertical meridian, while against the rule astigmatism, the cornea is more curved in the horizontal meridian.
• Astigmatism decreases with age and is not dependent on growth patterns.
• Convex positive lenses are used to converge the light rays and correct hyperopia, while concave negative lenses diverge the light rays and correct myopia.
• To correct astigmatism, spherical lenses are used to correct one meridian, and cylindrical lenses are used to correct the other meridian.
• New born babies are born about +2D hyperopic.
• There is a rapid decline in hyperopia from 6 months to 2 years, a process called emmetropisation, and a gradual decrease up to 6 years.
• The rapid infant stage is when the axial length of the eye changes from 18 - 23mm between 2 - 6 years, the slow juvenile stage is from 3 years to puberty, and the growth phase is when there is a co-ordinated process of emmetropisation, the process stops at around 14 - 15 years.
• Light perception is the threshold of vision.
• Discrimination is the ability to distinguish an object from its background.
• Form Vision and recognition is the ability to recognise letters and words.
• Resolution is the ability to see detail.
• Localisation is the ability to see that an object is to the side of another object.
• Higher Task is when the visual system stimulates other products.
• Unaided vision is determined by the smallest line of letters a person can read without optical aids.
• Visual acuity is determined by the smallest line of letters a person can read with optical aid.
• Testing for visual acuity has to be at a great enough distance not to stimulate accommodation, usually at 6m, near vision at 40cm but on the Snellen fraction you still use 6.
• If a person can't read at 6m, the distance would be moved to 3m, 2m and then eventually 1m away.
• Snellen Numerator is the distance you are viewing at)/ Denominator is the letter size or the distance at which the letter subtends 5 minutes of arc.
• The bigger the letter, the further away it has to be to subtend 5 minutes of Arc.
• Hermann Snellen created the Snellen Chart in 1862.
• The Snellen Chart is quick and easy to use, correlates to real life vision in most cases.
• The scale of the Snellen Chart is not linear, with different intervals between the lines.
• Some letters on the Snellen Chart are easier to read than others, and the legibility of the letters depends on the magnitude and axis of uncorrected astigmatism.
• The Snellen Chart has more letters on harder lines, meaning people with less acuity will have to read fewer lines than people who have better vision.
• LogMAR is the reciprocal of Snellen.
• The Bailey-Lovie Chart has 5 letters on each line and the letter spacing is dependent on the height and width of the letter respectively.
• Visual acuity is adversely affected by poor contrast as well as illumination.
• Point objects are not seen as points on the retina, they are instead spread over an area, this is called the point spread function.
• The width of the point spread function is influenced by focus, an out of focus image increases the point spread function, aberrations and diffraction force light to spread from a point to an extended pattern on the retina.
• If a person can't read anything from 6m, the distance would be moved to 3m, 2m and then eventually 1m away.
• If a person can't respond to or recognise letters, Landolt C or tumbling E can be used where the letters have a different orientation and the patient has to tell you which orientation the letter is.
• Preferential looking is when a child would rather look at a stimulus rather than a blank sheet.
• Teller and Keller Acuity cards are cards with a central peephole, one side has a black and white grating and the other side has just a blank stimulus.