Aperture, shutter speed and ISO decide how bright a photograph is. Focal length decides what it is a photograph of. It is the number on the front of the lens, written in millimetres, and most of what people believe about it is a little bit wrong. It does not bring things closer. It does not stretch faces. And the reason a 200mm shot of a mountain looks the way it does has nothing to do with the glass.
The number is an angle, not a zoom
Officially, focal length is the distance from the middle of the lens to the sensor when the lens is focused far away. True, and no help at all. Here is the useful version: the number decides how wide a slice of the world the camera keeps. That is all it does.
A 24mm lens on a full-frame camera sees about 84 degrees across. A 50mm sees about 40. A 200mm sees about 10. Stand in one spot and the world in front of you does not change. You are only choosing how much of it to keep. The drawing beside this paragraph is exactly that, drawn to scale, so drag it to the far end to see how narrow ten degrees really is.
This is why the most common sentence in photography, that a long lens brings you closer, is the wrong words for a real thing. The long lens keeps less. The picture ends up the same size on the wall either way, so 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 is the whole of the next section.
Compression is not something the lens does
Look at a portrait shot at 200mm and the background looks huge, close, piled up right behind the person. Photographers call this compression and usually blame the lens. The lens is innocent. Distance did it.
Try it in 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. And walking backwards changes how her distance compares with the hills’ distance. Stand two metres away and hills at sixty metres are thirty times further off than she is, so they look tiny. Walk back to forty metres and they are only two and a half times further off, so they loom.
Perspective is decided by where you stand. Full stop. The lens only decides how much of the view from there you keep. So the order that works is: pick your distance first, for the look you want between subject and background, then pick the lens that frames it from there.
What 24mm does to a face
Everyone has seen the wide-angle portrait: giant nose, domed forehead, ears gone missing, the whole face pushed forward like a fish in a bowl. The lens gets the blame, and the phrase wide-angle distortion turns up 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 stand about half a metre away. At half a metre the nose really is much closer to the camera than the ears are, so it really is much bigger. Step back to five metres with a 200mm and the nose is barely closer than the ears, so the face flattens out. Take the 24mm shot from five metres and crop it to match: the face looks perfectly normal, just soft. Same distance, same face.
That is why 85mm to 135mm is the classic portrait range. Not because those lenses are flattering, but because they let you stand two or three metres back, which is where a face looks like itself, and still fill the frame with a head. You pick the lens to get the distance.
The one stretch that is real
There is a stretch that genuinely does belong to wide lenses, and it lives in the corners. Most lenses are built to keep straight lines straight, which is what you want in a photo of a building. The price is that things away from the middle get pulled outwards. In the centre of the frame, nothing happens. In the corner of a 20mm frame, a round thing turns into an oval pointing at the corner.
It matters in one everyday case: the group photo shot wide, where the people on the ends come out broader than the people in the middle. The fix is not a different lens. 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
The focal length belongs to the lens, but the angle you actually get depends on the size of the sensor behind it. The lens throws a circle of light, and the sensor keeps a rectangle out of the middle of it. A smaller sensor keeps a smaller rectangle, which frames like a longer lens.
The multiplier is 1.5 on most APS-C cameras, 1.6 on Canon, and 2 on 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 genuine bonus and not marketing. Every focal length in this article is the full-frame one, so divide by your camera’s number to find the lens to look for.
Notice what does not change. A smaller sensor does not make a face look any different, because it only changes the framing, and framing is not perspective. Same rule as everywhere 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 ended up 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, jump by about double. A 35 and an 85 are two different ways of seeing. A 35 and a 50 are one way of seeing, bought twice.
Zoom or prime, honestly
A zoom covers a range of focal lengths. A prime covers one. The point of the zoom is obvious. The point of the prime is not that it is sharper, which stopped being reliably true years ago. It is 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 an f/2.8 zoom costs and still lets in more light, which matters in exactly the situations the ISO piece is about. If you have the kit zoom and want the cheapest upgrade in photography, it is a 50mm f/1.8.
Choosing it out in the field
In practice there is an order to this, and it is not the one most people use. Do not stand still and zoom until the framing looks right. Start with your feet.
- Decide what the photo is about, and how it should sit against the background. That is a question about distance, not about lenses.
- Walk to that distance. Closer for a background that falls away, further back for one that piles up behind your subject.
- Then pick 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, for the same reason. A bird will not let you walk up to it and a mountain cannot be walked up to, so the distance is fixed for you and the lens is the only thing left to change. 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 you keep.
- 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 your 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 back.
- On a smaller 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.