Photography basics

Updated 2026-07-09

A plain-language glossary of the terms the calculators and guides use. New to photography? Read it top to bottom once and the rest of the docs will make sense. Coming back for a refresher? Jump to the term you need.

Nothing here assumes you already know the jargon — each entry says what the thing is, then why it matters when you’re out shooting.

A “stop” — the unit of light

A stop is photography’s unit of brightness: one stop is twice (or half) as much light. It’s the common currency of the three exposure controls below — open the aperture one stop, or double the shutter time, or double the ISO, and each adds exactly one stop of brightness. Thinking in stops is what lets you trade one control for another without changing how bright the photo comes out.

Aperture & f-number

The aperture is the adjustable opening inside the lens that lets light through — like the pupil of an eye. Its size is written as an f-number: f/2.8, f/5.6, f/8, and so on.

The catch that trips up everyone at first: a bigger f-number means a smaller opening. So f/2.8 is wide open (lots of light, and only a thin slice of the scene in focus), while f/16 is a pinhole (little light, but much more in focus). Each full f-stop — f/2.8 → f/4 → f/5.6 → f/8 — halves the light, i.e. is one stop darker.

Shutter speed

How long the sensor is exposed to light, in seconds or fractions of a second: 1/500 s, 1/30 s, 2 s. Fast shutter speeds freeze motion; slow ones blur it (flowing water turns silky, car lights become streaks) and let in more light. Each doubling — 1/500 → 1/250 → 1/125 — is one stop brighter.

ISO

How strongly the camera amplifies the sensor’s signal. Low ISO (100) is the cleanest; raising it brightens the image in dark scenes but adds noise (grain). Each doubling — 100 → 200 → 400 — is one stop. Rule of thumb: use the lowest ISO that still gets the shot.

Exposure & EV

Exposure is the total light that reaches the sensor, set by aperture, shutter, and ISO together. EV (exposure value) rolls that into a single number, so two different settings with the same EV give the same brightness — handy for comparing them. As a feel for the scale (at ISO 100): bright sun ≈ EV 15, an overcast day ≈ EV 12, a moonlit landscape ≈ EV −3. See the Exposure calculator.

Focal length & zoom

Focal length (in mm) is how “zoomed in” a lens is. Short (wide-angle, e.g. 16–35 mm) takes in a broad scene; long (telephoto, 200 mm and up) magnifies a narrow slice. Focal length also affects how shallow the focus can go and how “compressed” the background looks behind your subject.

Field of view

The angle of the scene a lens takes in — wide for a wide-angle lens, narrow for a telephoto. It depends on both focal length and sensor size. The Panorama calculator uses it to work out how many frames cover a given sweep.

Sensor size & crop factor

Cameras come with different sensor sizes: full-frame (the reference size), APS-C (smaller), Micro Four Thirds (smaller still), and phone sensors (tiny). A smaller sensor “crops in” to the scene, so the same lens frames tighter.

The crop factor is that multiplier: full-frame is 1.0, APS-C about 1.5, Micro Four Thirds 2.0. A 50 mm lens on an APS-C body frames like a 75 mm on full frame (50 × 1.5) — the lens hasn’t changed, only how much of its image the smaller sensor keeps. Smaller sensors also give more depth of field at the same framing, which is why phone photos rarely have a blurred background. See the Focal match calculator.

Depth of field

How much of your photo is sharp, front to back — the zone around your focus point that still looks crisp, with everything nearer or farther falling into blur. A blurred-background portrait has a shallow depth of field; a landscape sharp all the way to the horizon has a deep one. Three things control it: aperture (smaller opening, bigger f-number → deeper), focal length (longer lens → shallower), and sensor size (smaller → deeper) — plus how close you stand (closer → shallower). Because focal length and aperture pull in opposite directions, a longer, “faster” lens doesn’t automatically keep more in focus — often it’s the opposite, which is why the right lens to bring depends on the shot. See the Depth of field calculator to see it on a real photo.

Circle of confusion

The “sharp enough” threshold. A point of light never focuses to a perfect dot — past a certain blur size your eye calls it unsharp in a normal-size print. That size is the circle of confusion, roughly the sensor’s diagonal ÷ 1500 (about 0.029 mm on full frame). The depth-of-field and hyperfocal maths use it as the cut-off between “sharp” and “not.”

Hyperfocal distance

The single focus distance that keeps everything from half that distance out to infinity sharp — the classic landscape move for front-to-back sharpness. See the Hyperfocal calculator.

Pixel pitch

The physical size of one pixel on the sensor, in microns (µm) — sensor width ÷ the number of pixels across. Tighter pixels (high-megapixel sensors) reveal star trailing sooner, which is why the more accurate NPF rule for night skies factors it in.

ND filter

A neutral-density filter — effectively sunglasses for the lens. It cuts light by a set number of stops without shifting colour, so you can use long shutter speeds in daylight (to smooth water or streak cloud) that would otherwise blow the image out. See Long exposure.

Nodal point

The point along the lens you should rotate around when shooting a panorama. Pivot there and near and far objects stay lined up between frames; pivot anywhere else and they shift against each other (parallax), which the stitcher can’t reconcile. See Panorama.

Celestial pole & declination

The night sky appears to rotate once a day around the celestial pole — marked by Polaris in the northern hemisphere; the southern sky has no bright pole star. A star’s declination is its angle from the celestial equator: stars near the pole trace tiny circles (barely move), while those near the equator sweep the widest arcs. This is why star trails curve around the pole.

Galactic core

The bright, dense centre of the Milky Way, toward the constellation Sagittarius — the detailed, photogenic part people mean by “shooting the Milky Way.” It sits low in the southern sky and is only above the horizon in a dark sky for part of the year (Milky Way season). See Sun, Moon & Milky Way.

NPF rule

A more accurate alternative to the old 500 rule for the longest exposure that still keeps stars as points rather than streaks. Where the 500 rule uses only focal length, NPF also factors aperture, pixel pitch and focal length — giving shorter, sharper limits on high-resolution sensors. See Spot stars.

Bortle scale & light pollution

A 1–9 rating of night-sky darkness: class 1 is a pristine sky where the Milky Way casts shadows; class 9 is an inner city where it’s invisible. It’s shorthand for light pollution — the artificial skyglow that washes out faint detail. A quick test: if you can see the Milky Way with dark-adapted eyes, your camera will record it well.

Azimuth

The compass direction to something on the horizon, in degrees — 0° north, 90° east, 180° south, 270° west. It’s how the app describes where the sun, moon or Milky Way core rises, sets, or sits relative to your subject. See Alignment planner.

Altitude

The height of something above the horizon, in degrees — 0° sitting right on the horizon, 90° straight overhead (the zenith). Paired with azimuth it pins a star, the sun or the moon exactly in the sky: azimuth says which way to look, altitude says how high. A negative altitude means it’s below the horizon — not yet risen, or already set.

Twilight: civil, nautical & astronomical

The graded darkness after sunset (or before sunrise), set by how far the sun sits below the horizon: civil (0° to −6°), nautical (−6° to −12°) and astronomical (−12° to −18°). Past −18° is full night — what you need for the faintest stars and the Milky Way.

Golden hour & blue hour

Two prime windows defined by the sun’s height, not the clock. Golden hour — sun roughly +6° to −4° — gives warm, low, directional light. Blue hour — sun roughly −4° to −8°, just below the horizon — gives even, cool twilight. Their length depends on your latitude and the season. See Sun, Moon & Milky Way.