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9/3/09

Sphere Establishment

ow we’ll start the first step of refracting, establish the sphere power.

The goal of this step is to establish the sphere power that gives the patient their best vision. If we go back and look at how light is focusing in the eye, our objective is to make an image focus as close to the retina as possible. If our patient doesn’t have any uncorrected astigmatism, when we’re through we should be able to get light to focus the whole image on the plane of the retina, like this…
However, many of our patients will have uncorrected astigmatism. And, of course, if there’s astigmatism that’s not corrected light from one meridian of the eye will focus at a certain point and light 90 degrees away will focus at a different point. In this case, our goal will be to make both meridians of the image focus as close to the retina as possible. When we’re done with the sphere establishment step, both parts of the image should be the same distance away from the retina, like this…

So let’s get to it…
Since this is a monocular refraction, we’re going to occlude the left eye so we can work with the right eye by itself. This is an easy thing to forget. If you do, it can really mess things up, so don’t forget!
Because we want to make sure the patient doesn’t get too much minus power (or too little plus power), it’s preferable to start with the patient slightly fogged. Adding plus .75 diopters by turning the sphere wheel down 3 clicks should do it.

At this point, we’ll ask the patient to read the chart. …

“Joe, please read the chart”

..and our patient reads most of the 20/30 line..




This is what we want. He should be no clearer than 20/30 to 20/40. If he were seeing better than that, we’ll add more plus power by turning the sphere wheel down further.

Now let’s tell Joe what we expect of him. We’ll say something like..
“Joe, I’m going to give you two choices. They may both be a little blurry, but I want you to tell me which is more clear. This is choice one…”

At this point we’ll take away plus or add minus by turning the sphere wheel upward, one step at a time.
“…or two…”

..so, Joe tells us he likes choice two better.

Because he likes the second choice, we’ll keep the sphere power where it is, at least for now. If he’d chosen option one, we would roll back the sphere wheel to its original position.

Even though our patient told us which lens he prefers, we’re going to check his vision.
“Please read the chart”

…and now he’s reading about 20/25.

To avoid giving him too much minus, make sure that each step in power yields an appreciable improvement in the patient’s ability to read the eye chart. In this case, Joe’s vision was originally 20/30ish, now it’s 20/25ish. This is pretty typical: A good rule of thumb is that for each .25 increase in minus power, the patient should improve by about one line on the chart.


If you add more minus but the vision doesn’t improve, you’re probably adding too much power.

As we said in the beginning, adding too much minus or too little plus is a no-no.
The bottom line is, be stingy! Think of minus power as if it were money. The patient can only earn more minus power if they can demonstrate to you that it improves their vision.

We’ll repeat this step until the patient’s vision is no longer objectively improving.
When we get to a point where more minus or less plus doesn’t improve our patient’s ability to read the eye chart, we’re done with this step.

Keep in mind that, since we have not refined the astigmatism at this point, the patient may not refract to a perfectly clear 20/20.


To summarize, the sphere establishment part of refraction starts with fogging the patient about 0.75 D and then adding minus (or removing plus), one step at a time. With each step, make sure the vision has objectively improved on the eye chart

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