A camera lens is a set of glass elements in a barrel that gathers light from the scene and brings it to a sharp image on the sensor or film. The lens decides how much of the world fits in the frame, how much light reaches the sensor and how much of the picture is sharp, so after where you stand, it is the biggest single choice behind how a photograph looks.
Lenses are sorted mainly by focal length: wide-angle lenses take in a broad view, normal lenses show about what your eye attends to, and telephoto lenses magnify distant subjects. Each is either a prime (one focal length) or a zoom (a range). A few specialty lenses, such as macro, fisheye and tilt-shift, do jobs the others cannot. The one idea to carry through all of it: the lens decides how much of the scene you see, and where you stand decides how near and far things relate.
How a Lens Makes a Picture
Every point in a scene sends light out in all directions. Hold a bare sensor up to the world and each point lights the whole sensor, so you get an even grey wash with no picture at all. Put a tiny hole in front and only a thin ray from each point gets through, so a dim, soft image appears. That is a pinhole camera, and it works, but the hole lets in so little light that exposures run to seconds or minutes.
A lens solves both problems at once. It collects a wide cone of light from each point in the scene and bends it so the cone comes back together at a single point behind the lens. A big opening gathers a lot of light, and the bending keeps the image sharp. Points at one distance come back together exactly on the sensor; points nearer or farther come together slightly in front of or behind it and land as small blurred patches. That is why a photograph has a plane of focus, and why depth of field exists at all.
Focusing means moving the lens, or a group of elements inside it, to change where those cones meet. For a simple 50mm lens focused at infinity, the image forms 50mm behind it. Focus on something 1 meter away and the image forms about 52.6mm behind it, so the lens has to move out about 2.6mm. To show a subject at life size the lens must sit twice its focal length from the sensor, 100mm for a 50mm lens, which is why lenses built for extreme close-ups have long focusing travel.
Reading the Numbers on a Lens
Every lens carries two key numbers on the barrel or front ring: the focal length in millimeters and the maximum aperture as a ratio. “1:1.8” means the widest aperture is f/1.8.
- “50mm 1:1.8” is a prime: one focal length, widest aperture f/1.8.
- “24-105mm 1:4” is a zoom from 24mm to 105mm that opens to f/4 at every focal length. This is called a constant-aperture zoom.
- “18-55mm 1:3.5-5.6” is a variable-aperture zoom. Its widest aperture is f/3.5 at 18mm and shrinks to f/5.6 as you zoom to 55mm.
Focal length is not the length of the lens. It is a measure of magnification: the distance at which a single simple lens, focused at infinity, would form an image of the same size. Real lenses are built to cheat that distance. A long lens is usually designed to be physically shorter than its focal length, and a wide-angle lens for a camera with a mirror has to sit farther from the sensor than its focal length to clear the mirror. So a 14mm lens can be ten times longer than 14mm, and a 300mm lens may be well under 300mm long. The number tells you how big a subject will appear, nothing about the barrel.
Maximum aperture is how wide the lens can open. A smaller f-number lets in more light, which allows faster shutter speeds in dim light and gives shallower depth of field. Lenses that open to f/2.8 or wider are often called fast lenses. Each full stop is a factor of about 1.4 in f-number (f/2.8, f/4, f/5.6), and each stop halves the light. The aperture guide explains f-numbers in full.
That makes the cost of a variable-aperture zoom easy to work out. Going from f/3.5 to f/5.6 is a factor of 1.6 in f-number, which is about 1 and 1/3 stops: at the long end you get roughly 40 percent of the light you had at the wide end, so in a dim room the camera must use a slower shutter or a higher ISO the moment you zoom in. A zoom marked f/4-5.6 loses one stop across its range.
Other markings include the lens mount (which cameras it fits), whether it has image stabilization, the minimum focusing distance, sometimes the maximum magnification (such as 1:1 or 0.25x), and the filter thread diameter, usually printed beside a circle with a line through it.
Decode your own lens
Three numbers turn a lens marking into what it will do on your camera: the focal range, the aperture range and your sensor’s crop factor (1 for full frame, about 1.5 or 1.6 for APS-C, 2 for Micro Four Thirds). Multiply focal length by crop factor to get the framing in full-frame terms. For a rough hand-held shutter speed without stabilization, use 1 divided by that equivalent focal length, rounded up to the next faster speed.
| Marking and camera | Frames like (full frame) | Light lost at long end | Hand-held guide at long end |
|---|---|---|---|
| 18-55mm 1:3.5-5.6 on APS-C (1.5x) | 27 to 83mm | about 1 1/3 stops | 1/100 second |
| 50mm 1:1.8 on Micro Four Thirds (2x) | 100mm | none (prime) | 1/100 second |
| 24-105mm 1:4 on full frame | 24 to 105mm | none (constant f/4) | 1/125 second |
| 70-300mm 1:4.5-5.6 on APS-C (1.5x) | 105 to 450mm | about 2/3 stop | 1/500 second |
The shutter guide is a starting point. Sensors with many megapixels show small shake that older ones hid, so many photographers double it, and stabilization can buy back several stops for a still subject. It does nothing for a subject that moves. The hand-holding rule page has the details, and the crop factor calculator does the multiplication for any sensor.
What Focal Length Changes, and What It Does Not
Focal length sets the angle of view: how wide a slice of the world lands on the sensor. The table gives figures for a full-frame sensor (36 x 24mm, diagonal 43.3mm) and ordinary rectilinear lenses, the kind that keep straight lines straight. The last column is how wide a strip of scene fills the frame from side to side when the subject is 10 meters away.
| Focal length | Diagonal angle | Horizontal angle | Width covered at 10 m | Usual name |
|---|---|---|---|---|
| 14mm | 114 degrees | 104 degrees | 25.7 m | Ultra-wide |
| 24mm | 84 degrees | 74 degrees | 15.0 m | Wide |
| 35mm | 63 degrees | 54 degrees | 10.3 m | Moderate wide |
| 50mm | 47 degrees | 40 degrees | 7.2 m | Normal |
| 85mm | 29 degrees | 24 degrees | 4.2 m | Short telephoto |
| 135mm | 18 degrees | 15 degrees | 2.7 m | Medium telephoto |
| 200mm | 12 degrees | 10 degrees | 1.8 m | Telephoto |
| 400mm | 6 degrees | 5 degrees | 0.9 m | Super telephoto |
Look at the width column. Double the focal length and the width covered halves exactly: 7.2 m at 50mm, 3.6 m at 100mm, 1.8 m at 200mm. The angle only roughly halves, because angles do not scale in a straight line: 14mm to 28mm takes you from 114 degrees to 75, not to 57. If you want to predict a frame, think in widths, not degrees.
The same arithmetic tells you how big a distant object will be. The sun and the full moon are each about half a degree across, so their image on the sensor is roughly the focal length divided by 108: about 0.2mm at 24mm, 1.9mm at 200mm, and 3.7mm at 400mm. On a 36mm-wide frame, even a 400mm lens makes the moon only about a tenth of the frame width. That is why big-moon pictures need very long lenses.
What focal length does not change is perspective. Perspective, meaning how big near things look against far things, depends only on where the camera stands. Stand in one spot and swap a 24mm lens for a 100mm lens, and the 100mm frame is exactly the middle of the 24mm frame, enlarged. Nothing inside it moves relative to anything else. You could crop the 24mm picture to its middle quarter of the width and get the same composition, only with far fewer pixels: a 24 megapixel frame cropped that way leaves about 1.4 megapixels.
The flattened look of long-lens pictures, where a mountain seems to loom right behind a person, is real. Its cause is the distance you stood back in order to fill the frame, not the glass. The same goes for the big noses of close wide-angle portraits: the camera was too close to the face. This is what lens compression really means, and that page has an interactive tool that lets you move the camera and change the focal length separately so you can watch which one changes the background.
The tool is here too. Choose Stand still and change the focal length, then choose Keep the person the same size and do it again, and watch which of the two changes how big the mountains look behind the person.
Wondering how a phone’s pinch-to-zoom fits in? Optical vs digital zoom shows why switching to a longer lens adds detail and cropping only enlarges what you already had.
Test your eye: one of these two photos was made with a much longer focal length. Pick it, then read why.
Telephoto focal lengths compress space so distant elements appear closer to the subject. Wide lenses exaggerate depth and stretch foregrounds.
The Same Lens on Different Sensors
A lens projects a circle of light called the image circle, and the sensor records a rectangle from its middle. A smaller sensor records a smaller rectangle, so the same lens shows a narrower view. A 35mm lens on an APS-C camera with a 1.5x crop factor frames like a 52mm lens on full frame, and like a 56mm on a 1.6x body.
It is still a 35mm lens. Its magnification, its perspective from a given spot and its aperture are unchanged; the smaller sensor just sees less of the picture. That matters for two reasons. First, focal length numbers only tell you the view once you know the sensor, which is why “full-frame equivalent” is the common language. Second, lenses made only for small sensors draw a smaller image circle. Put one on a full-frame body (diagonal 43.3mm) and the corners outside its circle, which was made for an APS-C diagonal of about 28mm, go dark, a strong form of vignetting.
See it for yourself below: keep one lens and switch between sensors to see how much of its picture each one records, and the equivalent focal length that gives.
Not sure what sensor your camera has? Full frame vs crop sensor compares the formats, and the sensor size page has an interactive tool for comparing them.
Wide-Angle, Normal and Telephoto Lenses
Wide-angle lenses
A wide-angle lens has a focal length shorter than normal, roughly 35mm and below on full frame, with anything under about 24mm called ultra-wide. It takes in a big slice of the scene, which makes it the natural choice for landscapes, architecture, interiors and any place where you cannot step back.
The wide view tempts you to stand close to something, and that is where the wide-angle look comes from. A rock a meter from the lens looms large while mountains behind it shrink, which gives a landscape strong depth. The same closeness makes a nose look big in a portrait. Tilt a wide lens up and vertical lines lean inward; objects near the frame edges look stretched. These are perspective distortion and the normal geometry of a rectilinear wide lens, not faults in the glass. Lines that bow outward are a different thing, barrel distortion, covered under lens distortion. At the same distance and aperture, a wide lens also keeps more of the scene acceptably sharp, so focus is more forgiving.
Normal lenses
A normal lens has a focal length close to the diagonal of the sensor, about 43mm on full frame. By convention 50mm is called normal, and anything from about 40mm to 60mm behaves alike. When you view a print at an ordinary distance, a normal lens gives perspective that looks natural, neither stretched nor flattened, which suits street scenes, travel, food and half-length portraits. Normal primes are usually small, sharp and fast. The 50mm lens guide covers this focal length in depth.
Telephoto lenses
A telephoto lens has a focal length longer than normal. Short telephotos, about 70mm to 135mm, are the classic portrait range: you stand back far enough that faces keep natural proportions, and a wide aperture blurs the background. From about 200mm up, long lenses bring distant subjects close for sports, wildlife and events where you cannot get near.
Long lenses do two other things worth knowing. Their narrow angle takes in only a small patch of background, so you can clean up a busy scene by moving a step sideways. And they magnify every tremor of your hands as much as they magnify the subject, so they need faster shutter speeds, stabilization or a support; camera shake explains why. Strictly, “telephoto” also names an optical design that makes a lens shorter than its focal length, which is how a 200mm lens fits in a bag. The telephoto photography guide covers technique.
Need more reach than your longest lens? Teleconverters explained covers what a 1.4x or 2x converter adds and what it costs in light.
Prime Lenses and Zoom Lenses
A prime lens has one fixed focal length, such as 24mm, 35mm, 50mm or 85mm. To change the framing you move your feet. In return, primes are usually smaller and lighter than zooms covering the same focal length, and they open wider: f/1.4 or f/1.8 is common, against f/2.8 to f/5.6 for most zooms. That extra light helps indoors and at night, and it gives the shallow depth of field and soft bokeh many portrait photographers want.
A zoom lens covers a range in one barrel: wide zooms (about 16-35mm), standard zooms (about 24-70mm, or 18-55mm on APS-C), telephoto zooms (about 70-200mm) and long all-in-one ranges, sometimes called superzooms. Zooms let you reframe without moving or changing lenses, which matters when the moment will not wait or when dust and rain make lens changes risky. The costs are size, weight and a smaller maximum aperture, and a very long range usually means more compromise in sharpness at the ends. The kit lens sold with many cameras is a compact standard zoom. A good zoom can match or beat an average prime, so judge the lens, not the category; prime vs zoom lens compares them side by side.
Specialty Lenses
Macro lenses are defined by how close they focus, not by their focal length. A true macro lens can project a small subject at life size onto the sensor, a magnification ratio of 1:1, so a 10mm insect covers 10mm of sensor. Most ordinary lenses stop at about a quarter of life size or less. Macro lenses come in short (about 50 to 60mm), medium (90 to 105mm) and long (150mm and up) versions; the longer ones give more working distance from skittish subjects. See macro photography for technique, or macro without a macro lens for cheaper ways in.
Fisheye lenses are ultra-wide lenses that do not try to keep lines straight. They cover up to 180 degrees or more and bend every straight line that does not pass through the center of the frame. The fisheye lens page explains the circular and full-frame kinds.
Tilt-shift lenses can move the optics relative to the sensor. Shifting keeps the verticals of a building parallel without tilting the camera; tilting swings the plane of focus so it can run along a tabletop or a road instead of facing the camera. See tilt-shift lens.
The Lenses in Your Phone
A phone with several cameras has several small fixed lenses, each with its own sensor, not one zoom. The “1x” camera is usually around 24 to 26mm in full-frame terms, so “0.5x” is an ultra-wide around 13mm, “3x” is a short telephoto around 75mm and “5x” is around 120mm. Exact values vary by model. The steps in between, such as 2x on a phone without a 2x lens, are crops from the nearest camera, sometimes sharpened with processing, and they hold less real detail than a frame from a dedicated lens.
Everything else on this page applies to phones too. Perspective still comes from where you stand, which is why a close selfie at 0.5x makes a face look wide and a portrait taken a few steps back at 3x looks more natural. The phone camera settings page covers the rest.
Which Lens for Which Kind of Photography
Focal lengths below are full-frame equivalents. They are starting points; once you know what each focal length does, choosing against the habit becomes a decision instead of an accident.
- Landscapes: 16mm to 35mm for broad scenes and strong foregrounds; 70mm to 200mm to pick out distant ridges and layers. See landscape photography.
- Portraits: 50mm to 135mm, most often an 85mm prime or a 70-200mm zoom, which lets you stand where faces look natural. A 35mm lens suits environmental portraits that show the setting. See portrait photography.
- Street and documentary: 28mm to 50mm primes, small enough to be discreet and wide enough to show context. See street photography.
- Architecture and interiors: wide and ultra-wide lenses, plus a tilt-shift when straight verticals matter.
- Wildlife and birds: 300mm and beyond, because animals will not let you close. See wildlife photography.
- Sports and events: telephoto zooms such as 70-200mm, longer for field sports, with a wide aperture for fast shutter speeds.
- Travel and everyday: a standard zoom of about 24-70mm covers most situations; a small fast prime adds low-light ability.
- Close-ups: a macro lens, usually 60mm to 105mm.
- Night sky: a fast wide prime, about 14mm to 24mm at f/2.8 or wider.
Want to see one zoom used to tell a story? How to tell a story using a zoom lens works through a single scene at several focal lengths.
What Is Inside a Lens
A single piece of glass cannot bring every color and every part of the frame to a sharp point at once. Blue light bends more than red, so a simple lens focuses colors at slightly different distances, which shows as colored fringes called chromatic aberration. Light through the edge of a spherical surface focuses a little short of light through the middle, which softens the image. A modern lens fights these flaws with anywhere from a handful to more than twenty pieces of glass, each a lens element, arranged in groups. Some elements are made of special low-dispersion glass for the color problem; others are ground to non-spherical shapes, the aspherical lens, so fewer elements can do the same correction.
Inside the barrel also sit the aperture blades, a focusing motor that moves one group of elements, and in many lenses a floating group that shifts to cancel shake. Many lenses also change their angle of view slightly as they focus, which is called focus breathing; it shows most in video when focus moves from far to near. The history of lenses traces how designers got from a single curved piece of glass to all of this.
Common Mistakes and Misconceptions About Lenses
- “A telephoto compresses and a wide-angle distorts.” Distance does both. If a face looks odd, step back and use a longer focal length; do not stay close and zoom out.
- “The mm number is how long the lens is.” It is a measure of magnification. Long lenses are often shorter than their focal length and wide lenses longer.
- “A crop sensor turns my 50mm into an 80mm.” It changes the framing to match an 80mm on full frame. The lens, its perspective and its aperture stay the same.
- “Primes are always sharper.” Usually brighter and smaller, often sharper, but a good zoom beats a poor prime. Judge real pictures, not the category.
- “A bigger zoom number means a better lens.” A “10x” or “30x” ratio is only the long end divided by the short end. It says nothing about image quality.
- “Macro just means a long lens.” Macro means close focus and high magnification. A 300mm lens that focuses no nearer than 2 meters is not a macro lens.
- Forgetting the light lost when zooming in. A variable-aperture zoom gives up 2/3 to 1 1/3 stops at the long end, and in dim light that shows up as blur or noise.
- Handholding long lenses at slow speeds. Shake grows with focal length. Use a faster shutter, turn on stabilization or brace the camera.
- Shooting wide for everything. A wide lens includes more clutter. If the subject looks small and lost, move closer or go longer.
Try This: Move Your Feet, Then Zoom
This takes about ten minutes with any camera or phone that has two or more focal lengths. Pick a person or an object with a building, tree or fence some distance behind it.
- Same subject size. Photograph the subject at your widest setting, then at about 50mm, 85mm and your longest setting (on a phone, 0.5x, 1x, 2x and your longest lens). Each time, walk forward or back so the subject fills the same height in the frame.
- Same spot. Now stay in one place and take the widest and the longest frames without moving.
- Crop to match. On your computer or phone, crop the wide frame from step 2 until the subject is the same size as in the long frame.
How to judge it. In the step 1 set, the subject should stay the same size while the background grows larger and fewer things fit behind it as the focal length rises: that is the effect of walking back. In step 3, the cropped wide frame and the long frame should line up almost exactly, background included, only with the crop looking softer. If they match, you have proved to yourself that the lens chooses how much you see and your position chooses how things relate.
Ready to check what you know? The lens quiz tests focal length, aperture and lens choice.
Questions About Camera Lenses
What are the different types of camera lenses?
By focal length, lenses are wide-angle, normal or telephoto. By design, they are primes with one focal length or zooms with a range. Specialty types include macro, fisheye and tilt-shift lenses.
What does the mm on a lens mean?
It is the focal length in millimeters, a measure of how much the lens magnifies. A smaller number gives a wider view and a larger number a narrower, more magnified one. What view you get also depends on your sensor size.
Why do my phone’s 2x pictures look softer than 1x?
On many phones there is no 2x lens, so the camera crops the middle of the 1x frame and enlarges it. You are using a quarter of the sensor area, so there is less real detail. Step closer at 1x, or switch to the phone’s real telephoto lens, for a sharper result.
Is a prime lens better than a zoom?
Neither is better in general. A prime is usually smaller and opens wider; a zoom lets you reframe without moving. Choose a prime when light and size matter most, and a zoom when you need to change framing quickly.
Why are some lenses so much more expensive than others?
Mostly because of maximum aperture and correction. Opening one stop wider means a much larger front element, and keeping corners sharp and colors clean at that opening takes more and costlier elements. Build, weather sealing and focus motors add the rest.
Can I use any lens on any camera?
No. A lens must match the camera’s mount, or be fitted with an adapter made for that pair. Lenses designed for smaller sensors may also show dark corners on a full-frame camera.