Understand Focal Length in 4 Easy Steps

One of the most important reasons why you buy a lens is down to the focal length. This little piece of information lets you know how close you need to be in relation to the subject you are capturing.

A longer focal length is necessary for objects further away. A smaller focal length captures more of the scene in front of you. To understand focal length in four, easy steps, read on.

Why You Should Know What Focal Length Means

Knowing what the focal length of a lens means in relation to your camera, is very important when it comes to buying lenses. Read this post to find out what different lenses are used for, which ones are right for you, how to use them creatively, and all the technical speak you’ll need.

Lenses are divided into two categories based on whether they can zoom. There are the ones that have a fixed focal length (prime lenses) and those that have a variable focal length (zoom lenses). In general, prime lenses are sharper and often have a wider aperture, great for low light conditions.

Zoom lenses allow you to use one lens to cover a range of different photography fields. One lens means less time spent searching for and changing lenses. There are advantages and drawbacks to both types of lenses, so having a mixture of them gives you versatility and power.

We can use the human eye as an example. Its focal length varies between roughly 17mm and 25mm, depends on who you ask and who’s eye you examine.

We have an approximate field of view of over 180°. This is different from the 90° angle of view from a lens, which is down to the fact that we have two eyes.

The area that we actively perceive is smaller, similar to a 40-50mm lens. That’s why the 50mm is called the ‘standard’ focal length.

On PetaPixel, you can find a great article explaining the human eye in-depth.

For more information on the two different types of lenses, read our Prime Vs. Zoom lens article here.

Section 1 – What Does It Mean?

The focal length of the lens determines how ‘zoomed in’ your photos are. The higher the number, the more zoomed your lens will be.

It is often misunderstood that the focal length is measured from the front or rear of the lens when, in reality, it’s the distance between the point of convergence in your lens to the sensor or film in your camera.

Take a look at the diagram below that explains this:

Section 2 – Different Focal Ranges and What They’re Used For

Ultra Wide Angle and Fisheye 14-24 mm

These lenses are often considered speciality items and not often included as part of a kit lens. They create such a wide angle of view that they can appear distorted. this is down to the lens having to fit more of the scene into film or sensor.

Ultra wide-angle lenses are often used in event and architectural photography. They help to get a lot into a photo when shooting in a confined space. Wide and ultra-wide lenses are about putting yourself in the middle of it all, rather than fitting in the whole of a scene.

These lenses are not suitable for portraits as they enhance the perspective so much that facial features can look unnatural.

Rectilinear wide angles project an image in which the straight lines remain straight.  Fisheye lenses distort the scene into a spherical shape.

Wide Angle 24-35 mm

This is where you’ll find most kit lenses for full-frame cameras start. 24 mm is the point at which the distortion that appears to stretch the side of an image stops appearing unnatural.

They are used by photojournalists for documenting situations. This is because they are wide enough to include a lot of the context, whilst still looking realistic.

Standard 35mm-70 mm

It’s in this range (at about 45-50 mm) that the lens will reproduce what our eyes see (excluding peripheral vision). I like to use this range when shooting on the street or with friends in a closed setting. Examples would be at the dinner table or the pub.

A standard lens such as a 50 mm f/1.8 is an excellent, inexpensive addition for a camera. It will provide excellent results. Prime lenses will always provide better results than your kit lens, as it is built with a single purpose. It does one job well.

Mild Telephoto 70-105mm

This range is often where kit lenses stop. Here,  you’ll start to get into the range of telephoto lenses and portrait primes (around 85mm). This is a good range for portraits as the natural perspective of the lens will separate the face from the background.

It does this without isolating it.

Telephoto 105-300mm

Lenses in this range are often used for distant scenes such as buildings or mountains. They’re not suitable for landscape photography because of the way they flatten the perspective of a scene. Lenses in a range higher than this are used for sport and animal photography.

Section 3 – How Does Focal Length Affect the Perspective of A Photo?

I tackled most of this in the previous section. To give you a better idea of how the focal length affects the perspective of a photo, I took four photos of the same subject at different focal lengths and compared them below.

The subjects (three soup cans) remained in the same position (about 10 inches apart from one another) in every photo. It’s worth noting that these photos are shot with a crop sensor. This means the actual focal length will be higher than listed – something I explain in Section 4.

To say it’s the focal length that changes the perspective is, however, quite misleading. You see, it’s the distance from the subject.

The focal length of a lens is an indicator of the distance from the subject: the images are all framed the same. Differences arise because the focal length is getting longer (zooming in) as the camera moves further away from the subject.

Remember, the distance from the subject is changing the perspective. The focal length is just used to compensate for this.

Section 4 – What About My Crop Sensor?

Shooting on a crop sensor has what’s known as the ‘crop factor’. What you’re doing is zooming in on an image, and avoiding the widest parts of the scene. The diagrams below show the way this works.

Even lenses built for crop cameras such as the EF-S range and DX range will still have this effect. This is because lenses are listed by their physical focal length rather than their field of view or equivalent focal length.

However, these lenses will not work on a full-frame body without heavy vignetting. This is because the image will not project onto the whole of the sensor.

That’s it!

Here are two focal length examples that were taken at very different focal lengths of the lens.

Before you go, check out this cool video we found.