Lenses and focal lengths
Focal length, expressed in millimeters, sets a lens's angle of view: the shorter it is, the more it takes in; the longer it is, the closer it brings distant things. But choosing a focal length isn't just about framing more or less: each one imposes a working distance and, with it, a way of seeing. Turn the zoom ring in the simulator without moving from the spot.
What focal length is
Technically, focal length is the distance, in millimeters, between the optical center of the lens and the sensor when it's focused at infinity. You don't need to remember the definition, but you do need its consequence: the longer the focal length, the narrower the angle of view and the bigger distant objects appear.
The relationship is proportional: if you double the focal length, at the same distance you take in half the width. With a 100 mm, a distant object appears twice as large as with a 50 mm.
Focal lengths and their uses
Values given as full-frame (36 × 24 mm) equivalents, the reference every camera is compared against:
| Focal length | Type | Typical uses |
|---|---|---|
| 8 to 15 mm | Fisheye | Creative effects, action sports and whole skies |
| 14 to 24 mm | Wide-angle | Landscape, architecture, interiors |
| 35 mm | Moderate wide-angle | Street photography, documentary, everyday life |
| 50 mm | Normal | General use, close to how the eye sees |
| 85 mm | Short telephoto | Portraits |
| 135 to 200 mm | Telephoto | Isolated portraits, sports, nature, details |
| 300 mm or more | Super telephoto | Wildlife and sports at long distance |
From the same spot, without moving.
Perspective depends on distance
People often say that a wide-angle distorts and a telephoto compresses, but what changes perspective is your distance from the subject. The focal length only decides how much of that scene you crop. A wide-angle very close to a face makes the nose bigger because it's proportionally much closer to the camera than the ears.
That's why portrait photographers prefer focal lengths from 85 to 135 mm: they force them to stand 6 to 10 feet (2 to 3 m) away, a distance at which the proportions of the face look natural and the background appears larger and more blurred.
It isn't the lens that distorts: it's the distance.
Primes, zooms and fast lenses
A zoom, like the 18-55 mm that comes with many cameras, covers several focal lengths with a single lens. A prime has only one focal length, but it's usually brighter, sharper and lighter, and, for the same quality, cheaper.
A lens's speed is its maximum aperture, and it appears in its name: a 50 mm f/1.8 opens up to f/1.8. A fast lens lets you shoot in less light and blur the background more. On budget zooms you'll see two figures, like f/3.5-5.6: the maximum aperture shrinks as you zoom to the longest focal length.
A 50 mm f/1.8 is the best investment after the kit lens.
Sensor size and crop factor
Many cameras use sensors smaller than full frame. An APS-C sensor uses only the central part of the image, so a lens behaves like a longer one: its focal length is multiplied by 1.5 (by 1.6 on Canon). A 35 mm on APS-C gives a framing equivalent to a 50 mm; on Micro Four Thirds the factor is 2.
The lens's physical aperture doesn't change, but with the same framing the depth of field is greater than on full frame: a small sensor blurs less.
The most common sensor formats
These are the approximate dimensions of the most common formats. The crop factor tells you what to multiply the focal length by to get its full-frame equivalent:
| Format | Approximate size | Crop factor | Where you'll find it |
|---|---|---|---|
| Medium format | 44 × 33 mm | 0.8 | Professional studio and landscape cameras |
| Full frame | 36 × 24 mm | 1 | Advanced and professional cameras |
| APS-C | 23.5 × 15.6 mm (Canon: 22.3 × 14.9) | 1.5 (Canon: 1.6) | Most entry-level and mid-range cameras |
| Micro Four Thirds | 17.3 × 13 mm | 2 | OM System (formerly Olympus) and Panasonic |
| 1-inch | 13.2 × 8.8 mm | 2.7 | Advanced compacts and some high-end phones |
| Phone, main camera | Between 5 and 10 mm wide | Roughly between 3.5 and 8 | Mid-range and high-end phones |
Sensor, pixels and quality
Sizes given in inches (1", 1/1.3"…) are a convention inherited from the old tubes in video cameras and don't match any real measurement: a "1-inch" sensor is about 13 mm (half an inch) wide.
More megapixels don't automatically mean more quality. At the same resolution, a large sensor has larger pixels, each of which receives more light: the result is less noise and more dynamic range, especially when light is short. More megapixels, on the other hand, let you crop and print large, at the cost of heavier files and greater demands on the lens.
In good light, almost any sensor performs well; the differences show up when it gets dark.
Your turn: one face, two focal lengths
With a zoom or with your phone's different lenses.
- Take a portrait with the shortest focal length (or the 0.5× lens) about 20 inches (50 cm) from the person.
- Step back and take another with the longest focal length (or the 3× lens), with the face filling the same space in the frame.
- Compare the nose, the ears and the size of the background.
- Decide which is more flattering and explain why in one sentence.