8.2  Subjective Refraction

• Begin by occluding the opposite eye.
• If the VA is 6 / 6… only very small adjustments will be required
• If the VA is 6 / 9 or worse… larger adjustments will be required
• As a rough guide a 0.25D change in refraction should lead to a one Snellen line improvement

• In pre-presbyopic patients, unless the patient is cyclopleged, active accommodation can result in a prescription that is “over-minused” (or too myopic) as their active accommodation absorbs additional minus. Therefore, the goal of the subjective refraction is to find the maximum plus (or minimum minus) that provides the maximum acuity (i.e. if additional minus is not improving visual acuity and just making the letters smaller and darker rather than clearer, it is not required)

• Add plus / minus spheres (as accommodation is not an issue) to maximise visual acuity, asking “Do you prefer lens 1 or 2” etc

• Add + 0.5D
• Ask “Does this make your vision worse?” (adding plus to blur the patient is referred to as “fogging”)
• “No” → Add the + 0.5D and repeat
• “Yes” → Add - 0.25D, Ask “Does this make your vision better?”
  • “No” → The sphere is already correct
  • “Yes” → Add the -0.25D and repeat

Notes:

• In a pre-presbyope, adding the - 0.25D won’t usually make their vision worse since the patient can accommodate to bring the image forward onto the retina. When changing plus lenses, always keep a plus lens over the eye (“protect the fog” by placing the new plus lens before removing the old plus lens) to ensure that the patient doesn’t accommodate

• Always refine the cylinder axis before the power (because the correct axis can be found in the presence of incorrect power, but the correct power cannot be found in the presence of an incorrect axis). Higher powered Jackson Cross Cylinders (JCCs) are used for axis determination; lower powered JCCs are used for power refinement
• Ask the patient to look at a round target (e.g. “O”) ~two lines above the maximum acuity read with the best sphere
• Align the handle of the Jackson cross cylinder along the axis of the cylindrical lens in the trial frame
• Flip the Jackson cross cylinder, telling the patient “These may both make it worse, but tell me which is the clearer of the two “1”… or "2”?” When flipping the Jackson cross cylinder, pause to give the patient sufficient time to appreciate each orientation, but make the “flips” sharp and quick
• After determining the orientation of the JCC that the patient prefers, rotate the cylinder in the trial frame towards the side of the same cylinder sign on the JCC (e.g. plus cylinder to plus cylinder on JCC). If the cylinder power is not high, rotations should initially be in ~15° steps, then reduced as the correct axis is approached. Be sure to re-align the handle of the JCC to the new axis before you re-test
• Continue until both orientations look the same to the patient, or you are switching between a narrow range of axes (in this case select the middle of the range)

• Align one axis of the Jackson cross cylinder along the axis of the cylindrical lens in the trial frame.
• Flip the Jackson cross cylinder slowly, asking the patient “Tell me which is the clearer of the two “1”… or “2”?” When flipping the Jackson cross cylinder, pause to give the patient sufficient time to appreciate each orientation, but make the “flips” sharp and quick
• If the patient finds it clearer when the “+” sign on Jackson cross cylinder is in line with cylinder axis, add + 0.25D. If the patient finds it clearer when the “-“ sign on Jackson cross cylinder is in line with cylinder axis, add - 0.25D.
• Continue until both orientations look the same to the patient

• For each 0.50D increase in cylinder power, - 0.25D will need to be added to sphere to maintain the position of the circle of least confusion. Similarly for every 0.50D reduction in plus cylinder, + 0.25D will need to be added to the sphere. The target is the most plus, or least minus sphere that yields maximum visual acuity (since you don’t want to overcorrect myopes, making them accommodate thus causing eye strain)

• Check this by ensuring that the patient is slightly on the red side of the duochrome visual acuity chart (“RAM-GAP”: “red add minus, green and plus”)
• (In other words, do NOT over minus a patient!)