Exposure
Exposing means deciding how much light reaches the sensor. If too much gets in, the bright areas turn pure white; if too little, the shadows go black and noise appears. Three controls set that amount (aperture, shutter speed and ISO), and each also has its own creative effect. Before you read, try them in the simulator.
Aperture: the opening light passes through
The diaphragm is a set of blades that opens or closes a hole inside the lens. Its size is given as an f-number, which is the result of dividing the focal length by the diameter of that hole. That's why it works the opposite way to what you'd expect: f/1.8 is a large aperture and f/16 a very small one.
The usual scale (f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16) isn't random: each value lets in half the light of the one before. As well as the amount of light, the aperture controls depth of field: the wider you open it, the more blurred everything in front of and behind the focus point becomes.
That's why a lens that opens very wide, like a 50 mm f/1.8 or an f/2.8 zoom, is considered bright; in photography jargon, a fast lens, because it lets in so much light that you can use faster shutter speeds. Even so, almost all lenses perform a little worse at their maximum aperture: they lose sharpness in the corners and darken the edges, an effect called vignetting. Stopping down one or two stops usually fixes both.
Small f-number: more light and less in focus. Large f-number: less light and more in focus.
Same scene and same final brightness. Only the aperture changes.
Shutter speed: the exposure time
Shutter speed tells you how long the shutter stays open. It's expressed in fractions of a second (1/1000, 1/125, 1/15) and, for long exposures, in seconds (1", 2", 30"). Each time you double the time, you double the light.
Its creative effect is motion: at 1/1000 you freeze a drop of water or a cyclist; at 1/30, whatever moves leaves a streak. There's also a practical handheld limit: as a guide, don't go below 1/focal length (1/50 with a 50 mm) or your hand shake will cause camera shake, meaning the whole photo comes out blurred. The camera's or lens's image stabilization lets you gain several stops.
A fast shutter freezes; a slow shutter draws the motion.
Same scene on a tripod. Only the shutter speed changes.
How to hold the camera
- Hold the grip with your right hand and put your left hand under the lens, palm up: it carries the weight and turns the rings.
- Tuck your elbows into your body and stand with your feet slightly apart, one a little forward: your body works like a tripod.
- Use the viewfinder whenever you can. Resting the camera against your face adds a third point of support that the screen doesn't give you.
- Shoot at the end of an exhale, when your body is stillest, and press the shutter button gently, without jabbing it.
- If there's no tripod, lean on a wall, a lamppost or a railing. In burst mode, the second or third photo is often sharper than the first.
ISO: amplifying the signal
ISO doesn't make the sensor capture more light: it amplifies the signal it has already received. That's why raising it brightens the photo but also amplifies the noise, that grain and those color specks that eat away at detail, especially in the shadows.
Each doubling of ISO (100, 200, 400, 800…) is worth one stop of light. Today's cameras give clean results up to ISO 1600 or 3200, but the professional rule hasn't changed: use the lowest ISO that the aperture and shutter speed you need will allow.
ISO is the last resort: decide on the aperture and shutter speed first.
At night and with the same brightness. Only the ISO changes.
Stops of light and equivalent exposures
All three controls are measured in the same unit: the stop, which doubles or halves the light. If you open the aperture one stop, you can speed up the shutter one stop and the exposure will stay exactly the same. These three combinations produce the same brightness, but three different photographs:
| Aperture | Shutter speed | ISO | Result |
|---|---|---|---|
| f/2.8 | 1/1000 | 100 | Blurred background and frozen motion |
| f/8 | 1/125 | 100 | Everything sharp; fast-moving things slightly blurred |
| f/8 | 1/1000 | 800 | Everything sharp and frozen, with a little more noise |
Exposure value and the light meter
Photographers sum up each combination of aperture and shutter speed in a single number: the exposure value, or EV. Combinations with the same EV produce the same brightness, like the three in the table above.
In the viewfinder or on the screen you'll see a scale from −3 to +3 with an indicator: that's the light meter. The 0 marks the exposure the camera considers correct, and each number is one stop of light above or below. Between them are smaller marks, because cameras adjust in third-of-a-stop steps: that's why f/3.2 and f/3.5 appear between f/2.8 and f/4, and 1/80 and 1/100 between 1/60 and 1/125.
In manual mode, your job is to move the indicator to the position you want. Remember that 0 isn't absolute truth but the light meter's opinion, and you already know it gets snow and black backgrounds wrong.
The light meter suggests; you decide.
The histogram
The histogram is a graph of the distribution of tones in the image: blacks on the left, midtones in the middle and whites on the right. You have it in the simulator, on your camera and in any editing program, and it's far more reliable than the screen, whose brightness changes with the ambient light.
There's no ideal histogram: a night scene is bunched up on the left and a snowfall on the right. What you need to watch are the edges. If the graph is pushed against the right edge, there are blown highlights, with no recoverable information; if it's pushed against the left, crushed shadows. In digital it pays to protect the highlights, because shadows can be lifted in editing.
To keep an eye on the edges, also turn on the highlight warning, which makes the blown-out areas blink on the screen; in video it's called zebras. Whatever is lost at that end is clipped highlights: there's no information left to recover, not even in RAW.
Watch the edges of the histogram and always protect the highlights.
How to read the histogram
These are the most common shapes and what they mean:
| Shape | What it means | What to do |
|---|---|---|
| Pushed against the left edge | Underexposed photo: crushed shadows with no detail. | Increase the exposure: open the aperture, slow the shutter or raise the ISO. |
| Shifted left without touching the edge | A dark, low-key scene, or slightly short exposure. | Fine for a night scene; otherwise, add a little exposure. |
| Spread across the whole graph | A balanced exposure, with detail in highlights and shadows. | Nothing: it's the usual starting point. |
| Shifted right without touching the edge | A bright scene or generous exposure. | Ideal in RAW: it's the expose to the right technique. |
| Pushed against the right edge | Overexposed photo: clipped highlights. | Reduce the exposure until the graph pulls away from the edge. |
Expose to the right
The sensor records far more information in the highlights than in the shadows. That's why many photographers who shoot RAW use a technique known by its acronym, ETTR: they give as much exposure as possible without blowing the highlights and then darken the photo in editing. The result has less noise in the shadows than if they'd brightened a dark photo.
It takes practice and a reliable histogram, and it doesn't work in JPEG, where the camera has already locked in the result. To start with, it's enough to keep the graph from touching the right edge.
In RAW, a little bright is better than dark, as long as you don't blow the highlights.
Your turn: one scene, three decisions
You need about 20 minutes and something moving: people walking, traffic or a fountain.
- Put the camera in M mode or your phone in Pro mode. If your phone doesn't have it, apps like Lightroom (iPhone and Android) or Open Camera (Android) add it.
- Take the same photo at 1/1000 and at 1/15, compensating with the aperture or ISO so both have the same brightness.
- Repeat at f/2.8 and at f/11 with a nearby object and the background far away.
- Turn on the histogram and check in each photo that the graph doesn't touch either edge.
- Compare the four images and note, for each one, what has changed besides the light.