Test 6

1. True statements about the lens effectivity include: a. it is equivalent to the back vertex power  b. it is affected the index of refraction of the lens material  c. in a hypermetropic patient wearing weak plus lenses, the effective      power of the lens can be increased by moving the lens forward  d. a patient who is fully corrected with -10.00D spectacles at a vertex      distance of 10 mm can be corrected a -9.00 D soft contact lens  e. a patient who is fully corrected with -10.00 D spectacles at a      vertex distance of 10 mm will need a -15..00 D lens if the      spectacles are now moved to 20 mm away from the eye. Answer1.    a.F    b.F    c.T    d.T    e.F Lens effectivity: refers to a change in the optical effect of a lens when its position before the subject's eye is altered is affected by the distance of the lens from the eye in a hypermetropia with weak plus lens, the image falls behind the eye. By moving the lens forward, the image is brought forwards the lens effectivity is increased  the formula below is used to calculate the lens effectivity: FX = F1/ (1-dF1) FX = power of lens needed at the lens position in dioptres F1= power of the original lens in dioptres d= distance moved in metres, it is negative when moved away from the eye and positive when moved towards the eye a patient who is fully corrected with -10.00D spectacles at a vertex distance of 10mm: the contact lens can be calculated with the above formula: FX = -10.00 / 1-(0.01)(-10.00) because the contact lens is apposed to the cornea the distance moved is +0.01m Fx = -9.10 D which is about -9.00D the lens required when the lens is moved to 20 mm FX= =10.00/1-(-0.01)(-10.00) because the lens is moved away from the eye the difference in distance is -0.01 FX= -11.1D which is about 11.0D 2. The following are true when a patient changes from spectacle to contact        lens: a. more accommodation is required when a myope changes from      spectacle to contact lenses  b. less accommodation is required when a hypermetrope changes from      spectacles to contact lenses  c. a myope with esophoria at near will experienced increased esophoria      when changes from spectacles to contact lens  d. a myope requires more fusional convergence when using spectacle      than using contact lenses  e. contact lenses are better than spectacles for presbyopic myopia Answer2.    a.T    b.T    c.T    d.F    e.F Contact lenses vs spectacles: when a patient changes from spectacles to contact lenses there is a change in the demands on accommodation and convergence a hyperope who wears glasses will need more accommodative effort per dioptre of accommodation than a myope who wears glasses. When contact lenses are used, a hyperope needs to use less accommodation whereas a myope requires more accommodation a myope with esophoria will requires more accommodation and therfore more convergence when using contact lenses rather than glasses. This results in more esophoria glasses for myope can be regarded as having a base-in prism and therefore less fusional convergence is needed because contact lenses in myope requires more accommodation and convergence it can be precipitates problem for presbyope   3. The following are true about indirect ophthalmoscopy: a. the aerial image of the retina is formed between the subject's eye and      the condensing lens  b. the aerial image of the retina is an inverted real image  c. in an emmetrope, a 30D condensing lens produces a larger retinal      images than a 20D lens  d. in an emmetrope, a 30D condensing lens gives a larger visual field      than a 20D lens  e. for a given condensing lens, the aerial image of a hypermetropic retina      is larger than a myope. Answer3.    a.F    b.T    c.F    d.T    e.T Indirect ophthalmoscopy: the aerial image of the retina is an inverted real image which is situated between the condensing lens and the examiner's eye the stronger the condensing lens, the smaller the magnification of the retinal image the stronger the condensing lens, the larger the field of view myopic eye results in a lower magnification but a wider field of view than a hypermetropic eye   4. The following are true about ophthalmoscope: a. the image is laterally transposed with the indirect ophthalmoscope  b. the teaching mirror on the indirect ophthalmoscope works on the      principles of total internal reflection  c. the magnification of the retina with the direct ophthalmsscope is about      15X  d. refractive error has a more significant effect with the direct      ophthalmoscope  e. the field of view is larger with he indirect ophthalmoscope Answer4.    a.T    b.T    c.T    d.T    e.T the image in indirect ophthalmoscope is vertically and horizontally transposed the teaching mirror works on the principal of internal reflection the magnification with the direct ophthlamoscope is about 15X the magnification with a indirect ophthalmoscope using a 20D is about 3X. refractive error has a greater effect with direct ophthalmoscope the field of view is larger with indirect than direct ophthalmoscope ( 6 degrees vs 25 degress)   5. With regard to Bagolini's glasses: a. the test is only useful for adult  b. contain two lenses one with striation set at 90 degrees and the other      at 180 degrees  c. amblyopia in one eye always result in perception of only one line  d. a cross is only seen in subjects with binocular single vision  e. can be used to test the presence of abnormal retinal correspondence      when combined with cover / uncover testing Answer5.    a.F    b.F    c.F    d.T    e.T Bagolini's glasses contain two lenses with striation (optical principles similar to Maddox rods) one with striations at 45 degrees and the other at 135 degrees it can be used in children who can describe what they see a spotlight stimulus is used, an if there is a no suppression, the subject will see two diagonal lines crossing at above or below the light source binocular single vision is needed for the perception of a cross amblyopia  depending on the severity will result in either absence of one line (if dense) or part of the line (if mild) if a cross were seen when the cover / uncover test shows an ocular deviation, the subject has harmonious, abnormal retinal correspondence.

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