Fan and Block

• once the best vision sphere is in place measure the visual acuity and estimate the cylinder using the table
• 2. half the value of the estimated cylinder is added as a positive sphere. this brings the posterior focal line onto the retina helping us find the principle meridian of astigmatism
• 3. the patient looks at the fan chart and indicates the position of the clear lines
• 4.the Maddox v is rotated into the clear lines to determine the principle meridians
• 5. when the limbs of the v are equally blurred the point of the v indicates the principle meridian of the posterior focal lines
• 6. if one limb is clearer than the other rotate the v in the direction of of the more blurred limb to equalise the appearance
• 7. Add +0.50DS sphere to move the posterior line focus in front of the retina (fogging sphere)
• 8.Place your estimate of cylinder power into the trial frame and ask your patient to look at the blocks. Which one of the blocks appear darker or are they both the same?
• 9.if the blocks are the same – cyl power should be correct (check by adding –0.25DC, the block perpendicular to axis should become darker)
• 1. Add cylinder power if the block parallel to the arrow direction is darker
  2. Decrease cylinder power if the block perpendicular to the arrow direction is darker
  3. Each cylinder increment pushes the anterior focal line towards the retina.
  4. When the blocks appear equally clear the astigmatism has been corrected
  5. Refine the sphere with Plus/minus method, Duochrome
• • Check tests:

  +1.00DS for sphere:

  • It prevents overcorrection in the sphere with negative lenses. Vision will drop 4 lines

  +0.50 DS for cylinder:

  • It prevents overcorrection of the astigmatism.
  • When you add +0.50DS and the cylinder power is correct, both focal lines will move in front of the retina and blocks will be equally blurred. If not, the cylinder power is not correct