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 “Magnifying power” 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


Click to enlarge.

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If i understood, so, if i’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.


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 don’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 it’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 don’t know, I haven’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.



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.

See below for more information.

That article doesn’t account for changes in axial length on the receiving end of light rays.

I’m too lazy to work out the math but basing it on personal experience.
The more myopic you are, the same object of visual interest takes more space in your field of vision. (And this blur simulator confirms my bias Blur simulator: What you'll see without your glasses (depending on prescription: sphere, cylinder and axis) . Hyperopia decreases size of the snellen chart. Myopia increases size of snellen chart.) My guess behind why that is the case because the eye is longer the retina is less flat/more oval-round, and therefore the wideness of your field of vision decreases. Putting on glasses counteracts this by pushing space away from you, and bringing objects’ images appear further than they actually are.
So, there are two counteracting forces, however because your base myopic eye has a decreased field of view, and since the minus lens glasses don’t actually increase your field of view and only push objects farther away. Because the effect is an overall decreased field of view, objects subtend a larger angle.

Mirroring the logic, hyperopic eyes should have objects subtend a smaller angle overall, even with glasses on. Although the math for hyperopic eyes is not as clear, and past +3.99 hyperopia seems to indicate everything should be blurry but my high hyperopic friends seem to still see fine so IDK. I think they just are blur adapted. Need an actual hyperopic person that understands what I understand but sees what they see to confirm but no such person exists.