Take a flashlight and tape a cross to it(make sure you don't let any light through the taped section, this is very important).

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I figured the focal length of magnifying glass by experimenting with a light source, but I want to calculate it on a paper.

(1) I didn't calculate it, I just drew the magnifying glass in Illustrator with its actual values Index of refraction doesn't matter. The result doesn't have to be 100% correct. 1.52 can be used as the index.

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To calculate their magnification power, I've read several Wikipedia pages, Lens (Optics) for calculating focal length, and Magnification for calculating magnification power, but I end up with wrong results. I must be doing something wrong.

I believe that the difference between the radius of curvature and the thickness is great enough to use the thin lens equation [if not I have another formula at the end that will work with this method].

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Aspheric lenses have much lower f-numbers, allowing them to perform better than spherical lenses in light collection, projection, illumination, detection and condensing applications. Since the focal lengths are often very short, compact designs are frequently possible. Precision polished aspheric lenses are computer-optimized to achieve diffraction limited performance over high NAs and are available uncoated or with broadband visible or broadband NIR antireflection coatings. For more economic or OEM applications, our aspheric condenser lenses are fabricated from Schott B 270® glass and are available uncoated or with a single-layer MgF2 antireflection coating. If small diameter lenses are required, our molded glass aspheric lenses with visible or NIR antireflection coatings are available mounted in a stainless steel threaded holder or unmounted. Aspheric objective lenses are also available that offer the same magnification and on-axis performance as microscope objectives in a more compact package.

The resulting magnification, $M$, will be equal to the ratio between $h_1$ and $h_2$, which when expressed in terms of $v$ and $b$ looks as follows,

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where $n$ is the refractive index of the lens material, $d$ the thickness of the lens, $R_1$ and $R_2$ the radius of curvature of the two sides of the lens.

Here $\text{F}_1$ and $\text{F}_2$ are the two focal points of the lens, with $f_1$ and $f_2$ as their respective focal lengths (these are often equal to each other, which is also assumed in the first equation).

At the very least, even if the lens isn't thin enough to utilize the thin lens criterion, you will be able to calculate a magnification, by dividing the image's distance to the lens into the cross' distance to the lens (unless you used the height method I gave you first). After you have the magnification, you can use the relationship to calculate the focal length by the relationship: Magnification = (focal length)/(focal length - cross' distance from the lens). You have 1 equation and 1 unknown now. I am curious as to what you measure the focal length to be with each method. If you get different results, the lens was likely too thick to use the thin lens equation, however, you will still be able to calculate the focal length no matter what, and can therefore calculate the power by dividing that into 1 and multiplying it by 100.

Aspheric lenses have much lower f-numbers, allowing them to perform better than spherical lenses in light collection, projection, illumination, detection and condensing applications. Since the focal lengths are often very short, compact designs are frequently possible. Precision polished aspheric lenses are computer-optimized to achieve diffraction limited performance over high NAs and are available uncoated or with broadband visible or broadband NIR antireflection coatings. For more economic or OEM applications, our aspheric condenser lenses are fabricated from Schott B 270® glass and are available uncoated or with a single-layer MgF2 antireflection coating. If small diameter lenses are required, our molded glass aspheric lenses with visible or NIR antireflection coatings are available mounted in a stainless steel threaded holder or unmounted. Aspheric objective lenses are also available that offer the same magnification and on-axis performance as microscope objectives in a more compact package.

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Make an optical setup aimed at the wall so that the flashlight is static and exactly 90 degrees to the center of your lens (make sure that you know how close your lens is to the wall!).

When you have a lens with a given focal length then you have two equations with three unknown. So, when you want to calculate the magnification you would not have a unique solution. However manufactures probably want to add a label to their lenses which a layman can understand. For this they probably will use eyepiece magnification,

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where the numerator is equal to the least distance of distinct vision, which is roughly 250 mm for a human with normal vision.

A lens does not have have one specific magnification, it depends on the positioning of the lens. When neglecting aberrations, the workings of a lens can be simplified with the following equation,

I have various magnifying glasses and I'm using them when I take macro photos with a phone or a camera. I want to group/label my magnifying glasses by their magnification power. And by magnification power I mean something like 10x.

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where $f$ is the focal length of the lens, $v$ is the distance from the object to the lens and $b$ the distance from the lens to the image of the object. This is demonstrated in the image below, including three principal rays (these only apply for thin lenses).