Exposure basics: aperture, shutter speed and ISO as three trades

Aperture, shutter speed and ISO all change how bright the picture is. If that were all they did, one dial would be enough. There are three because each buys its brightness with a different currency, and choosing which to spend is the whole of exposure.

• Aperture is the hole in the lens, written as an f-number. A smaller number is a wider hole, more light, and less depth of field: less of the scene in front of and behind the point of focus is acceptably sharp.
• Shutter speed is how long the sensor is exposed. Longer means more light and more motion recorded, whether that is a subject moving across the frame or your own hands moving the camera.
• ISO is how much the signal is amplified before it becomes a file. Higher means a brighter result from the same light, and more visible noise, because there were never enough photons to average the noise away.

Two of those costs are creative: depth of field and motion blur are decisions about what the picture looks like, and the right answer is often the blurred one. The third, noise, is never wanted. So the working order is: choose the aperture or the shutter speed the picture needs, let the other follow, and use ISO to close the gap.

A stop is a factor of two in light. One stop more is twice the light; one stop less is half. Because all three controls are measured in the same unit, a stop taken away from one can be handed straight to another, and that is what makes the three interchangeable.

• Shutter speed is the easy one: 1/125 to 1/250 is one stop less light, because the sensor is open for half as long.
• ISO is just as easy: 400 to 800 is one stop more.
• Aperture looks strange because it is a ratio, not an area. The full-stop series runs f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16. Each step changes the light by a factor of two even though the numbers only grow by about 1.4, because light passes through an area and area goes as the square of the diameter.

Once the series is memorised, the arithmetic is addition. Going from f/5.6 to f/2.8 at the same brightness is two stops more light, so take two back elsewhere: 1/125 becomes 1/500, or ISO 800 becomes ISO 200. It is also how the aperture argument in your first three lenses works out: f/1.8 against f/5.6 is about three and a third stops, which is a different photograph, not a slightly better one.

There are two blurs with different cures. Camera shake is you; subject motion is them. Stabilisation fixes only the first.

The reciprocal rule is the starting point for shake: to hand-hold without visible blur, use a shutter speed of at least one over the focal length, in full-frame-equivalent millimetres. A 50 mm lens on full frame wants 1/50 or faster. The same lens on APS-C frames like 75 mm, so it wants 1/80. On Micro Four Thirds it frames like 100 mm and wants 1/100. The rule predates high-resolution sensors and large screens, so treat it as a floor rather than a target.

Stabilisation, whether in the body or the lens, is rated in stops. A body claiming five stops is claiming that 1/50 becomes about 0.6 seconds. In the field, halve the claim and you will be about right: technique, breathing and the lens all matter. Stabilisation also does nothing whatsoever for a moving subject, which is the blur that ruins most indoor photographs of children.

• A person walking: 1/250 keeps the feet sharp.
• A child indoors or a dog at play: 1/250 to 1/500.
• Running, cycling, field sport: 1/1000.
• Birds in flight and motorsport: 1/2000 and upwards, and at that point the limit is usually the autofocus system rather than the shutter, which is the subject of autofocus and burst .

Noise is not caused by the ISO setting but by there being too little light. Light arrives as a countable number of photons, and counting anything few times gives a noisy count. Raising ISO amplifies a weak signal, making visible the noise that was always there.

So raising ISO is not a sin. A sharp, noisy photograph is a photograph; a clean, blurred one is not. On most modern sensors, setting a higher ISO in the camera is very slightly cleaner than shooting dark and brightening the raw file afterwards, because the amplification happens before some of the electronic noise is added. The difference is small. What matters is always how much light reached the sensor.

Set a ceiling rather than avoiding high numbers. Auto ISO with a cap and a minimum shutter speed is the most useful feature on a modern camera: you tell it the slowest shutter you will tolerate and the highest ISO you will accept, and it works within your rules. A larger sensor collects more light for the same framing and aperture, so it shifts the ceiling up.

The priority modes are not training wheels. Most working photographers live in one of them, because the camera recomputes a meter reading faster than a person can.

• Aperture priority: you set the aperture, the camera sets the shutter. The default for anything where depth of field is the decision - portraits, landscapes, still subjects. Watch the shutter speed it picks and raise the ISO when it drops below your hand-holding floor.
• Shutter priority: you set the shutter, the camera sets the aperture. For anything where motion is the decision - sport, children, water, panning. Beware of it silently running out of aperture in dim light and underexposing.
• Manual with Auto ISO: you fix both the aperture and the shutter, and the camera moves ISO. This is the mode for changing light and fixed requirements - a stage, a bird in flight, an indoor sports hall. It is manual where it counts and automatic where it does not.
• Full manual: for light that does not change while the subject does. Studio flash, astrophotography, a tripod in one place for an hour, or any situation where the meter is being fooled and you want the exposure to stop moving between frames.

Exposure compensation is what makes the priority modes work. The meter aims for an average mid-tone, so snow comes out grey and a spotlit face comes out blown. Dial in plus one for snow, minus one for a dark stage, then check and adjust.

These are places to begin, not answers. Set them, take one frame, look at the histogram, adjust.

The rear screen lies about brightness. It has its own backlight setting, and it is being viewed in whatever light you happen to be standing in. The same frame looks underexposed at noon and blown at midnight. The histogram does not care about any of that: it is a count of how many pixels fall at each brightness level, left for black, right for white.

• A bar piled against the right edge means clipped highlights. There is no detail there to recover, because the recorded value has hit its ceiling. This is the only failure worth panicking about.
• A bar against the left edge means crushed shadows. Recoverable, at the price of noise, and often entirely intentional in a night scene.
• A histogram bunched in the middle with gaps at both ends is a flat, low-contrast scene. It is not a mistake. Fog looks exactly like this.
• There is no correct shape. A snow field belongs on the right and a black cat on a coal heap belongs on the left, and forcing either into the middle is how you get grey snow.

Turn on the highlight warning as well - the blinking overlay on the areas that have clipped. Together with the histogram it tells you in half a second whether the frame is recoverable. If you shoot raw there is usually a little more highlight headroom than the display suggests, because the histogram is computed from a processed preview rather than from the raw data.

Once exposure is under control, the next thing that changes your pictures is where you stand: see focal length and framing . If you are still buying, how to choose a camera covers which specifications matter and the camera finder ranks all 61 against what you shoot. For weather and cold, see camera care and travel .