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What Is Focal Length? A Beginners Guide

Why Is Focal Length Important? 

Focal length is important because it has the greatest effect when it comes to the overall look of the photos that you take.

The main things to consider are the following:

Before we cover the 3 topics above, let us quickly describe how focal length is measured.

How Is Focal Length Measured In Photography?

The term focal length measures the distance between the centre of the lens and the camera sensor or film (this is where the light rays are focused). Focal length is normally measured in millimetres (mm).

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This isn’t necessarily something you need to know for your photography, but you DO need to know the effect that it can have. 

Why is focal length measured in millimetres (mm) in camera lenses?

Even though lenses nowadays will be shown in millimetres (mm), there are older lenses that were expressed in centimetres (cm) or inches (“). 

Since the focal length in cameras is such a small measurement, mm would seem the most practical because it can be used without using decimals and fractions. 

For example if you had an 18mm lens it would be the following in cm and inches: 

1.8cm 

0.708661 inches

Hopefully the above examples show why mm is a more practical way to show focal length. 

The focal length advertised on lenses is always the 35mm equivalent focal length.

What Is 35mm Equivalent Focal Length?

The 35mm equivalent focal length is the focal length of a lens if it were to be on a full frame sensor camera. This is because old 35mm film was the standard for film cameras.

On APS-C cameras which are standard for most crop sensor cameras the sensor size is smaller than full frame so it is cropped. 

Hence there is a crop factor which alters the effective focal length of a lens. You find the effective focal length by multiplying the crop factor for the camera with the 35mm equivalent focal length of the lens. 

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Where to find the 35mm equivalent focal length of a lens?

The focal length of a lens should be denoted clearly in mm on the lens take a look at the 50mm lens example below.

Photo by Jeff Eaton

Now lets get into the effects of focal length…

Focal Length and Field of view

Field of view describes how much of a scene a camera can capture. 

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

The shorter the focal length the wider the field of view.

Lets try and use an analogy:

You have 10 people standing side by side in a line in front of you. 

You stand at a fixed position with a camera and you use 2 different lenses. One is a longer focal length, say 80mm. The other lens is a shorter focal length, say 24mm. 

You take a picture of the group with the 80mm lens and you can only fit 3 people in the picture. 

But you take a picture with the 24mm lens and you can fit all 10 within the frame. 

As you can see here the field of view on the shorter 24mm lens is ‘wider’. 

Angle of view and field of view is used interchangeably at times and short focal length lenses can be called wide angle lenses. 

The following video is useful in showing you the difference between different focal lengths and how they look.

Skip to 2:42 to start seeing the different focal lengths and keep an eye on the framing as it goes through the different focal lengths.

Focal length, Compression and Perspective Distortion

This can be a very confusing subject, but the video below explains it well. just remember it is not the focal length alone that determines perspective.

To summarise, perspective distortion is determined by the distance between the camera and the subject and background.

It’s just that the field of view of different focal lengths makes it seem that it is the lens itself which is causing the distortion.

The main takeaway is that with wide angle lenses, you can fit more into a scene from one position than you would with a telephoto lens from the same position.

As described above, a longer focal length lens can give you the effect of zooming in or cropping in.

Focal Length and Magnification

In camera lenses the longer the focal length the more magnified the subjects become and the bigger they will appear.

Let’s say you want to take a picture of a bird in a tree 100 metres away from you. If you used a wide angle lens like a 18mm lens, then the bird would look really small in your image since the field of view is so wide. 

On the other hand if you used a longer focal length lens like a 120mm lens then the bird will seem a lot larger as your field of view is smaller and the image is a lot more zoomed in. 

Different focal length ranges 

The following table outlines the different focal length ranges in terms of 35mm equivalent focal length – so the focal length if the lenses were on full frame cameras.

CategoryFocal Length
Ultra Wide AngleLess than 24mm
Wide Angle24-35mm
Normal 35-70mm
Mild Telephoto 70-135mm
Long Telephoto135-300mm
Super TelephotoGreater than 300mm
Focal length ranges

Since there are so many focal length ranges different lenses are used for different purposes. 

It is important to note that some lenses will be prime lenses covering just one focal length whereas other lenses will be zoom lenses covering various focal lengths.

What are Zoom lenses and Prime Lenses?

A zoom lens is a lens which can cover a range of focal lengths. They can be versatile in achieving various looks while having to carry less gear. 

However the complexity of the lenses can make them more expensive. They will also tend to be slower lenses meaning they won’t have maximum apertures as wide as prime lenses.

A prime lens is a lens with a fixed focal length. There is a restriction in terms of varying the focal length ,but in return they have a simple build which is cost effective and high quality.

They will tend to be “fast” meaning they have really wide maximum apertures. 

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