Fan and Block Technique

• What does light travelling in the vertical meridian form?

  Horizontal focal line
• What is the term for astigmatism where the vertical focal line is closer to the retina?

  With the rule astigmatism
• What shape do images take within the interval of Sturm?

  Blur ellipses

  
• What is the closest thing to a point image in astigmatism?

  The circle of least confusion

  
• What is the first step in retinoscopy for astigmatism?

  Find the two principal meridia
• What should the reflex in the pupil be aligned with during retinoscopy?

  That over the eye

  
• Which meridian should be neutralized first in retinoscopy?

  The most positive meridian

  
• What is the advantage of neutralizing the most positive meridian first?

  It allows the use of a negative cylinder for the second meridian

  
• What is the classification of ocular astigmatism?

  Simple Myopic, Compound Myopic, Simple Hyperopic, Compound Hyperopic, Mixed Astigmatism
• How is astigmatism corrected?

  By introducing cylindrical lenses to reduce the interval of Sturm to zero

  
• What is the purpose of sphero-cylindrical lenses in astigmatism correction?

  To correct astigmatism by providing both spherical and cylindrical power

  
• What is the Best Vision Sphere (BVS)?

  A spherical lens giving the best vision

  
• What does the BVS do in relation to the circle of least confusion (COLC)?


  Places COLC on the retina

  
• If a patient's prescription is +2.00/-5.00 x 90, what is the spherical equivalent (SE)?

  • 0.50 D
• How can the likely amount of astigmatism be estimated from visual acuity obtained with the BVS?

  Using the rough rule of thumb: Snellen denominator / 9

  
• What does the Plus/Minus technique help determine in astigmatism correction?

  The best visual acuity (VA)

  
• What should be considered when refining ret findings in astigmatism correction?

  Patient history, level of unaided vision, and response to lens changes
• What are the two main clinical techniques used to determine astigmatism in subjective refraction?

  The Fogging Method and the Jackson Cross Cylinder Method
• What type of charts are used in the fogging method?

  Internally illuminated charts: fan and block
• What is the purpose of the fan chart in the fogging method?

  To locate the approximate axis
• What should happen when the COLC is on the retina during the fogging method?

  All lines on the fan chart should be equally blurred

  
• What does adding positive spheres do in the fogging method?

  Moves both astigmatic foci forward

  
• What is the aim of the fogging method?

  To place both focal lines in front of the retina

  
• How is the amount of fog determined in the fogging method?

  By estimating the astigmatism from the level of VA after BVS

  
• If a patient's VA with BVS is 6/9, how much fog is required if their estimated astigmatism is -1.00DC or less?

  Fog VA by 2 lines (usually +0.50DS)

  
• If a patient's VA with BVS is 6/15, how much fog is required if their estimated astigmatism is more than -1.00DC?

  Add +0.50DS plus half the patient’s estimated degree of astigmatism

  
• What should be done with the fogging lens during axis determination using the fan chart?

  Ask the patient if any lines appear blacker and clearer
• What does a single line on the fan chart indicate?

  The meridian of the posterior focal line
• What is the purpose of the V chevron in axis refinement?

  To locate the meridian of the posterior focal line

  
• What should be done if the limbs of the V chevron are not equally blurred?

  Move V in the direction of the blurred limb until both limbs equal clarity

  
• What is the purpose of the block chart in power determination?

  To determine the power of the correcting cylinder
• What should be done for every -0.50 cylinder added during power determination?

  Add +0.25 to your sphere to maintain the fog
• What should be confirmed after refining the result in astigmatism correction?

  That the fog has been maintained

  
• What is the final step in the astigmatism correction process?

  Reduce fog in 0.25 D steps until best vision is achieved

  
• What must be accurate for successful astigmatism correction?

  BVS and estimate of astigmatism

  
• What should the COLC and posterior focal line be in relation to the retina during fogging?

  COLC in front of the retina and posterior focal line nearer to the retina

  
• How much sphere should be added for each -0.50D cylinder added to maintain fog?

  Add +0.25 D sphere
• What should be done if starting with ret in astigmatism correction?

  Aim to have an understanding of the process by the end of the semester