Why You Should Know What Focal Length Means

Knowing what the focal length means, especially 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.

Section 1 – What Does it Actually Mean?

The focal length of your lens essentially 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:

Diagram explaining focal length in photography

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

Ultra Wide Angle 14-24mm

These lenses are often considered specialty items and the range is not often included as part of a kit lens. They create such a wide angle of view that they can appear distorted as our eyes aren’t used to seeing that sort of range.

Ultra wide angle lenses are often used in event and architectural photography for getting 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 simply fitting in the whole of a scene.

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

Wide Angle 24-35mm

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

They are used widely by photojournalists for documenting situations as they are wide enough to include a lot of the context whilst still looking realistic.

Standard 35mm-70mm

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

A standard lens such as a 50mm f1.8 is an excellent, inexpensive addition for a camera and will provide excellent results. Prime lenses (lenses with a fixed focal length – can’t zoom) will always provide better results than your kit lens as it is built with a single purpose in mind. It does one job well rather than multiple jobs poorly.

Mild Telephoto 70-105mm

This range is often where kit lenses stop and you’ll start to get into the range of telephoto lenses and portrait primes (around 85mm). This is a good range for portrait lenses. The natural perspective of the lens will separate the face from the background without completely 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 mostly 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 actually the distance from the subject.

The focal length 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.

A four photo grid of soup cans, showing the impact of using different perspectives - focal length in photography

Section 4 – What About My Crop Sensor?

Shooting on a crop sensor has what’s known as the ‘crop factor’. This essentially means that any full frame lenses (EF, FX, etc.) that you put onto a crop sensor body will have a cropping effect.

The crop factor is approximately 1.6. In real terms, this means that if you shoot at 35mm, the actual result will be closer to a 50mm image.

The diagrams below show the way this works. What you’re effectively doing is zooming in on an image, and avoiding the widest parts of the scene.

diagram explaining focal length cross section comparison

Diagram explaining crop sensor and full frame lens projection

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 actual length rather than their field of view.

These lenses will not work on a full frame body without a heavy vignetting effect. The image will not project onto the whole of the sensor.

That’s it! Finally, here are two example shots taken at very different focal lengths. The first is shot at 24mm and the second at 300mm (both on a crop sensor).

A photo of three soup cans

A still life of three soup cans arranged to demonstrate the impact of using different focal lengths in your photography