Lenses, focal length and perspective

The lens decides what part of the world reaches the sensor and how it is drawn. Change from a wide-angle lens to a telephoto and the same scene can look spacious or compressed, intimate or distant, dramatic or calm. Yet much of what people attribute to lenses actually comes from where the photographer stands. This lesson explains focal length and angle of view, how sensor size changes them, the real cause of perspective effects, the families of lenses and what each is good for, the difference between primes and zooms, close-up photography, and the optical flaws worth recognizing. The aim is to choose a lens, and a position, for the picture you want rather than out of habit.

Focal length and angle of view

Lesson 1 defined focal length as the distance behind the lens at which distant subjects come into focus. For a given sensor, it determines the angle of view, how much of the scene the camera sees. A short focal length projects a small image of each object, so more of the scene fits on the sensor; a long focal length projects a large image of a small part of the scene. For a sensor of dimension d (width, height or diagonal) and a lens of focal length f focused at a distance, the angle of view is:

A "normal" lens has a focal length close to the diagonal of the sensor, about 43 mm for full frame (50 mm in practice). Pictures from it look natural because their angle of view resembles the part of the scene we attend to, not because the eye has a focal length of 50 mm.

Sensor size and equivalent focal length

The same lens gives a narrower angle of view on a smaller sensor, because the sensor captures only the middle of the image the lens projects. To compare framing across formats, photographers use the full-frame equivalent focal length: the actual focal length multiplied by the crop factor (Lesson 1). A 35 mm lens on an APS-C camera (crop factor 1.5) frames like a 52 mm lens on full frame; a 25 mm lens on Micro Four Thirds (crop 2.0) frames like a 50 mm. Phone makers quote equivalent focal lengths, so a phone main camera described as "24 mm" actually has a lens of about 6 to 7 mm in front of a small sensor.

Equivalence affects only framing (and, as Lesson 3 shows, depth of field). It does not change the lens's actual focal length or its f-number's effect on exposure: f/2.8 gives the same exposure on any format.

Perspective comes from position

Wide-angle lenses are often said to "exaggerate" depth and telephotos to "compress" it. Strictly, perspective, the relative size and position of near and far objects, depends only on where the camera is. From a given spot, a wide-angle picture cropped to the same framing as a telephoto picture shows exactly the same relationships between near and far objects. What differs is how the photographer uses each lens:

• With a wide-angle lens, photographers move close to fill the frame. Nearby objects are then much closer than distant ones, so they look large and the background small and far away: depth seems exaggerated.
• With a telephoto lens, photographers stand far back. Near and far objects are then at similar distances relative to the camera, so their sizes are similar: the background looms close behind the subject, and depth seems compressed.

Distance also affects faces. Photographed from 50 cm with a wide lens, a nose is noticeably closer to the camera than the ears, so it looks enlarged and the face distorted, the effect familiar from close-up selfies. From 2 to 3 m with a short telephoto, the proportions look natural, which is why 85 mm and similar lenses are favoured for portraits. Wide lenses also make vertical lines converge when tilted up at buildings (keystoning); keep the camera level, use a tilt-shift lens, or correct it in editing (Lesson 10).

Prime and zoom lenses

A prime lens has a single focal length; a zoom covers a range, such as 24–70 mm or 70–200 mm. Each has advantages:

• Primes are usually smaller, lighter, sharper and faster (wider maximum aperture, such as f/1.4 or f/1.8) for their cost, which helps in low light and for shallow depth of field. Working with one focal length also trains you to move and to see in that lens's angle of view.
• Zooms are versatile: one lens covers many framings, which suits events, travel and situations where you cannot move or change lenses. Professional zooms with a constant maximum aperture (for example 24–70 mm f/2.8) are excellent but large and costly; consumer zooms are lighter and cheaper with variable, smaller maximum apertures (for example f/3.5–5.6).

A lens's name gives its focal length and maximum aperture: "50 mm f/1.8" or "70–200 mm f/4". Other markings indicate image stabilization, the lens mount and special features such as macro.

Lens families and their uses

Beyond focal length, some lenses are designed for particular jobs:

• Macro lenses focus close enough to reproduce small subjects at life size (1:1 magnification) on the sensor: insects, flowers, products, textures.
• Tilt-shift lenses can shift to correct converging verticals in architecture and tilt the plane of focus, for landscapes sharp from front to back or for a miniature-model effect.
• Fisheye lenses capture 180° or more with strong curved distortion, used for creative and scientific purposes.
• Teleconverters fit between camera and lens to multiply focal length (1.4× or 2×), at the cost of one or two stops of light.

Lens image stabilization (and in-body stabilization, which moves the sensor) counteracts small hand movements, allowing slower handheld shutter speeds (Lesson 4). It does not freeze a moving subject.

Close-up photography

Magnification is the ratio of the size of the image on the sensor to the size of the subject. A typical lens at its closest focus reaches about 0.1 to 0.25×; a macro lens reaches 1×, so a 20 mm insect spans 20 mm on the sensor, more than half the width of a full-frame sensor. Close up, depth of field becomes very shallow (Lesson 3), lighting becomes difficult because the lens and photographer cast shadows, and tiny movements blur the image, so macro photographers use tripods, focus rails and flash, and often combine several frames focused at different distances (focus stacking). Cheaper routes to close-up work include extension tubes, which move a lens farther from the sensor so it focuses closer, and screw-on close-up lenses.

Optical flaws to recognize

No lens is perfect. Recognizing common flaws helps you choose equipment, avoid problems and fix them in editing:

• Softness: images less sharp at the edges than the middle, often worst at the widest aperture; stopping down two stops usually helps (Lesson 3).
• Vignetting: darkening toward the corners, also strongest wide open; it can be corrected or used deliberately.
• Distortion: straight lines bowing outward (barrel distortion, common in wide lenses) or inward (pincushion, common in telephotos); editing software corrects it with lens profiles.
• Chromatic aberration: coloured fringes along high-contrast edges, because the lens focuses different wavelengths slightly differently.
• Flare and ghosting: reduced contrast and bright artefacts when a strong light source is in or near the frame; a lens hood and shading the lens help.

Lens profiles in raw processing software (Lesson 10) correct distortion, vignetting and some chromatic aberration automatically. Phones correct them in processing before you ever see the image.