# What is the resultant acuity?

Hello, I’ve been wondering what is the ‘resultant acuity’ when you actually put lenses that is NOT your full prescription.

Let’s say you have an acuity of
OD -3.00
OS -3.00

If you decide to wear a pair of -2.00, does this mean you have an acuity of -1.00 of spherical? Is the resultant acuity just x – y?

What if you wear a pair of -4.00 glasses? What is your resultant acuity? A +1.00 hyperopia? Since -3.00 – (-4.00) = +1.00. Yes?

Now, let’s say you wear your full prescription of -3.00s. This means you have 20/20 vision, or an acuity of 0. And you decide to wear a +1.00 lens ON TOP of your glasses again. What would happen? Will you see the world in -1.00?

I hope you’re able to follow my thought process here. What do you think?

This is what I have been wondering as well. I wonder how I can work out my actual acuity from distance to blur with a differential lens. It does not seem as straightforward as x-y. I found measuring distance to blur with a differential lens over the space of a year has given me an increased distance to blur that does not correlate with the improvement in my naked eye distance to blur measurements. It must be a measure of some improvement, but what?

Yes.

I think it has to do more with the subjectivity of the cm measurement, where blur adaptation can also make a big difference. Theoretically (so by optics) it should be a simply x-y.

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Thanks, I had forgotten that as soon as I put on a lens I lose some clinical blur adaptation. I think another problem is that my differential lens for the left eye has insufficient cylinder correction. This also makes it difficult to measure Snellen lines with what I guess would be a norm lens (also insufficient cylinder). I can read the line through the astigmatic distortion, but as with naked eye cm measurements, it is difficult to separate spherical blur from astigmatism effects. This is quite apart from the fact that being able to read the 20/20 line does not mean there is no myopia left. I think that none of my measurements are very accurate on account of astigmatism. Three ways of measuring, none of them really accurate.

I assume that increase in distance to blur with a differential lens must be some indication of improvement, and that is all that really matters, even if it does not correlate well with the other two measures…

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In contact lenses, it is.
That is why in physics it is called “thin lens formula”.