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by Jamie Harrison

Choosing and achieving the correct exposure for your images is at the very heart of photography. Together with focus, exposure is pretty much the only thing you need to get right in order to record what’s in front of you. I’m not saying it will be the greatest picture – composition, communication and creativity need to be added into the mix for that – but none of those matter if you can’t record the scene onto the camera sensor in the first place.

The great American photographer Ansell Adams devoted his life to developing the perfect exposure system and wrote a whole series of books outlining the Zone System, along with producing a vast body of outstanding landscapes. Luckily with modern digital cameras we can achieve perfect exposures far easier, but it helps to have a basic understanding of how shutter speed, aperture and ISO work together.

Shutter Speed

Strictly speaking, shutter speed is the only factor that determines exposure. Exposure is really determined by the length of time the sensor is exposed to light and the shutter is the only timed part of the operation. However, the aperture and ISO setting have become part of the language of exposure and are now accepted as parts of the exposure triangle.

The shutter is a mechanical blind that protects the sensor and opens when the shutter release opens. This exposes the sensor to incoming light, which records the image. The length of the exposure, or the shutter speed, is selected from a choice of options from B (for Bulb), which allows long timed exposures of minutes, or even hours, to split second exposures of up to 1/8000 of a second.

Each stop between shutter speeds doubles or halves the exposure. For example 1/30 sec is double that of 1/60 and vice versa. Use slower shutter speeds in poorly lit conditions, but be careful of camera shake blurring the image or subject movement. Similarly use fast shutter speeds when you want to freeze action or in bright light.

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Theatre photography can be challenging, requiring fast enough shutter speeds to capture fast moving action and high ISO to allow for dark conditions. Exposure f/5.6, 1-60sec, ISO 1600

Aperture

The aperture is a small variable iris within the lens that can be adjusted to be small or large, and thus letting less or more light through the lens and on to the sensor at the time of exposure. Generally speaking, the less light there is, the wider the aperture needs to be.

The aperture size is indicated by a number, known as an f-stop. The larger the number, the smaller the size of the aperture. Typically, aperture controls begin at a large aperture (low number) and move to a smaller aperture (higher number), so a lens may have a maximum aperture of f/2, then f/4, f/5.6, and further down to f/22. Each full f/stop effective halves the size of the aperture and thus halves the exposure. Vice versa, starting from a small aperture and ‘opening up’ by 1 stop doubles the exposure. Therefore in low light you may be restricted to a large aperture to gather what light is available, and in bright light use a smaller aperture to reduce too much light entering the camera.

Just to make it even more confusing, each full stop of the aperture is also split into half or third stops depending on the option you chose in the camera menu.

Aperture also controls the depth of field of an image, which is the amount of focus sharpness from the front to the back of the picture. A small aperture such as f/11 will provide deep depth of field, with objects close to the camera and far from the camera remaining sharp and is used for subjects such as landscapes. A large aperture, such as f/2 will have shallow depth of field, with the main subject sharp, but distant or close objects out of focus. This is useful for portrait or close-up macro photography where you want a blurred background, for example.

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I used the lens’ maximum aperture of f/1.8 and focused on the models eyes to blur the background and produce focus fall off towards the back of her head. Exposure f/1.8, 1-160sec, ISO 200

ISO Speed

The ISO equivalent setting on your camera relates back to pre-digital film speeds. ISO stands for International Standards Organisation, and in photography relates to the light sensitivity of the film or sensor as defined by the standard ISO 12232:2006.

With film, the ISO speed is fixed throughout the whole roll of film and is determined by how quickly the silver within the film reacted to light, usually dependent on the size of the silver grains. Larger grains of silver react fast, so are good for low light, for example ISO 1000; while small, more tightly packed silver grains react slowly so are used for bright light conditions, such as ISO 200 rated film for daylight.

With digital and Olympus cameras, the ISO sensitivity is usually optimised at ISO 200 for general daylight use and can be boosted electronically by increasing the digital signal of the sensor and processor up to ISO 25600. The advantage with digital cameras over film is that ISO ratings can be varied for each image, and the high sensitivity for low light is far higher than that of film.

Each full increase of an ISO setting will double the sensitivity of the sensor and a reduction will halve the sensitivity.

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Shooting in dark conditions in a bar, this image of a singer required a high ISO to capture the action. Exposure f/10, 1/30sec, ISO 1600

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The image reveals digital ‘noise’ starting to appear as the signal is boosted but turning the image monochrome reduces the effect somewhat.

Reciprocity

Each of the exposure controls work together to produce a correctly exposed image across a whole range of lighting conditions, from candlelight to midday summer sun and beyond. Each time you change one the controls, you change the exposure of the image to be brighter or darker. A stop – or the differential between each setting, such as f/5.6 and f/8, 1/500 sec and 1/1000 sec or ISO 200 and ISO 400 – determines a doubling or halving of the amount of light recorded by the camera. This is known as reciprocity – each setting has a reciprocal value across all three controls.

This is useful when you want to change a specific control for a specific effect. For example, imagine you are shooting a portrait with an aperture of f/5.6, at a shutter of 1/125 sec and ISO 400. You want to blur the background so change the aperture to f/4 (1 stop), which also doubles the exposure. To compensate for this, you can either change the shutter by 1 stop to 1/250 sec to halve the exposure or change the ISO by 1 stop to 200, which doubles the light sensitivity of the sensor.

By using combinations of aperture, shutter speed and ISO ratings you can begin to not only achieve perfectly exposed images, you can also start to take creative decisions and push your photography to realize your vision.

In part two, we’ll look at how to combine the exposure controls using the manual and automatic functions of your camera to achieve creative results.

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Taken at dusk, using a tripod allowed me to shoot at a slow shutter speed and low ISO, while still retaining a small aperture for deep focus throughout the image. Exposure f/11, 1/15sec, ISO 100By using combinations of aperture, shutter speed and ISO ratings you can begin to not only achieve perfectly exposed images, you can also start to take creative decisions and push your photography to realize your vision.

In part two, we’ll look at how to combine the exposure controls using the manual and automatic functions of your camera to achieve creative results.