How To Find Focal Length of Lens?

How To Find Focal Length of Lens featured photo

If you are shopping for a new lens, you probably heard about the focal length. It determines how much of the scene you can capture. However, it can be challenging to find and understand if you are a beginner.

You can use a ruler or meter stick to find the focal length of a lens. Measure the distance of the image formed by the lens when you place it near the light source.

You can also consider other methods to find the focal length of a lens. This article will cover the different ways to measure the distance between the lens’ optical center and the image sensor. You will also learn more about the focal length and the factors affecting it.

What Is the Focal Length of a Lens?

The focal length refers to the distance between the optical center of the lens and the digital sensor. It is expressed in millimeters (mm). A large number denotes a long focal length, whereas a small number has a short focal length.

The lens’s focal length describes how much of the scene it can capture. It also affects the magnification or how large the subject appears in the frame.

A lens with a short focal length offers a wide field of view. It also makes the subject appear smaller than its actual size in real life.

Meanwhile, a long focal range captures a narrower angle of view. It also magnifies objects in the scene, even if they are far away.

Why Is the Focal Length Important?

The focal length is an essential part of photography. It affects how the final image will look.

Below are some of the reasons why focal length is important. 

Field of View

The lens’s focal length affects the field of view. Also known as perspective, it determines how much of the scene you can capture.

A lens with a short focal length can provide a wide angle of view. It can help you take landscape, real estate, and group photos without fear of cropping or losing detail.

Meanwhile, a long focal length offers a narrow perspective. It can capture objects at a far distance from the lens. It is ideal for tightly cropped portraits and wildlife photography.


The focal length can also impact the magnification of the image. In simple terms, it describes how large or small the object appears within the frame.

Short focal lengths make the subject look small. On the other hand, long focal lengths can magnify the scene.

Depth of Field

The lens’s focal length can affect the depth of field. It refers to the area within an image that appears sharp and in focus. 

Lenses with short focal lengths have a deep depth of field. They can make most elements in the frame look sharp. 

In contrast, longer focal lengths offer a shallow depth of field. They create a small focus area that isolates the subject from the background. 

Image Quality

Finally, the focal length of a lens can influence the final image quality.

Longer focal length lenses usually produce sharper images.

However, that does not mean that shorter focal lengths have low-quality photos. The former simply has an advantage because of its magnification rate and depth of field.

How to Find Focal Length of Lens? 4 Easy Ways to Try

Now that you understand the importance of focal length, you may be curious about finding it. Below are four easy ways we use when finding focal length of lenses.

Get a Ruler or Meter Stick

You can calculate focal length of a lens by using a ruler or a meter stick.

First, you must determine whether you have a convex lens or a concave lens.

Convex lenses are thicker in the middle and thinner at the edges. They can bend parallel rays traveling at the optical axis of the glass. This converging lens forms a real image on the opposite side of the lens.

Meanwhile, concave lenses have a curved surface that bends inwards. They can direct light rays to meet at a point in front of the mirror.

After knowing if you have convex and concave mirrors, ensure your timing is right. Calculate focal length of the lens in the early morning or before the sun sets.

Next, bring the lens and a white sheet of paper outside. Place the lens appropriately to face the sun on one side and the paper on the other. Doing so allows light rays converge and enter the lens. They produce an inverted and virtual image of the sun on the sheet.

Then, adjust the position of the lens until the virtual image is sharp.

Finally, grab your ruler to measure the distance between the lens and the paper. The number you will get is the focal length of the lens.

Use a Focal Length Formula to Calculate the Focal Length

You can also use a formula to get the focal length of the lens. However, you must know the distance of the object and the inverted image created by the lens.

Determine the focal length using this formula:

1/f = 1/do + 1/d

The f represents the focal length of the lens. The do in the focal length equation is the distance of the object from the lens. Finally, the di is the image distance produced by the lens. 

Find Markings on the Lens

Find Markings on the Lens

Another way to find the lens’s focal length is to look at the markings on the barrel or the glass end.

You will notice several letters and numbers on the front end of the lens. Find the figure with the letters “mm” following it. It describes the focal length of the lens in millimeters (mm).

Go to the Manufacturer’s Website

You can visit the website of the lens manufacturer. Search for the specific model of your lens and go to the product page. There, you will find essential product details, including its focal length.

What Are the Different Types of Lenses?

Photographers usually group lenses based on their focal lengths.

Below are the different types of lenses.

Wide-Angle Lens

Wide-angle lenses have a short focal length of 35mm or less. They offer a wide field of view, hence their name. They can let you capture more of the scene in front of you.

Wide-angle lenses can also make the subject appear smaller and the depth of field deeper than standard lenses. They can suit group photos, landscapes, architecture, and real estate.

Standard Lens

Standard lenses feature a 35mm-70mm focal length. They provide the most natural field of view, similar to what our naked eyes see. They can also minimize distortion in the image.

A standard lens is also among the most versatile lenses you can buy. It can handle travel and action photography. However, it is popular for portraiture.

Telephoto Lens

Telephoto lenses have long focal lengths of more than 70mm. They are further divided into sub-categories.

  • Short Telephoto Lens – It offers a focal length between 70mm and 135mm.
  • Medium Telephoto Lens – It has a 135mm-300mm focal length.
  • Super Telephoto Lens – It features a focal length of more than 300mm.

These lenses can offer a narrow angle of view. They can bring distant objects close without moving physically near them. They are suitable for action, bird, sports, and wildlife photography.

Fisheye Lens

Fisheye lenses have an extremely short focal length of 16mm and less. Also known as ultra-wide-angle lenses, they capture a very broad perspective that feels unnatural.

They are considered specialty lenses, which emphasize distortions within the image. They are ideal for architectural photography and other creative photography genres that require a dramatic angle.

What Is the Recommended Focal Length Based on the Photography Genre?

The focal length of a lens can also determine the type of photography that you do.

Below are the most popular photography genres and the recommended focal length examples for each of them.

Portrait Photography

The best focal lengths for portraiture are between 50mm and 135mm. They can capture the most natural view close to what our eyes see. 

The 50mm prime lens has the least distortion. Meanwhile, the 135mm lens can isolate the subject from the background and create stunning bokeh.

Zoom lenses with the 24-70mm or 70-200mm focal ranges are also worth considering. They are great versatile lenses if you like to take photos of people and family.

Architecture and Real Estate Photography

Architecture and real estate photography are all about showcasing the unique properties of a particular structure. 

Consider wide-angle lenses with a 35mm focal length or less. These lenses can capture more of the scene in front of you. Hence, they are great at taking photos of a tiny room or a majestic cathedral. 

Street and Travel Photography

Get a standard lens with a 35mm-70mm focal length if you like to travel and take street photos. It lets you capture a wide field of view. At the same time, it allows you to get close and show details of the scene.

Landscape Photography

A wide-angle lens with a 16-35mm focal length is ideal for landscape photography. It can give you stunning panoramic shots.

However, some landscape photographers also like to use a telephoto lens, such as the 100-400mm. This lens can capture more details in the scene.

Action and Sports Photography

Action and sports photographers are unable to get close to their subjects. Fortunately, a telephoto lens with a focal length of more than 70mm exists.

This type of lens can magnify the subject and make them appear close, even if you are far from them. 

Bird and Wildlife Photography

Telephoto lenses are also ideal for bird and wildlife photography. They can get you closer to the subject without being near them physically. Hence, they can guarantee your safety, especially when photographing wild and dangerous animals.


The focal length is among the most important elements of the lens. It can affect how the final image will look.

You can use different ways to find the focal length of the lens. Get a ruler and use a formula for a more accurate measurement. Or you can simply look at the lens markings to determine the focal length.

Do you have other questions about focal length? Visit our contact page to send your queries!