Diopter refers to the curvature of the lens. As the diopter increases, the lens becomes thicker and the curvature greater. As the curvature increases, light rays are redirected to fill a greater portion of the viewer’s retina which makes the object look bigger. Powers refers to how much larger an object is made to look through a magnifying lens. Power is typically indicated by an x such 2x or 4x. The most common formula to convert the diopter of a lens into its power is:

For example, a +3 diopter lens makes an object look 75% times larger, over and above what the unaided eye already sees. As you increase the magnification the lens size effectively decreases. It is not possible to get a high power lens in a large diameter. Optical combinations are a compromise for the “ideal” magnifier. The following table is a guide for magnification levels as compared to Diopter:

Here is an example of a lens with a +3 diopter, its magnification is 3/4 + 1=1.75, and a lens of +4 diopter has magnification 4/4 + 1 = 2. By this way, you can easily calculate the magnification power of a lens from its diopter.

As you climb in magnification power, you will lose viewing area but get more pinpointed detail at a closer range. Usually, if your vision is severely damaged, higher magnification powers could help immensely.

Some optical users automatically assume the diopter of a unit is not as important as its magnification. Well, they’re wrong ... the following explains these two measurements, and illustrates how to use one to calculate the other.

Magnification is simply a measurement ratio produced by an optical system. In the world of optics, magnification is the comparison between the actual object size and the size of its image. Mathematically, that would be illustrated as:

But you also need to remember this; if you keep pushing the magnification above the useful range, you’ll end up with what is called an empty magnification. A situation where no additional detail is resolved despite the fact that the image keeps on getting bigger. There are different types of magnification. Namely:

• Relative-Size Magnification: In this type of magnification, you utilize the size of a larger screen to enlarge the size of whatever object you’re interested in magnifying.

• Relative-Distance Magnification: This is all about reducing the distance between the user and the object so as to be able to view more details.

• Optical Magnification: Using different lenses, as in the case of a telescope, to enlarge a specimen under observation.

• Angular Magnification: When you compute the tangential ratio of an object, and its image-subtended angles, you get the angular magnification.

• Electronic Magnification: Increasing the size of an object using electronic magnifies.

A diopter is also a measurement and an optical power of a lens. It’s easy to figure out what the diopter strength of a lens is by looking at its focal length. And by definition, the focal length of a lens refers to that distance between its center point, and the point at which you get a clear image after focusing on an object. The relationship between a lens’ focal length and its diopter strength is somewhat inversely proportional.

For example, if your lens’ diopter is +2, the focal length will be ½ m. If it’s +3 or +4, it’s going to be ⅓ or ¼ m, respectively. But a focal length of 1m is just the same as a +1 diopter lens.

It’s really important to understand this relationship because the focal length is still a factor that influences the magnification of any optical device. A longer focal length significantly decreases a lens’ magnification ability, and that’s why optical users can’t stop complaining about getting low-quality or dull images, anytime they use lenses that have low diopter strength.

You’ll also hear the ophthalmologist talk about diopters a lot. It’s the reason why the human eye is able to clearly focus on any object within a reasonable distance. If your eyes are healthy, they’ll have 40 diopters. Meaning, you can comfortably focus on something that’s 2 cm from your eyes.

The age of the individual is yet another factor worth taking into account. Say, he or she is younger than 25. They’ll naturally have the ability to make adjustments, and enjoy an additional 20 diopters. However, if they’re older, that focusing flexibility gradually drops to 10 diopters. And that’s caused by two things: the hardening of the lens, and the weakening of the eye muscles due to aging. By 45, you’ll only be left with a 1 diopter, and most likely be suffering from a condition known as presbyopia—an age-related loss of your eyes’ ability to focus on a close object.

So, magnification measures the change of an object’s size whenever it’s being viewed through an optical device, while the diopter measures the lens’s strength to bend light. And seeing as the lens’ ability to bend that light is what’s responsible for the magnification of the object, these two variables are related in a way. Calculating the diopter of your lens is very much possible, as long as you know its magnification power, and vice versa.

For the sake of clarification: The first thing that you do, is to take a note of the lens’ or device’s magnification power. If the lens has a magnification power of 1x, the size of the image that you’ll see through it will be twice the size of the object ... an increase of 100 percent. Of course, that’s if it’s being viewed with a naked eye. With a higher magnification power of 2.5x, and an object that’s 3 inches in size the object size will be by 250 percent, the image size will appear 10.5 inches big ... 7.5 inch increase, plus the object size, which is 3 inches. To calculate the diopter, you multiply that magnification by four, and here’s why: When a 1d lens (1 diopter) bends light, the size of the object being viewed increases by 0.25. So if you had a 4d lens, the size of our object would increase by 100 percent. Hence, a magnification of 1x. Going back to the above example, the lens’ diopter will be: 2.5 multiplied by 4, which equals to 10d.

Simple Lens

This is the type of lens that you’d typically go for if you’re trying to read tiny writings in a book or a newspaper. Its strength to magnify any object is incredibly low, as it ranges from 2x to 6x. This is not the type of lens for anyone looking for something that has a high magnification power or can produce clear, detailed images. 

Compound Lens

Telescopes and are the only optical devices that come to mind whenever anyone talks about compound lenses. Both devices often rely on multiple lenses to increase their magnification powers. 

So just to recap, this is how you’ll be able to differentiate the two: If the lens makes the object appear 4 feet closer, that’s a simple lens. But if it appears 4 feet closer six times, it’s definitely a compound lens.

Optical devices with simple lenses like reading glasses and Optivsors are limited in the amount of magnification they can provide with a single lens over a large area. Readers would simply be too thick with a restricted viewing area. Reading glasses are measured in diopters.

Optivisors have a smaller lens and are able to provide increased magnification but they are limited to 3.5x with the typical single lens. Further magnification requires the additional monocle. Optivisors achieve a workable distance by spacing the lenses well in front of the eye surface therefore limiting the viewing field. Optivisors are rated in magnification values.

Loupes worn by dentists, surgeons and jewelers are measured in magnification. They use a series of lenses and prisms to provide a high degree of magnification at a workable distance while still close in to the eye.

When selecting a high magnification optical device, be mindful of the field of view and depth of field. It is easy to go crazy selecting a high magnification only to find you have difficulty zeroing in on what you want to look at. It takes practice and some people never feel comfortable. As a general rule, most people can easily handle up to 2.5x.