"Difficult" Refractions

phoropter (4)Most refractions are straight-forward but what happens when you're faced with one that's not?  Knowledge is power. With the proper training and guidance you can handle even the most challenging  refractions. Let's discuss some common challenges you may face and how to meet them head on. First, let's learn an important formula to know when scrutinizing your refraction. Generally, a one diopter change in spherical equivalent should equal approximately three lines of improved VA on the Snellen eye chart. A spherical equivalent is calculated by taking half of the cylinder and adding it to the sphere power, then dropping the cyl and axis.  For example, the spherical equivalent of -1.00 +1.00 x 180 is -0.50.

Patient Refracts To a Lot More Plus Cyl AND a Lot More Minus Sphere:                  Probable Cause:  Too much minus sphere, inaccurate refraction.                                                        Try This: Decrease spherical equivalent (0.25 sph to each 0.50 cyl)                                            Example:  WRx: plano +1.00 x 180 20/30   MRx: -2.50 +3.00 x 180 20/20

Explanation: This refraction doesn't make sense.  The spherical equivalent of this patient's WRx is +0.50. The spherical equivalent of their MRx is -1.00. They have only 2 lines of improvement on the chart with 1.50 D change in spherical equivalent.  You wouldn't expect that big a change in Rx for only two lines improvement of VA.

What to Do:  Decrease the spherical equivalent until BVA is obtained.  This is accomplished by removing +0.50 cyl and 0.25 sphere at the same time and checking VA after each change of the lenses.  The end point is when you determine the least change in spherical equivalent which gives the best VA.  Then, recalculate the change in spherical equivalent vs. the improvement in VA and see if it makes sense.

Patient Refracts to  > 3.00 D Difference Between Their Two Eyes.                                                Probably Cause: Anisometropia                                                                                                                    Try This: With both of patient's eyes open: Show pt changes in sphere of 0.50 or pulling sensation in RF increments so that both eyes’ sph powers are brought closer together.  Example: Pt refracts to:  -3.00 +1.00 x 180 OD   +1.00 +1.00 x 180 OS                                                                     Show the pt:   - 2.50 +1.00 x 180 OD   +0.50 +1.00 x 180 OS                                                  Ask the patient to compare this Rx with the refracted Rx.   “Does this lens look about the same?” Remember to show them binocularly - not monocularly.  Recheck binocular VA.

Explanation: Anisometropia means the patient has > 3D difference in refractive errors between their two eyes. Minus lenses minimize images, plus lenses magnify them. Patients who have anisometropia may have a difference in perceived image size known as anesekonia. Our brains can only fuse images that are of similar size, so when the patient has anisometropia they may have double vision or a pulling sensation.

What To Do: Give the patient the Rx which gives them the sharpest vision possible without causing a pulling sensation or diplopia.  It's critical to trial frame the new Rx. Be certain to center the lenses in the trial frame so the patient is looking through the optical center to avoid induced prism. With the trial frame on, encourage the patient wander around the office and look outdoors to make sure they don't have any diplopia or pulling sensation in the Rx.


Every refraction is a learning opportunity for the technician. Follow up with your doctor after he or she sees these challenging refractions and ask the doctor why he Rx'd what he did so you can learn from it. With practice and exposure to many different types of special situations you will become a seasoned refractionist. Happy Refracting!