Aperture, shutter speed and ISO decide how a photograph is exposed. Focal length decides what it is a photograph of. It is the number on the front of the lens, it is written in millimetres, and almost everything people believe about it is slightly wrong: it does not bring things closer, it does not distort faces, and the reason a 200mm photograph of a mountain looks the way it does has nothing to do with the glass.
The number is an angle, not a magnification
Focal length is the distance from the optical centre of the lens to the sensor when the lens is focused on something far away. That definition is true and almost useless. The useful translation is this: the number decides the angle the camera can see, and nothing else.
A 24mm lens on a full-frame camera takes in about 84 degrees across. A 50mm takes in about 40. A 200mm takes in about 10. Stand in one place and the world in front of you does not change; you are only choosing how wide a slice of it to keep. The drawing beside this paragraph is exactly that and nothing else, and the boxes are to scale, so it is worth dragging to the far end to see how small ten degrees really is.
This is why the most common phrase in photography, that a long lens brings you closer, is a bad description of a real thing. It keeps less. Because the frame ends up the same size on the wall either way, keeping less means what is left is bigger, and that feels like being closer. It is not the same as being closer, and the difference between the two is the whole of the next section.
Compression is not something a lens does
Look at a portrait taken at 200mm and the background looks enormous, close, stacked up right behind the subject. Photographers call this compression and usually blame the lens for it. The lens is innocent. Distance did it.
Here is the experiment, and it is the drawing next to this paragraph. Keep one person the same size in the frame at every focal length. To do that with a longer lens you have to walk backwards, because the longer lens keeps less of the scene. Walking backwards changes the ratio between how far away she is and how far away the hills are. At two metres, hills at sixty are thirty times further away than she is, and they look it. At forty metres they are only two and a half times further, so they loom.
Perspective, in other words, is decided entirely by where you stand. The lens only decides how much of what you can see from there gets kept. Which is why the advice worth remembering runs in this order: choose your distance first, for the relationship you want between the subject and its background, then pick the focal length that frames it from there.
What 24mm does to a face
Everyone has seen the wide-angle portrait: the nose enormous, the forehead domed, the ears vanished, the whole face pushed forward like a fish in a bowl. It gets blamed on the lens, and the phrase wide-angle distortion appears in every camera review ever written.
It is the same trick as compression. To fill the frame with a head using a 24mm lens you have to get about half a metre from it, and at half a metre the nose genuinely is much nearer the camera than the ears are, so it genuinely is much larger. Step back to five metres with a 200mm and the nose is a rounding error against the distance to the ears, so the face flattens out. Take the 24mm photograph from five metres instead and crop it to the same framing: the face looks correct, just soft. Same geometry, same picture.
Hence 85mm to 135mm as the classic portrait range. It is not that those lenses are kind. It is that they let you stand two or three metres away, which is where a face looks like itself, and still fill the frame with a head. The lens is chosen to permit the distance.
The one distortion that is real
There is a stretch that genuinely belongs to wide lenses, and it lives at the edges. A rectilinear lens keeps straight lines straight, which is what you want in a building, and the price of that is that objects away from the centre get pulled outwards. In the middle of the frame nothing happens. In the corner of a 20mm frame, a round thing becomes an oval pointing at the corner.
It matters in exactly one common situation: the group photograph taken wide, where the people on the ends come out broader than the people in the middle. The fix is not a different lens, it is composition. Keep faces near the middle and leave the corners for sky, floor and wall, or step back and use something longer.
Why the same number frames differently on two cameras
Focal length is a property of the lens, but the angle you get out of it depends on the size of the sensor behind it, because the sensor keeps a rectangle out of the middle of the circle of light the lens throws. A smaller sensor keeps a smaller rectangle, which frames like a longer lens.
The multiplier is 1.5 for most APS-C cameras, 1.6 for Canon, and 2 for micro four thirds. So a 35mm lens on APS-C frames like a 50mm, and the 300mm you bought for birds reaches like a 450mm, which is a real advantage rather than marketing. Every focal length quoted in this article is the full-frame one, so divide by your crop factor to find the number to look for on the lens.
Note what does not change. A crop sensor does not make a face look any different, because it only changes the framing, and framing is not perspective. Same rule as everything else here: perspective belongs to your feet.
Where to start
There is no correct focal length, but there are conventions, and they exist because thousands of people arrived at the same distances for the same subjects. These are full-frame numbers.
| Focal length | What it is for | Where it puts you | Verdict |
|---|---|---|---|
| 14 to 20mm | Interiors, big skies, tight spaces | Inside the scene, and it shows | Specialist |
| 24mm | Landscape with a foreground, rooms | Close to something, the rest behind it | Wide |
| 35mm | Street, reportage, one lens for a trip | In the conversation | One lens |
| 50mm | Almost anything | Where you were standing anyway | Normal |
| 85mm | Portraits, details | Two metres, which flatters | Portrait |
| 135mm | Portraits with room, stage, candid | Across the room, unnoticed | Portrait |
| 200mm | Wildlife at a distance, sport, mountains | Across a field | Long |
| 300 to 600mm | Birds, small animals, far ridges | Further than you would like | Specialist |
If you own one lens and want a second, the useful rule is to jump by a factor of two. A 35 and an 85 are two different ways of seeing. A 35 and a 50 are one way of seeing, bought twice.
Zoom, prime, and the honest difference
A zoom covers a range of focal lengths and a prime covers one. The advantage of the zoom is obvious. The advantage of the prime is not that it is sharper, which stopped being reliably true some years ago, but that it takes the decision away from you. Standing still and turning a ring is not composing. Walking is composing, and a prime makes you walk.
The other real difference is the aperture. A prime at f/1.8 costs a fraction of what a zoom at f/2.8 does and lets in more light than one, which matters in exactly the situations the ISO article is about. If you own a kit zoom and want the cheapest upgrade available anywhere, it is a 50mm f/1.8.
Choosing it in the field
In practice the decision has an order, and it is not the one most people use. Do not start at the camera and zoom until the framing looks right. Start at your feet.
- Decide what the photograph is about, and how it should sit against its background. That is a question about distance, not about lenses.
- Walk to that distance. Nearer for a background that falls away, further back for one that stacks up behind the subject.
- Then choose the focal length that frames the subject from where you are standing. That is the whole job the number does.
Two subjects break the rule, and both for the same reason. A bird will not let you approach and a mountain cannot be approached, so the distance is fixed by the world and the lens is the only variable left. Everywhere else, your feet are the better control.
The short version
- Focal length is an angle of view. It sets how much of the scene the frame keeps.
- A long lens does not bring you closer. It keeps less, so what is left looks bigger.
- Perspective belongs to where you stood. It never belonged to the lens.
- Compression is the background looming because you walked back to fit the subject in.
- The wide-angle face is half a metre of distance, not the glass.
- 85 to 135mm flatters because it lets you stand two or three metres away.
- On a crop sensor, divide these numbers by 1.5 or 2 to find the lens to buy.
- Choose the distance first. Then choose the lens that frames it from there.