Every lens is described by a number in millimetres: 24mm, 50mm, 200mm. That number, the focal length, is the single most important fact about a lens. It decides how much of the scene fits in your frame and how big your subject looks. In this lesson you will learn what focal length really means, how it controls angle of view, and why the same lens shows a different view on different cameras.
What is focal length?
Focal length is an optical measurement of a lens, given in millimetres. Technically, it is the distance from the optical centre of the lens to the sensor when the lens is focused on something very far away. You do not need to measure it; what matters is what it does.
- A short focal length (for example 16mm or 24mm) shows a wide view, and things look smaller.
- A long focal length (for example 200mm or 400mm) shows a narrow view, and things look bigger, as if you moved closer.
Focal length is fixed by the lens design. A 50mm lens is always a 50mm lens, whatever camera it is on. What changes between cameras is how much of its image the sensor records, explained later in this lesson.
What is angle of view?
Angle of view (also called field of view) is how wide a slice of the scene the camera captures, measured in degrees. A short focal length gives a wide angle of view; a long focal length gives a narrow one. They are two sides of the same coin: when focal length goes up, angle of view goes down.
| Focal length | Approximate angle of view | What it feels like |
|---|---|---|
| 14mm | About 114° | Extremely wide; takes in almost everything in front of you |
| 24mm | About 84° | Wide; a whole room or a big landscape |
| 35mm | About 63° | Moderately wide; natural with some context |
| 50mm | About 47° | Close to a natural, everyday view |
| 85mm | About 29° | Narrower; a person from the chest up from a few metres away |
| 200mm | About 12° | Narrow; distant subjects look close |
| 400mm | About 6° | Very narrow; a bird across a field fills much of the frame |
Remember:Angles of view are usually quoted diagonally, corner to corner. The numbers above are standard approximate values for full-frame cameras.
Focal length and magnification
Focal length also controls how big things appear. Double the focal length from the same position and your subject appears twice as tall in the frame. Going from 50mm to 100mm makes a person who filled half the frame height fill the whole frame height.
This is why focal length is often described as "zoom". But remember: changing focal length from the same spot only crops in or out of the scene. It does not change how near and far objects relate to each other in size. That relationship, called perspective, depends on where you stand, and is explained in perspective, camera position and subject distance.
Focal length categories
Photographers group focal lengths into broad families. The exact boundaries vary slightly between sources; these are common full-frame ranges:
| Category | Approximate range | Typical uses | Lesson |
|---|---|---|---|
| Ultra-wide | Below about 24mm | Interiors, architecture, dramatic landscapes | wide-angle and standard lenses |
| Wide-angle | About 24–35mm | Landscapes, street, environmental portraits | wide-angle and standard lenses |
| Standard (normal) | About 35–70mm, centred on 50mm | Everyday photos, street, food, portraits with context | wide-angle and standard lenses |
| Short telephoto | About 70–135mm | Portraits, details | telephoto lenses |
| Telephoto | About 135–300mm | Sports, events, compressed landscapes | telephoto lenses |
| Super-telephoto | Above about 300mm | Wildlife, birds, distant sports | telephoto lenses |
A "normal" lens is often defined as one whose focal length is about the same as the diagonal of the sensor. For full frame, that is about 43mm, which is why lenses around 50mm are called standard.
Why the same lens looks different on different cameras
Focal length descriptions are usually based on full-frame (35mm-film-sized) sensors. Smaller sensors record only the centre of the lens's image, so they show a narrower angle of view, as if the focal length were longer.
| Sensor | Crop factor | Looks like this on full frame |
|---|---|---|
| Full frame | 1× | 50mm |
| APS-C (most brands) | About 1.5× | About 75mm |
| APS-C (Canon) | About 1.6× | About 80mm |
| Micro Four Thirds | 2× | About 100mm |
The lens has not changed; the sensor simply sees less of its image. This is called crop factor or equivalent focal length, and it has its own lesson: crop factor and equivalent field of view. Phone cameras often describe their lenses this way too, as "24mm equivalent".
How to choose a focal length
- Decide how much of the scene to include. The whole place, or just the subject?
- Decide where you can stand. If you cannot get closer, you need a longer focal length; if you cannot step back, you need a shorter one.
- Decide on the look. Wide lenses used close-up feel immersive; long lenses used from far away feel compressed and calm.
Common mistakes
- Always zooming instead of moving: your position changes the look of the photo; zoom only changes the crop.
- Forgetting crop factor: a 50mm lens on APS-C does not give the same view as on full frame.
- Thinking a long lens is only for far subjects: short telephotos are excellent for portraits and details.
- Using ultra-wide lenses for everything: they can make scenes look empty and stretch things near the edges.
How professionals think about focal length
Professionals choose focal length deliberately. First they choose where to stand to get the perspective they want, then they choose the focal length that frames the scene from that spot. Many also learn to "see" in a few favourite focal lengths, so they can picture the frame before lifting the camera. Their phrase for this is often "zoom with your feet first".
Practical examples
A small hotel room
- Situation
- At 50mm, you can only photograph the bed, even with your back against the wall.
- What to do
- You change to 16mm.
- Why it works
- You cannot step back further, so a shorter focal length gives a wider angle of view.
- Result
- The bed, window and desk all fit, showing the whole room.
A bird in a tree
- Situation
- At 50mm, the bird is a tiny dot among the branches.
- What to do
- You use a 400mm lens from the same spot.
- Why it works
- Eight times the focal length makes the bird appear eight times taller in the frame.
- Result
- The bird fills a large part of the photo, with its feathers clearly visible.
Key points
- Focal length, in millimetres, decides angle of view and how big subjects appear.
- Short focal lengths give wide views; long focal lengths give narrow, magnified views.
- Doubling focal length from the same spot doubles the subject's size in the frame.
- Focal lengths are grouped from ultra-wide to super-telephoto; around 50mm is standard on full frame.
- Smaller sensors show a narrower view from the same lens (crop factor).
Frequently asked questions
What is focal length in photography?
What is the difference between focal length and angle of view?
Why is 50mm called a normal lens?
Does a 50mm lens look different on APS-C?
Conclusion
Focal length is the lens number that controls angle of view and magnification. Short lenses show more, long lenses bring things closer, and sensor size changes the final view. Next, you will compare the two families of lenses: primes with one focal length and zooms with many.
Finished reading? Track your progress through Lenses and filters.