Zoom Lens: Versatility in a Single Lens

A zoom lens is a lens whose focal length you can change by turning a ring, so one lens covers a range of views, for example from 24mm to 70mm. It matters because it lets you choose how much of the scene fills the frame without changing lenses, and because the way it does that, and what it cannot do, shapes almost every photo you take with one.

A lens with a single focal length is a prime lens. A zoom is many focal lengths in one barrel. This page is about how that works, how to read the numbers on the lens, what changes as you turn the ring and what stays the same, and how to get sharp, well-framed photos at both ends of the range.

What turning the zoom ring changes, and what it does not

Turning the zoom ring changes the angle of view. At the short end (24mm on a full frame camera) the lens takes in a horizontal angle of about 74 degrees. At 70mm that narrows to about 29 degrees, and at 200mm to about 10 degrees. Everything that stays in the frame is magnified to fill it.

The clearest way to picture this: if you stand still, zooming in gives you the same picture as cropping the middle out of the wide frame. The diagram shows what a lens keeps of its 24mm view at each longer setting, all from one spot. The width kept is simply 24 divided by the new focal length, so 200mm keeps 12% of the width, which is 1.44% of the area.

What zooming does not change is perspective: how big near things look compared with far things, and how they line up. That depends only on where the camera stands. If a tree is twice as far away as a person, the tree looks the same size relative to the person at 24mm and at 200mm; the 200mm frame just shows less around them. The “flattened” look of long-lens photos comes from standing far back, which the long focal length makes possible. The lens compression page has a tool where you can test it: keep a person the same size and watch the background grow as you step back with a longer lens, then stand still and see that the longer lens only crops.

So a zoom gives you two separate controls. Your feet decide perspective. The zoom ring decides framing. Choose where to stand first, for the relationship you want between foreground and background, then turn the ring until the frame edges fall where you want them.

Try the two controls one at a time. Stand still and change the focal length, and the picture only crops; then keep the person the same size by stepping back with a longer lens, and watch the mountains grow behind them.

Want to see how one subject changes as you walk back and zoom in to keep it the same size? Lens compression shows it with real photos and a crop test.

Reading the numbers on a zoom lens

A zoom lens is named by two pairs of numbers, such as 24-70mm f/2.8 or 18-55mm f/3.5-5.6.

• The focal length range (24-70mm) is the shortest and longest setting. It tells you how wide and how long the lens goes.
• The maximum aperture is the widest the aperture opens. One number (f/2.8) means a constant aperture zoom: f/2.8 is available at every focal length. Two numbers (f/3.5-5.6) mean a variable aperture zoom: f/3.5 at the short end, f/5.6 at the long end, in between on the way. You can still stop down to f/8 or f/11 anywhere in the range; the numbers only set the widest opening.
• The zoom ratio is the long end divided by the short end. A 24-70mm is a 2.9x zoom (70 ÷ 24), a 70-200mm is also 2.9x, and a 28-300mm is 10.7x. The ratio says how much the view can change, not how long the lens is: a 16-35mm is a 2.2x zoom and a wide angle lens all the way through.

The table puts common ranges side by side. Angles are horizontal, worked out as 2 × arctan(half the sensor width ÷ focal length), using 36mm wide for full frame and 24mm wide for an APS-C sensor with a 1.5x crop factor.

Two things stand out. The same numbers on different sensors give different views, so an 18-55mm on APS-C frames much like a 27-83mm on full frame. And the zoom ratio and the angle tell you more about what a lens is for than the word “zoom” does. Zooms with a ratio of around 10x or more are often called superzooms: one lens from wide to long, at the cost of a slower maximum aperture and, usually, softer results at the ends.

Using a camera with a smaller sensor and wondering what your zoom really frames like? The crop factor calculator converts any focal length and shows it on each sensor.

Inside the barrel: groups of glass that slide

A zoom lens is built from several groups of lens elements. Turning the zoom ring drives some of those groups along curved slots, called cams, so the spacing between them changes. One moving group changes the magnification. Another, the compensator, moves along with it to keep the image in focus on the sensor. A separate focusing group handles distance. The lens designer has to keep all of that sharp and well corrected at every focal length and every distance, which is why a zoom usually has many more elements than a prime.

From the outside you will meet three kinds of zoom:

• Extending zooms get longer as you zoom, usually toward the long end. Most standard and kit zooms work this way.
• Internal zooms stay the same length; all the movement happens inside. Many 70-200mm lenses are built like this, which keeps the balance of the camera steady as you zoom.
• Push-pull zooms change focal length when you slide the front of the barrel in or out instead of turning a ring. A few long telephoto zooms still use this.

On an extending zoom, gravity can slide the barrel out when the camera points down. That is zoom creep, and many lenses have a lock switch for carrying.

Why the maximum aperture shrinks as you zoom in

An f-number is a ratio: focal length divided by the diameter of the opening as seen from the front of the lens (the entrance pupil). To give the same f-number at a longer focal length, the opening has to be wider in proportion.

Take an 18-55mm f/3.5-5.6. At 18mm, f/3.5 needs an opening 18 ÷ 3.5 = 5.1 mm across. At 55mm, f/5.6 needs 55 ÷ 5.6 = 9.8 mm. Holding f/3.5 at 55mm would need 55 ÷ 3.5 = 15.7 mm, three times the width of the wide end and a much bigger and heavier lens. Letting the maximum aperture slide to f/5.6 keeps the lens small. The cost is light: f/3.5 to f/5.6 is about 1⅓ stops, so the long end gets about 2.6 times less light wide open.

The same arithmetic explains why constant aperture telephoto zooms are big. A 70-200mm f/2.8 needs an opening of 200 ÷ 2.8 = 71 mm at its long end, so its front glass has to be at least that wide.

What you see depends on your exposure mode. In aperture priority or with Auto ISO, the camera quietly lengthens the shutter speed or raises the ISO as you zoom in wide open. In full manual with a fixed ISO, the picture simply gets darker as you zoom in, by up to the 1⅓ stops above. If you set f/5.6 or smaller, the aperture stays put across the whole range and nothing changes.

Shooting a variable aperture zoom in low light and wondering how to keep exposure steady? Variable aperture lenses explains what f/3.5-5.6 means for exposure, flash and video.

Zooming can move your focus and your framing

A lens that holds focus exactly while you zoom is called parfocal. Lenses built for film and video crews are made that way. Most zooms made for still photography are not guaranteed to be: focus can shift slightly as you zoom, too little to see at the wide end and enough to soften the subject at the long end. With autofocus this rarely matters, because the camera refocuses when you half-press the shutter. It matters when you focus manually, or focus once and then recompose by zooming.

The fix is a habit: zoom first, then focus. The old trick of zooming all the way in to focus precisely, then zooming out to frame, only works if your lens is parfocal. The “Try this” section below shows how to check yours.

Many lenses also change their angle of view slightly as you focus, an effect called focus breathing. In stills you will mostly notice it on telephoto zooms: many set to 200mm frame a little wider at close focus than they do at long distance. In video it shows as the frame pulsing during a focus change.

How a zoom behaves across its range

A zoom is not equally good everywhere in its range, and the weak spots are predictable:

• Sharpness is often best somewhere in the middle of the range and a stop or two down from wide open, and weakest in the corners at the extreme ends. Every lens is different, so test your own; the sweet spot page shows how.
• Distortion often changes character across the range. Many standard zooms bow straight lines outward at the wide end (barrel distortion) and pinch them inward at the long end (pincushion), with a setting in between where lines are close to straight. See lens distortion for how to spot each one.
• Vignetting, darker corners, is usually strongest wide open and at the extremes, and fades as you stop down.

Most of the distortion and vignetting, and the color fringes of chromatic aberration, are corrected by lens profiles in the camera or editor. Lens corrections covers what they fix and what they leave alone.

The other thing that changes is your steadiness. The longer the focal length, the more any wobble of your hands is magnified. The rule of thumb for handheld shooting is a shutter speed of at least 1 over the full frame equivalent focal length: 1/25 second at 24mm, but 1/200 at 200mm, and 1/300 at 200mm on a 1.5x crop body. A shutter speed that was safe at the wide end can blur the same scene at the long end. The hand-holding rule has the full chart, and image stabilization explains how much slower you can go with it.

Try it at the long end in the camera simulator: zoom to 200mm or longer, hold the camera by hand with stabilization off, and find a shutter speed fast enough for a sharp frame.

Getting soft photos at the long end of your zoom? Camera shake shows how to tell shake from missed focus and how to fix each.

Zoom on a phone, and zoom versus prime

Most phones with a “2x”, “3x” or “5x” button do not have a zoom lens. They have two or more separate fixed lenses, and the steps between them are digital: the phone crops and enlarges the image from the nearer camera. A true zoom lens changes the view optically and uses every pixel of the sensor at every focal length; a crop throws pixels away. Cropping a 24-megapixel frame at 24mm down to what 200mm would frame leaves 1.44% of it, about 0.35 megapixels. Optical vs digital zoom goes through the difference in detail.

Against a prime lens, a zoom set to 50mm frames exactly like a 50mm prime from the same spot. The differences are elsewhere: a prime usually opens wider (f/1.8 against f/2.8 or f/5.6), and is smaller and simpler. A zoom saves lens changes and lets you frame precisely without moving, which counts most when you cannot move or cannot predict what comes next.

Deciding whether to carry a zoom, a prime, or both? Prime vs zoom lens weighs aperture, sharpness, size and convenience.

Common mistakes with zoom lenses

• Zooming instead of moving. Standing in one spot and zooming in to get closer gives you a crop of the same perspective. If the background is cluttered or the subject looks flat, the fix is a different position, not a different focal length.
• Thinking zooming in “compresses” the background. Distance does that. Zoom in from where you stand and the background stays exactly as big relative to the subject. Back away and zoom in to keep the subject the same size, and the background grows.
• Focusing, then zooming. On most still photography zooms focus shifts as you zoom. Zoom to your framing, then focus.
• Keeping the wide end’s shutter speed at the long end. 1/30 second may be fine at 24mm and blurry at 200mm. Check the shutter speed every time you zoom in a long way.
• Shooting manual exposure with a variable aperture zoom wide open. Zooming in darkens the photo by up to 1⅓ stops. Either set an aperture of f/5.6 or smaller, or let Auto ISO absorb it.
• Living at the two ends. It is easy to twist the ring until it stops. The ends are where many zooms are weakest, and the framing you wanted is often in between.
• Treating zoom ratio as reach. “10x” says the long end is ten times the short end. A 10x lens that starts at 18mm reaches 180mm; a 4x lens that starts at 100mm reaches 400mm.

Want to check you can read a zoom’s numbers and tell it from a prime? Try the Camera Lens Quiz: Focal Length, Prime vs Zoom.

Try this: one spot, one walk, one focus test (10 minutes)

You need a zoom lens (or a phone, for the first part) and a scene with something near, such as a post, bench or sign, and something far behind it, such as a building or trees.

1. Stand still. Put your toe on a mark about 10 steps from the near object. Shoot at the shortest focal length, then at the longest, without moving.
2. Walk instead. Set the lens to its shortest focal length and walk toward the near object until it is the same size in the frame as it was in your long end shot. Take the photo.
3. Compare. On a computer, crop the wide shot from step 1 down to match the long shot. The framing should line up almost exactly, with less detail in the crop. Now compare the walked shot with the long shot: the near object is the same size, but the background is far smaller and you see much more of it. That is perspective, and only walking changed it.
4. Test focus. On a tripod or steady surface, switch to manual focus. Aim at fine detail, such as printed text a few meters away. At the shortest focal length, magnify the live view and focus carefully. Without touching the focus ring, zoom all the way in and take a shot. Then focus again at the long end and take a second shot. Magnify both on the screen. If the first is as sharp as the second, your lens holds focus while zooming; if it is softer, it does not, and “zoom first, then focus” is your rule.

Judge it this way: you got it right if the cropped wide frame and the long frame match, the walked frame looks clearly different, and you know whether your lens holds focus.

Want to use zooming as an effect, not just for framing? How to tell a story with a zoom lens includes zooming during a slow exposure.

Zoom lens questions

Is a zoom lens the same as a telephoto lens?

No. “Zoom” means the focal length can change; “telephoto” means the focal length is long. A 16-35mm is a wide angle zoom, and a 300mm prime is a telephoto that does not zoom. A 70-200mm happens to be both. Telephoto photography covers using long lenses, zoom or prime.

What does “3x zoom” mean?

It is the ratio of the longest focal length to the shortest. An 18-55mm is about 3x, and so is a 70-200mm, though one is a general lens and the other a telephoto. The ratio tells you how much the view changes, not how far the lens reaches.

Does a zoom at 50mm give the same picture as a 50mm prime?

From the same spot on the same camera, the framing and perspective are the same. The prime will usually open to a wider aperture, which gives more light and a shallower depth of field, and the two lenses may differ a little in sharpness and how they render out-of-focus areas.

Why does my photo get darker when I zoom in?

Your lens has a variable maximum aperture, you are shooting wide open, and your exposure is set manually. As you zoom, the widest aperture narrows, for example from f/3.5 to f/5.6, and less light reaches the sensor. Raise the ISO or slow the shutter, or set an aperture the lens can hold across the range.

Is zooming in the same as cropping later?

For framing and perspective, yes: from one spot the result covers the same part of the scene. For detail, no. An optical zoom spreads the narrower view over the whole sensor, while a crop keeps only the pixels in the middle; matching 200mm from a 24mm frame keeps 1.44% of them.

Does zooming in change depth of field?

From the same spot at the same aperture, yes: zooming in magnifies the subject, and depth of field gets shallower. If you instead step back to keep the subject the same size in the frame, depth of field at the same f-number stays roughly the same at short and long focal lengths; what changes is how much background you see and how blurred it looks.