Chapter 7Part 2: Focus and depth
A lens decides how much of the scene fits in the frame, gathers light, and brings one distance into focus. Learn to read its markings and choose one for the photograph you want to make.
A camera lens gathers rays from each point of a scene and bends them toward the sensor. Several shaped elements, often arranged in groups, help bring the image into focus while reducing unwanted colour fringes, distortion and blur. The lens projects an image circle; the sensor records a rectangle inside it. Sensor size decides how much of that circle is captured. [2, 3]
The barrel holds the elements in alignment. A focus ring or autofocus motor moves a lens group to focus; in a zoom, moving groups change focal length. The mount attaches the lens and, on many systems, carries electronic signals. Some lenses add optical stabilisation, moving a group to counter camera shake. Stabilisation cannot freeze a moving subject. [3]
One simple element cannot correct every optical error. Designers trade image quality against size, weight, cost and aperture. Curved and specially shaped elements can reduce aberrations; coatings and a hood help control stray light and flare. A perfectly corrected image is not everyone's goal: some photographers choose older lenses for lower contrast, glow or unusual flare. Test the particular lens rather than assuming all vintage glass behaves alike. [2, 6]
Front element, hood and filter thread. The front element admits light. A fitted hood blocks stray light; a threaded front accepts compatible filters. Some very wide lenses have a curved front and no ordinary front filter thread.
Zoom ring and focus ring. The zoom ring changes focal length by moving optical groups. The focus ring changes focus distance; on many autofocus lenses it sends an electronic command rather than mechanically moving the glass.
Switches, mount and contacts. A lens may offer AF/MF, stabilisation or focus-limit switches; some add custom buttons. The rear mount determines which camera bodies it fits; contacts share settings and lens data with the camera. Check the actual lens manual because controls differ. [3]
Focal length is an optical property, marked in millimetres. With the camera in one place and the same sensor, a shorter focal length takes in a wider angle; a longer one records a narrower slice. Drag the slider or use its arrow keys. The diagram uses a rectilinear lens model, focused far away. [1]
Same camera position. Full frame, 36 × 24 mm.
About 40° horizontally
Illustration of the angle from one fixed position. The shapes are landmarks, not a photograph or a simulation of perspective.
On full frame, 24 mm is wide, 50 mm is a standard view, and 200 mm is telephoto. A 24–70 mm zoom spans several views; a 50 mm prime stays at 50 mm. These labels are approximate. On APS-C, the same 50 mm lens frames more tightly because the smaller sensor crops the image circle; the lens itself has not become longer. [1] See the crop factor chapter.
A long lens used from far away often shows a compressed relationship between near and far subjects. The change comes from where the camera stands. If you keep the camera fixed and crop a wide image to match a telephoto frame, the perspective remains the same. Step closer with a wide lens and nearby objects grow large relative to the background.
Overlapping diaphragm blades make an adjustable opening. The f-number is focal length divided by the effective opening diameter: at 50 mm, f/2 means an entrance pupil about 25 mm across. A smaller f-number admits more light and can give shallower depth of field. The maximum aperture printed on a lens is the widest it can open; a zoom might say f/3.5–5.6 because its maximum changes through the zoom range. [3]
Focusing changes the lens's optical arrangement so rays from your chosen distance converge on the sensor. In many modern lenses, a motor moves internal elements. The camera may estimate which direction to move with phase detection, or search for peak image contrast; some systems combine both. Choose the eye for a portrait, and check it after the shot when depth of field is thin. [4]
A lens can have more than one label: a 90 mm macro may also be a prime and a short telephoto. First ask how wide it sees, then whether it zooms, then whether it has a special job. The focal lengths below are examples for a full-frame sensor, not rigid boundaries. On APS-C, use the crop factor to compare framing. [1, 10]
About 14–20 mm · full frame
Use it for: interiors, architecture, night skies and landscapes where you want a strong foreground. It fits more from one position.
Watch for: a tiny distant subject, stretched people near the edges and converging building lines if you tilt the camera up. A rectilinear ultra-wide aims to keep straight lines straight, though edge stretching can still be conspicuous. [11]
About 24–35 mm · full frame
Use it for: street scenes, travel, environmental portraits and landscapes with both subject and setting. Move close when the foreground should feel prominent.
Watch for: faces becoming unflattering if you stand very close, and clutter slipping into the edges. Step back or use a longer lens for a tight headshot. [11]
About 40–60 mm · full frame
Use it for: everyday scenes, food, documentary work and learning composition. Around 50 mm on full frame, or 35 mm on APS-C, is a useful starting view.
Watch for: limited reach for distant subjects and limited room coverage in tight interiors. A standard prime makes you choose camera position deliberately. [12]
About 85–135 mm · full frame
Use it for: portraits, stage details and isolating a subject from a busy background while standing at a comfortable distance. An 85 mm prime is a common portrait choice.
Watch for: needing more room to step back indoors. At wide apertures, the in-focus zone can be thin, so place focus carefully on the eye. [13]
About 200 mm and longer · full frame
Use it for: wildlife, sports, aircraft and faraway details you cannot approach. A 70–200 mm zoom covers changing action; 400 mm and longer can fill the frame with small distant subjects.
Watch for: camera shake, fast subject motion, weight and heat shimmer over long distances. Stabilisation helps hand shake; moving subjects still need fast shutter speeds. The “compressed” look comes from the distant camera position used to keep a subject the same size. [13]
Anatomy: one fixed optical focal length, a focus mechanism and an aperture; no zoom ring. Many primes offer a wide maximum aperture, but this varies by model.
Use it when: you know the angle you want, value a particular size or aperture, or want a simple constraint for practice. A 35 mm prime is handy for life around you; an 85 mm prime gives room for portraits.
Tradeoff: to change framing substantially, move your camera or change lenses. Moving also changes perspective. [1, 12]
Anatomy: a zoom ring or lever moves groups to change focal length; the lens still has a separate focusing mechanism. A 24–70 mm lens covers wide to short telephoto views.
Use it when: subjects move unpredictably or you cannot change position, as at events, travel stops or the edge of a sports field.
Tradeoff: maximum aperture may be constant, such as f/2.8, or vary through the range, such as f/3.5–5.6. Size and image quality depend on the particular design; “prime” alone does not mean sharper. [3]
Read the name before buying: “24–70 mm f/2.8” is a zoom with one stated maximum aperture; “18–55 mm f/3.5–5.6” has a smaller maximum opening at the long end. Neither number tells you its minimum focus distance, mount or sensor coverage. [3]
Inside: a focusing design that reaches high magnification at close distances. At 1:1, a 10 mm subject projects 10 mm onto the sensor; a close-focusing ordinary zoom may reach only a fraction of that.
Use it for: insects, jewellery, textures and tiny product details. Check both maximum magnification and working distance, measured from the front of the lens to the subject. Minimum focus distance is measured from the sensor plane; it can sound generous while leaving little room for light or a live insect. [14]
Inside: an ultra-wide optical projection that deliberately curves many straight lines, unlike a rectilinear wide-angle design.
Use it for: an immersive, bent-horizon look, unusual interiors or dramatic action close to the lens. Keep a face away from the edge unless that distortion is intentional. The precise view depends on the lens and sensor coverage. [15]
Outside: controls move and rotate the optical assembly relative to the camera. The lens needs a larger image circle to allow shift.
Use shift for: architecture: keep the camera level, then move the frame upward within the image circle to include the top of a building without leaning verticals. Use tilt for: changing the orientation of the focus plane, useful when a plane of products or landscape detail recedes away from you. The two movements do different jobs. [8]
Inside: optics record a horizontally squeezed image. A matching de-squeeze in the camera, monitor or edit restores its proportions; the final aspect ratio also depends on sensor area and cropping.
Use it for: a wide cinema frame or a particular rendering of highlights and edges. Flare streaks and oval bokeh vary by lens; they are not guaranteed. Check the lens's squeeze factor, sensor coverage and de-squeeze setting before filming. [9]
Field exercise: photograph one subject from a fixed position at your shortest, middle and longest focal lengths. Then make a fourth frame by walking closer with the short focal length. Compare what changed in framing and in the size of the background.
The optical diagrams are original teaching illustrations; the lens anatomy infographic is from Tamron Americas and credited in its caption. Claims about products and optical behaviour are linked at the point of use.