Focal Length: Understanding Your Lens’s Field of View

Focal length is the number in millimeters on every lens, and it decides how much of the scene your photo takes in and how big things appear in it. A short focal length shows a wide view with everything small; a long one shows a narrow slice with everything large. It is the first thing to know about any lens, because it shapes how you frame, where you stand, how steady you must hold the camera, and how much background ends up behind your subject.

The moon above, shot at 300mm, is a clear disc where a phone's main camera would draw a speck. What focal length does not do is change perspective. That comes from where you stand, and untangling the two is most of what there is to understand about lenses.

What the Millimeter Number Measures

For a simple lens focused on something very far away, focal length is the distance from the lens to the point where it brings that light to a sharp image. Modern lenses are made of many elements, so the number is an optical property of the whole design, not a measurement of the barrel. A 14mm wide-angle lens is far longer than 14mm, and its rear element sits more than 14mm from the sensor, because its design (called retrofocus) moves the effective optical center back behind the glass. A 400mm telephoto lens is usually shorter than 400mm, because a true telephoto design moves that optical center forward, out in front of the lens. So do not read the length of the lens. Read the number printed on it.

A prime lens has one focal length. A zoom lens covers a range, such as 24 to 70mm, and turning the zoom ring changes the focal length optically. A phone's digital zoom is different: it crops and enlarges, as the optical vs digital zoom page explains.

Angle of View and Magnification, in Numbers

Focal length sets two linked things. The angle of view is how wide a cone of the world reaches the sensor. Magnification is how large a given object appears in the frame. Both follow from simple geometry: the angle is 2 × arctan(sensor size ÷ (2 × focal length)), and the size of a distant object's image is roughly focal length × object size ÷ distance.

Here are the angles a full-frame sensor (36 × 24mm) records at common focal lengths:

Two patterns are worth carrying in your head. First, magnification is linear: double the focal length and a distant subject becomes twice as tall and twice as wide in the frame, which means it fills four times the area. Going from 50mm to 100mm is the same step as going from 100mm to 200mm. Second, the angle shrinks fast at the short end and slowly at the long end. The 10mm between 14mm and 24mm changes the horizontal view by 30 degrees; the 200mm between 200mm and 400mm changes it by 5. That is why a few millimeters matter so much on a wide lens and hardly at all on a long one.

The moon makes the magnification rule easy to check, because it is always the same angular size, about half a degree. Its image on the sensor is about the focal length divided by 110. At 50mm that is 0.45mm, about 1 percent of a full-frame width. At 300mm, as in the photo at the top, it is about 2.7mm, close to 8 percent. At 600mm it is 5.5mm, about 15 percent. To make the moon fill a full-frame image from top to bottom would take about 2,600mm.

Wide, Normal and Telephoto

The names describe angle of view, so they depend on the sensor. A lens is called normal when its focal length is close to the diagonal of the sensor, which on full frame is 43mm. That gives a diagonal angle of about 53 degrees and, at a typical print or screen size and viewing distance, a picture whose perspective looks neither stretched nor squeezed. The 50mm lens became the standard because it is close to that and was easy to make well. It is not "what the eye sees"; the eye takes in far more than 47 degrees, and it sees fine detail only in a tiny central spot. The normal lens page has the history.

On full frame, roughly:

• Ultra wide, under 20mm. Huge coverage. Anything near the lens dominates the frame and the background shrinks fast. See ultra-wide lenses.
• Wide, 20 to 35mm. Rooms, landscapes, streets, groups in tight spaces. The wide-angle lens page covers using one well.
• Normal, 40 to 60mm. A view that reads as natural on a print.
• Short telephoto, 70 to 135mm. Portraits, details, isolating a subject from a moderate distance.
• Telephoto and beyond, over 135mm. Subjects you cannot or should not walk up to: wildlife, sports, distant landscape detail. See telephoto lens.

How Far Back You Have to Stand

Because image size is focal length × subject size ÷ distance, you can turn it around and work out where you will be standing for a given framing: distance ≈ focal length × subject height ÷ frame height. These figures are for full frame and are close enough to plan with.

This table is the real reason focal length matters for portraits. A head-and-shoulders portrait at 24mm puts your lens 40 centimeters from someone's nose. At 85mm you are 1.4 meters away. The next section is about what that distance does.

Perspective Comes From Where You Stand, Not From the Lens

Perspective, meaning how big near things look compared with far things, depends only on the distances from the camera to each of them. Stand in one place and zoom from 24mm to 200mm, and nothing about the relationships in the scene changes: the 200mm frame is exactly the middle of the 24mm frame, enlarged. Now walk toward your subject with a 24mm lens until it fills the frame, and the background shrinks away behind it. Walk back with a 200mm lens until the subject is the same size, and the background looms up behind it. The lens told you where to stand. Standing there changed the picture.

That is why wide lenses are said to "stretch" space and long lenses to "compress" it. Both are effects of distance. A wide lens lets you work close, and close is where near objects get huge relative to far ones. A long lens lets you fill the frame from far away, and from far away near and far objects are at nearly the same distance, so they render at nearly the same scale. The lens compression page goes further, and this tool lets you try both moves: keep the person the same size and watch the mountains grow, then stand still and watch a longer lens do nothing but crop.

If the same idea shows up as converging buildings or a stretched face in your photos, perspective distortion treats it as its own subject.

Why faces change with focal length

Take the head-and-shoulders distances from the table and assume the tip of the nose is about 10 centimeters closer to the camera than the ears. At 0.4m (a 24mm headshot) the nose is at 40cm and the ears at 50cm, so the nose renders about 25 percent larger relative to the ears than it really is. At 1.4m (85mm) the difference is about 7 percent. At 2.3m (135mm) it is about 4 percent. Nothing in the 24mm glass made the nose bigger. Being 40 centimeters away did, and if you stood 1.4m back with the 24mm lens and cropped, the face would have the same proportions as the 85mm shot, just with far fewer pixels.

Viewing distance decides whether a photo looks natural

A photo shows the scene in its true proportions when you look at it from the same angle the lens saw. On full frame, that distance is the focal length multiplied by how many times the 36mm sensor width was enlarged. A 36cm wide print is ten times the sensor, so a 50mm shot looks right at 50cm, about where people hold prints. A 14mm shot printed at that size would need to be viewed from 14cm, which nobody does, so we see its near objects as stretched. A 200mm shot looks right from 2 meters, and at arm's length its background seems unnaturally close. The lens is not at fault; your viewpoint does not match the camera's.

Crop Factor and Equivalent Focal Length

A lens's focal length does not change when you put it on a different camera, but the angle of view does, because a smaller sensor records only the middle of the image the lens projects. Crop factor is how many times smaller the sensor's diagonal is than full frame's, and multiplying by it gives the full-frame focal length with the same angle of view, called the equivalent focal length.

The photo above was shot at 12mm on an APS-C camera. Multiply by 1.5 and it frames like 18mm on full frame, about 90 degrees across. The aerial photo in the comparison further down was taken at 10.3mm on a 1-inch sensor, which frames like 28mm. Neither number is wrong; the real focal length describes the lens, and the equivalent describes the picture.

Crop factor changes framing only. It does not add "reach" to the lens in an optical sense: a 200mm lens on an APS-C body shows the same thing you would get by cropping the middle out of a full-frame 200mm shot. It can still be useful in practice when the crop-sensor camera puts more pixels on that middle area than the cropped full-frame file would have. The crop factor calculator converts any lens to any sensor, and sensor size explains what else changes between formats.

Put one lens on five sensors below and watch how much of the same picture each one records, and what focal length on full frame would frame it the same way.

Phones print equivalent focal lengths in their camera apps, and the real numbers are tiny. A typical main camera has a real focal length of only a few millimeters, usually well under 10mm, on a small sensor, which frames like about 24 to 26mm on full frame. The "0.5x", "2x" and "5x" buttons are relative to that main camera. Some of them switch to another lens with a different focal length; some crop the main camera's image. Your phone's settings or specifications tell you which, and phone camera settings covers working with them.

Comparing two cameras with different sensors? Full frame vs crop sensor sets out what each size gives and takes away.

Three Kinds of "Distortion" That Get Blamed on Focal Length

Learners use one word for three different things, and they have three different fixes.

1. Barrel and pincushion distortion. This is a property of a particular lens design. Straight lines near the frame edges bow outward (barrel, common in wide lenses and the wide end of zooms) or pinch inward (pincushion, common at the long end of zooms). It is the same in every shot from that lens at that setting, so software lens profiles correct it well. See barrel and pincushion distortion.
2. Edge stretching on wide lenses. A normal wide-angle lens keeps straight lines straight, and the price is that objects near the corners are drawn stretched outward from the center. At 14mm on full frame something round in the extreme corner is drawn about 1.8 times as long as it is wide; at 24mm about 1.35 times; at 50mm about 1.09 times, too little to notice. This is geometry, not a defect, and correcting barrel distortion does not remove it. Keep faces out of the corners of wide shots.
3. Perspective effects from distance. The big nose, the tiny mountains, the buildings that lean back when you tilt the camera up. These come from where the camera is and which way it points, and they look the same through any lens from that spot. Step back, or keep the camera level and crop.

Fixing one of these in an editor? Lens distortion walks through which tool corrects which kind.

Focal Length and a Steady Shot

A long lens magnifies your shaking along with the subject. A tremor that moves the frame by a tiny angle shifts a 400mm image eight times as many pixels as a 50mm image. The traditional guide is to keep the shutter speed at least as fast as 1 ÷ the equivalent focal length when handheld: 1/50 second at 50mm, 1/200 at 200mm, 1/500 at 500mm. On a crop sensor use the equivalent, so a 200mm lens on APS-C needs about 1/300. High-resolution sensors and large prints show smaller blur, so faster is safer. Image stabilization typically buys several stops against your own shake, though it does nothing for a moving subject. When your photos are soft and you are not sure why, camera shake shows how to tell.

This challenge is one of 21 in the full camera simulator, across eight scenes, if you want to try a long lens in other light.

Focal Length and Background Blur

It is often said that longer lenses give shallower depth of field. Framed the same way, they mostly do not. If you keep your subject the same size in the frame and the same f-number, a 200mm lens from far away and a 50mm lens from close up give nearly the same depth of field on the subject. What changes is the background. The 200mm lens takes in a narrow slice of it and magnifies that slice, so the out-of-focus background is enlarged into big soft shapes, and there is less of it to be busy. That is why long lenses are good at isolating a subject. The full story is on the depth of field page, and bokeh covers what the blurred areas look like.

Even at 200mm the heron is small in the frame. From the same spot a 50mm lens would have drawn it a quarter as tall, and walking four times closer to make up the difference would very likely have flushed the bird, so 200mm was a matter of distance, not style. The out-of-focus shapes in front become a soft wash, the background is a narrow, plain slice of water, and 1/2000 was far past the 1/200 handheld guide.

Choosing a Focal Length for the Picture You Want

Choose by the problem in front of you, not by a rule for your genre. Ask three questions.

• Where can I stand, or where should I? In a small room or a narrow street you cannot back up, so you need a short focal length. With wildlife, a stage or a sports field you cannot walk in, so you need a long one.
• How big should the background be next to my subject? Decide this first, then pick a distance that gives it, then pick the focal length that frames your subject from there. Near and wide makes the background small and the place visible; far and long makes the background big and simple.
• How should the viewer feel placed? Close and wide puts the viewer inside the scene. Far and long makes them an observer. Normal sits between.

Landscape photographers work wide for a strong foreground and at 100 to 200mm to pull one ridge out of a valley. Portrait photographers favor 85 to 135mm for the distance it puts them at. Street photographers often stay between 28 and 50mm to remain part of the scene. Each habit is a choice of distance first.

A prime lens or a zoom will get you there. Primes are usually smaller and offer wider apertures; zooms let you reframe without moving. Many lenses also change focal length slightly as you focus close, which is called focus breathing, so a "100mm" lens can frame a little wider at its closest focus. The prime vs zoom page weighs the two.

Long enough lens but still not close enough? A teleconverter multiplies focal length; the moon photo at the top used one to reach 300mm.

See It Side by Side

Which photo was shot at a longer focal length?

A B

Common Mistakes With Focal Length

• Thinking zooming in is the same as walking closer. Zooming crops the same perspective. Walking changes it. Decide which one you need before you touch the zoom ring.
• Crediting the lens with "compression". Distance does it. The long lens just lets you fill the frame from where it happens.
• Blaming a wide lens for a distorted face. The same 24mm lens used for a full-length portrait from 1.3m away hardly changes a face at all. The distortion was 40 centimeters.
• Believing a crop sensor makes a lens longer. A 50mm lens is 50mm on any camera. The sensor records a smaller part of its image, so it frames like 75 or 80mm.
• Reading a phone's "26mm" as its real focal length. It is an equivalent. The real lens is a few millimeters.
• Expecting a longer lens to give shallower depth of field at the same framing. It gives a bigger, softer-looking background; the depth of field on the subject is about the same.
• Expecting lens corrections to fix a wide shot's stretched corners or leaning buildings. Distortion profiles fix curved lines. Corner stretching and tilted verticals need a different fix, or a different camera position.

Try This: One Spot, Then One Size

Take ten minutes with any camera that zooms, or a phone with two or three lenses. You need a subject, such as a friend, a bin or a bike, with something tall well behind it, like a tree or a building.

1. One spot. Stand still and take three shots: widest, middle and longest setting. Do not move your feet.
2. One size. Now make the subject the same height in the frame in three shots: at the widest setting from close up, at the middle setting from further back, at the longest setting from further back again. Use something in the scene as a guide, such as the subject's head touching the top third line.
3. Compare. Crop the middle of your first wide shot from step 1 to match the long shot. Apart from sharpness, the relationship between subject and background should be the same. Then line up the three shots from step 2.

How to judge it: in step 1, the only difference should be how much is in the frame. In step 2, the subject stays the same and the background grows several times larger from the wide shot to the long one. If the background did not grow, you probably did not step back far enough. Doubling the focal length should mean roughly doubling the distance.

Questions About Focal Length

Is focal length the length of the lens?

No. It is an optical measurement of how strongly the lens bends light, given as the distance at which it focuses light from very far away for an equivalent simple lens. Wide-angle lenses are usually physically longer than their focal length, and telephotos are usually shorter.

Does a higher number mean more zoom?

Yes. A higher focal length means a narrower view and larger subjects. Magnification goes in proportion, so 200mm makes a distant subject four times as tall in the frame as 50mm does.

What does 18-55mm mean on a lens?

It is a zoom that covers every focal length from 18mm to 55mm. The ratio, about 3x here, is what compact cameras call "3x zoom". A phone's "3x" button means something else: three times the focal length of its main camera. The numbers are the real focal lengths, so on a crop sensor multiply by the crop factor to compare with full frame.

Which focal length is closest to human vision?

No single one. Your eyes cover a field far wider than any normal lens, but see sharply only in a small center. A 40 to 50mm lens on full frame gives prints whose perspective looks natural at a normal viewing distance, which is why it is called normal.

Why do my phone's ultra-wide photos make people at the edges look stretched?

Ultra-wide lenses keep straight lines straight by stretching objects near the corners outward. Put people near the middle of the frame, or step back and use the main camera.

Test Your Knowledge

Check what stuck with the focal length and lens choice quiz, which explains each answer.