Omni calculator
Last updated:

Diopter Calculator

Table of contents

What is a diopter? — Diopter definitionHow to find the power of a lensHow to use the diopter calculatorConverting diopter to focal lengthDiopter with respect to magnification and focal lengthFAQs

Have you ever wondered what the optical power of the human eye is or wanted to convert it to focal length; our diopter calculator is at your service.

Whether you need it for research purposes or are just curious by nature, our diopter calculator is the best option.

You can also use it as a focal length to diopter converter. But don't think it's all about conversion because our article is full of information revolving around optical power, diopter, and focal length.

To give you an idea of what we have in store for you, here are some of the topics we will be discussing:

  • Diopter definition;
  • How to find the power of a lens; and
  • Diopters to focal length.

What is a diopter? — Diopter definition

To understand what a diopter is, first, we need to know about the power of a lens. The power of a lens is its ability to converge a beam of light falling on it. For example, a powerful lens will focus parallel light rays closer to itself and into a smaller and more intense spot.

The diopter (D) is the SI unit of lens power and is defined as one divided by the focal length of the lens measured in meters. You might have seen the term diopter on eyeglasses and contact lens prescriptions issued by optometrists.

⚠️ Don't confuse this power with the concept of power you read about in work and power.

How to find the power of a lens

The power of a lens PP is expressed as the reciprocal of its focal length ff:

P=1f\small P = \frac{1}{f}

To find the power in diopter (D), we need to convert the focal length into meters.

P[D]=1f[m]\small P [{\rm D}] = \frac{1}{f [\rm m]}

You may use this as the diopter formula, as the optical power is in diopters.

For example, if the focal length of a lens is 20 cm. We can calculate its power in diopters as:

P[D]=120×102 m=5 D\small \begin{align*} P [{\rm D}] & = \frac{1}{20 \times 10^{-2}\ \rm m} \\[1em] & = 5 \ \rm D \end{align*}

How to use the diopter calculator

Considering the significance, one might think that using a diopter calculator would be a piece of work. But it couldn't get any simpler. The only thing you need to determine optical power is the focal length in meters. But fret not; we also have a few unit options for that.

The steps to use the focal length to diopters converter are:

  1. Input the focal length, preferably in meters, but other unit options are available (select the desired unit first, then input the value).
  2. The result is the optical power in diopters.
  3. You can use the tool the other way around as well. Input the diopter measurement to get the focal length.

For example, the focal length of a lens is 70 cm. You will choose the unit as centimeters and then enter the focal length as 70. The result is the optical power as 1.4286 diopters.

Converting diopter to focal length

Up until now, we have seen how we can determine the power of a lens in diopters using the focal length. But you should know that this calculation is a two-way street. You can use the diopter formula to determine the focal length if you already know the power of the lens.

Simply rearrange the diopter formula, and you get:

f[m]=1P[D]f[{\rm m}] = \frac {1} {\text P[\rm D]}

Taking the reciprocal of optical power gives the focal length.

For instance, the optical power is 45 diopters. We can calculate the focal length as:

f=145 D=0.02222 m\small \begin{align*} f & = \frac{1}{45\ \rm D} \\[1em] & = 0.02222 \ \rm m \end{align*}

Diopter with respect to magnification and focal length

Now that we understand how to find the power of a lens, let's look at the relationship between diopters, magnification, and focal length.

Relationship between diopters and magnification:
Magnification and diopters are two distinct aspects of the same optical system, with magnification representing the relative increase in the size of an object and diopters indicating the visual or refractive power of a lens, e.g., in corrective eyewear. The higher the diopter value, the greater the optical power, translating into greater magnification.

Relationship between diopters and focal length:
In optics, the focal length is the distance between the lens and the point where parallel light rays converge or diverge. Diopters are the reciprocal of the focal length in meters. For example, a lens with a focal length of 0.25 meters (25 centimeters) has a refractive power of 4 diopters.

You may want to try our magnification of a lens calculator. It is sure to come in handy.


How do I determine the diopter of a lens from focal length?

To determine the diopter of a lens from its focal length, follow these instructions:

  1. Find out the focal length f of the lens in meters.

  2. Take the reciprocal of the focal length: 1 / f.

  3. You will get the power of the lens in diopters.

Or, if you want to make it hassle-free, use Omni Calculator's diopter calculator.

How many diopters is the human eye?

About 60 D. The focal length of the human eye is about 1.70 cm or 0.017 m. Hence, the optical power of the human eye is:

P = 1 / 0.017 m ≈ 60 D

What magnification is 4 diopters?

With the rule of thumb that one diopter equals 0.25 magnification, 4 diopters roughly translate to 1× or 100% magnification.

In case you are calculating these for medical purposes, professional guidance is recommended for accurate assessments.

What magnification is the equivalent of 5 diopters?

5 diopters could yield an approximate magnification of 1.25× or 125%.

Keep in mind these are rough estimates, and visual experiences can differ. If you're using these calculations for medical purposes, consulting a healthcare professional for accurate assessment is crucial.

Check out 28 similar optics and light calculators 🔍
Angular resolutionAperture areaBinoculars range...25 more