When you’re starting out in photography, the numbers engraved on a lens can seem abstract and not entirely clear. 50 mm, 35 mm, f/1.8… What do they actually mean? And, more importantly, how do they affect your photos?
Focal length is one of the most fundamental concepts to grasp if you’re to make real progress. It determines what you frame, how far away you stand from your subject, and the overall mood of your photo. Once you’ve got the hang of it, everything becomes clearer and your lens choices far more deliberate.
What is focal length and what do the numbers on a lens mean?
Before we talk about framing or choosing a lens, there’s one concept that underpins everything else. Focal length is the starting point. Fully understanding what it represents means understanding why two lenses can produce such different results, even under the same shooting conditions.
What is focal length? What is the focal length f?
Focal length is the distance between the optical centre of the lens and your camera’s sensor when the focus is set to infinity. It is measured in millimetres and denoted by ‘f’. You’ll find it engraved directly on your lens: 35 mm, 50 mm, 85 mm, 200 mm…
The lower this value, the wider the angle of view. The higher it is, the more the lens magnifies the subject and narrows the frame. It really is as simple as that.
So this figure does not refer to the physical length of the lens. A 50 mm lens is not 50 mm long. It is an optical value that reflects the way the lenses converge light onto the sensor.
What does a focal length of 50 mm mean?
The 50 mm is generally referred to as the ‘normal’ focal length. And you’ll see that this nickname is no coincidence. In fact, its angle of view is very close to that of the human eye. What you see through the viewfinder resembles what you perceive naturally, without any exaggeration or compression.
This is why it is often recommended for beginner photographers. It forces you to move around, to compose with your feet, and to think about the frame rather than relying on a zoom lens. On a full-frame sensor, the 50mm will produce natural, balanced images, with the subject standing out beautifully against the background.
In portraiture, the result will be authentic, almost documentary-style. In street photography, it allows you to blend into the scene without being noticed. It is a versatile, understated focal length that doesn’t try to do too much.
How do you calculate focal length and understand a lens’s specifications?
A lens’s specifications tell a story, but you still need to know how to read them. Behind each value lies a precise optical principle that directly influences your photos. Here’s how to make sense of this data, without needing an engineering degree.
How do you calculate the focal length? How can you work out the focal length?
The basic formula in optics is that for thin lenses. It is written as follows: 1/f = 1/v + 1/u. In this equation, f represents the focal length, v is the distance between the lens and the sensor, and u is the distance between the lens and the subject being photographed.
In practice, this formula is mainly used in theoretical optics, but in the field, you don’t need to calculate it. To put it simply, the focal length is a fixed characteristic of your lens, specified by the manufacturer and engraved on the mount. On a zoom lens, it varies between two values—for example, 24–70 mm—depending on your setting.
What really matters to you is knowing how to interpret this value to anticipate the framing you will achieve.
Is the focal length f or 1/f?
This is a common source of confusion, particularly amongst those with some experience of physics. In optics, the vergence of a lens is denoted as 1/f and expressed in dioptres. It is, in fact, the inverse of the focal length.
The focal length, on the other hand, is denoted by f and expressed in metres in optical calculations, or in millimetres in the world of photography. When we say that a lens has a focal length of 50 mm, we are indeed referring to f, not 1/f.
Vergence, on the other hand, is used to describe the power of a lens. The shorter the focal length, the higher the vergence and the more powerful the lens. To explain, a 50 mm lens has a vergence of 20 dioptres. A 100 mm lens has a vergence of 10 dioptres.
The two concepts are related, but they do not refer to the same thing. In photography, we always refer to ‘f’, the focal length in millimetres.
Does f/1.8 correspond to the focal length?
No, and this is a very common source of confusion. The notation f/1.8 does not refer to a focal length. It actually indicates the maximum aperture of the lens’s diaphragm.
The aperture, also known as the f-number, expresses the ratio between the focal length and the diameter of the light entrance. To put it simply, an f/1.8 lens lets in a lot of light. An f/5.6 lens lets in much less. The smaller the number after the ‘f’, the brighter the lens.
On a camera lens, you will therefore find two distinct types of information. The focal length, expressed in millimetres, indicates the field of view. The aperture, expressed as f/x, indicates the brightness. A 50 mm f/1.8 is a lens with a focal length of 50 mm and a maximum aperture of f/1.8. The two values go hand in hand, but they do not refer to the same thing.
How do you choose and use the right focal length for your style of photography?
Choosing a focal length isn’t just a technical matter. It’s also a creative decision that influences how you position yourself in relation to your subject and what you want to convey through your image.
Short, long and standard focal lengths: a lens for every purpose
We generally distinguish between three main categories of focal lengths, each with its own characteristics and preferred uses.
There are short focal lengths, also known as wide-angle lenses, which cover focal lengths below 35 mm. They offer a wide field of view and allow you to capture a lot of detail within the frame. They are best used for landscape, architectural or travel photography. They lend a sense of scale to scenes, emphasise vanishing lines and create a feeling of immersion. A 24 mm or 28 mm lens, for example, will allow you to capture an entire panorama without having to step back several metres.
Be aware, however, of an effect specific to wide-angle lenses. The closer you get to the subject with a short focal length, the more the proportions become distorted. Foregrounds are magnified, whilst backgrounds appear pushed back. This is a creative choice that many photographers use deliberately, but honestly, it’s best to master it before using it.
Standard focal lengths, around 35 mm to 55 mm, therefore reproduce a view similar to that of the human eye. They are versatile, unobtrusive and will suit a wide range of situations. The 35 mm is particularly popular in street photography and photojournalism. The 50 mm is equally suited to portraiture and everyday photography. These focal lengths are often the first to be recommended to photographers wishing to refine their eye, as they do not introduce any significant distortion.
As for long focal lengths, beyond 70 mm, they allow you to magnify the subject and reduce the apparent distance between planes. The longer the focal length, the closer the background appears to the subject, creating a compression effect. For example, an 85 mm lens is highly prized in portraiture for its flattering rendering of faces. A 200 mm or 400 mm lens is essential in wildlife or sports photography, where it is obviously impossible to get physically close to the subject.
To explore the subject a little further, we can also discuss macro photography. Here, we move slightly beyond the usual range of focal lengths. Macro lenses generally have a focal length of between 60 mm and 180 mm, but their distinctive feature is their very short minimum focusing distance, which allows you to photograph subjects just a few centimetres away. In this case, the focal length will influence the working distance between the lens and the subject. A 100 mm macro lens will allow you to stay further away than a 60 mm macro lens, which is very useful for avoiding scaring insects or blocking the light.
When it comes to travelling, versatility is obviously key. An optical zoom lens covering 24–70 mm or 18–55 mm will give you plenty of scope to switch between landscapes, portraits and candid shots. If you want to travel light whilst retaining flexibility, bear in mind that a fixed-focal-length lens around 35 mm may be sufficient to cover a wide variety of situations.
Choosing a lens is therefore always a question of striking a balance between your photography style, your practical constraints and your creative aspirations. Taking the time to understand what each focal length offers means equipping yourself to photograph with intention, rather than by default.
Focal length: a compass for your vision
Focal length isn’t just a technical specification engraved on a lens; it’s a genuine storytelling tool. It determines your relationship with your subject, what you include in the frame, and the emotion you convey.
There is no such thing as a ‘right’ or ‘wrong’ focal length. There are focal lengths suited to what you want to express, to the moment you want to capture. The more you experiment, the more you develop a natural instinct for choosing the right lens at the right moment.
Technique can certainly be learnt, but your photographic eye is developed over time, through practice and curiosity.
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