What is focal length in photography and how to use it (5 facts)

Focal length in photography is talked about a lot, because it has a big impact on the appearance of an image. If used correctly, it can enhance your subject. However, if used badly or without thought, the wrong focal length can negatively distort the appearance of your subject.

That goes for people, products, buildings, landscapes…anything.

I say negatively distort, because sometimes the distortion is a positive thing. An example of positive distortion in a photo is making a model’s legs seem longer. Bad distortion is when you make your model look wider or just plain weird, which is very easily done when using the wrong focal length for portraits.

We could get really technical with graphs, charts and diagrams to explain what is focal length in photography, but I’m going to keep it as simple as possible and answer these five frequently asked questions:

1. What is focal length?
2. What focal length looks normal?
3. What effect does focal length have on an image?
4. Why use different focal lengths?
5. What is field of view?

1. What is focal length in photography?

The focal length of a lens for digital cameras, is the distance between the focal point of a lens and the sensor when the subject is in focus.

It doesn’t refer to the size of the lens, because it’s not the actual length of the lens. The focal point is inside the lens, at the point where light rays converge.

Lens focal length is indicated by the numbers on the lens, such as 50mm, 75mm etc, which is why we refer to a lens as being, for example a 50mm lens. 

A prime lens has one focal length, such as 85mm.

A zoom lens can be used at different focal lengths and this is part of the lens description, such as 24-70mm.

2. What focal length looks normal to the human eye?

The closest we can get to how the human eye sees the world is with a 50mm lens on a full frame camera. On a crop frame camera the normal focal length for the human eye is a 35mm lens.

The reason for the difference is that a crop frame camera magnifies the scene by roughly 1.5 times. Any type of magnification in photography distorts the image.

This leads us to the next most obvious question…

3. What effect does focal length have on an image?

The simple answer is that the wider the lens, the further apart objects within a scene will appear. Longer focal lengths compress perspective in the image and makes objects appear closer together.

These photos show you what I mean far better than I could ever say in words.

You may have heard the expression “zoom with your feet”? In other words move back and forth rather than standing still and just changing the focal length on a zoom lens to get closer or further away during a shoot.

Well, this experiment just showed us why.

Did you notice how with the shorter focal lengths you can see more to the side of the lenses? We’ll talk about that in a moment.

So, you might think we’ve answered the next question, but there’s more to it than that…

4. Why use different focal lengths?

Apart from distortion, your choice of lens focal length has more of an impact on photos than you first realise.

Depth of field and focal length

Focal length affects depth field in an image and is one of the key ways for creating a blurry background.

The longer the focal length, the blurrier your background will be. A long telephoto lens of 200mm for example, is great for compressing perspective and blurring out the background.

Portrait focal lengths

You can change your subject’s appearance with your choice of focal length for portraits (click the link to see). It affects how much of the background is in shot, as well as the shape and proportions of your subject’s face.

For example, when using a wide angle lens, like a 24mm lens:

• You don’t want to make a person’s forehead and nose seem bigger than it is, or change the shape of their face by getting up close and using a lens with a focal length of say 24mm for a head and shoulders shot.
• However, for an environmental portrait showing your subject in their environment, a 24mm focal length could be the right choice if they’re small and in the middle of the frame.

An 85mm lens on a full frame camera is considered the ideal focal length for flattering portrait photos.

A quick guide to best portrait focal lengths to use:

• 50mm – best for full length and waist upwards photos. (If used closer the face becomes too thin and the nose too big.)
• 85mm – ideal for portraits from the knee up and also for as close as head and shoulders photos
• 135mm – head and shoulders photos and headshots, although the face is slightly wider. This is a great focal length for full length shots with a narrow field of view and background blur

You might also be interested in reading my article on what lenses to use for portraits.

Architecture focal lengths

When you tilt your camera upwards to photograph a building, you’ll experience distortion as the building’s lines converge.

This can create very dramatic photos.

To capture a building accurately, use a telephoto lens and stand further away so that the building’s lines are straight.

Landscape focal lengths

Using a wide angle lens, so 14-35mm, works well on a full frame camera for landscapes.

If, however, you want to create a panoramic image by stitching several shots together, a longer focal length would be better. The images won’t be distorted, so will be easier to stitch together.

5. What is field of view?

We can’t talk about focal length without mentioning field of view (angle of view), because as you change your focal length, you also change the field of view. Again, I’m not going to get massively technical.

Before I go further – field of view and angle of view are the same thing. Just different ways of saying it.

Easy field of view exercise

The easiest way to understand field of view is to try this:

• Stretch your arms out straight in front of you
• Join the tips of your thumbs and the tips of your forefingers so that you form a triangle shape with your hands
• Look through that triangle as you bring your hands closer to your eyes
• Do you see how the closer your hands get the more you can see through the triangle?

Or imagine yourself in a tunnel. As you get closer and closer to the tunnel exit you have a wider view of what’s outside of the tunnel.

That’s the field (angle) of view changing.

With a longer focal length your focal point is further away from the sensor. As you decrease the focal length you bring the focal point closer to the sensor so the field of view increases.

The longer the focal length, the narrower the field of view.

• Wide angle lenses are below 35mm
• Standard lenses range from 50mm – 60mm
• Telephoto lenses are from 75mm upwards

Summary

So now we know that as you change your focal length three things happen:

• It affects depth of field
• Aspects of an image move closer together or further apart
• Field of view changes

You might also enjoy my 7 tips for better portrait photography.

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