# How to calculate mag power?

Focal length in (m) is 1/diopters or 100/diopters for (cm).

How is Magnifying power calculated?

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I ran a quick search and found something in this external-forum topic.

It seems that there are different formulas for calculating this, and they give different results, as apparently the definition of â€śMagnifying powerâ€ť can be different. I ran a couple of lines of your table through the following formula mentioned in the linked forum topic, and it looks like your table is using exactly this formula:

`Diopters / 4 + 1 = MagPower`

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Click to enlarge.

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If i understood, so, if iâ€™m a -4.00 myopic i can magnify 2x a close object?

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Yes, that is what I understand.

i wondering if there is a chance to change the diopter just by puting away glasses from eyes, in order to move the focal plane. If i put the glasses at the tip of my nose, i can add a little of blur challenge, so i only would order new glasses when i start to see well with the glasses at the tip of my nose.

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So with the magnification worked out here shows the percentage reduction for each .25 diopter. As you can see the reduction gets bigger and bigger at you approach 20/20.

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Seems to work, although I donâ€™t put it all the way at the tip of my nose. Also I push the frame closer to my face in the beginning of a reduction if necessary, to get a signal of clarity. It makes the lens appear 0.125-0.25D stronger than it is. Seldom do I need it but is good to know. That kind of focal plane change is good? I assume yes cos itâ€™s so shortly done. But with plastic frames they slide up and down all day long.

I am under the impression that the myopic eye subtends the inverse of the magnification, i.e., the myopic eye subtends a smaller angle of a body behind the focusing lens. I donâ€™t know, I havenâ€™t really sat down to work out the math from the article. It just seems logical to me based on the mental models I have of how the eye functions.

Kent

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Not exactly, magnification is not so simple. The article is referring to plus lens prescriptions which are converging aka convex lens, these in general magnify the image because they have a prism effect that brings the perspective inwards, that is make objects appear closer and take up a larger field of view.

However, the magnification for minus lens aka diverging aka concave lens. That is, these have a prism effect that bring the perspective outwards, that is make objects appear farther and take up a smaller portion of the field of view. This is why at high levels of myopia, you may be able to correct to 20/20 vision, but your vision may be worse than someone with actual 20/20 vision, because although the clarity is on point, the image of the object may be too bitesize for you to resolve the detail.