Intermediate Tutorial 4
The ISO setting with which you take a photograph is the level of sensitivity to light. In different circumstances it can be set manually, or left to your camera’s automatic systems. In this tutorial we look at ISO, its relation to shutter speed and the issue of picture ‘noise’.
Exploring Light Sensitivity – ISO
Before digital photography, it was necessary to choose the type of film you were going to use for your day’s shooting based on how bright the shooting situation would be and therefore how sensitive to light you needed your film to be. Photographic film was given a rating according to the speed at which it could absorb light. When the American Standards Association (the body originally in control), was replaced by the global International Organization for Standardization, they adopted ISO – derived from the Greek īsos, meaning equal – as their Light/Speed reference.
Film stock was rated on a scale from 25 (low sensitivity) to 3200 (high), with shooting in very bright light requiring the lowest sensitivity film in order that a wide aperture could be used without the danger of overexposure. In low-light situations, high-sensitivity film was required in order that fast shutter speeds could be used to eliminate blur.
When digital cameras replaced film with an image sensor, light sensitivity could be electronically controlled, thereby allowing flexibility – with the ISO setting changeable from shot to shot.
The ISO setting has a bearing on the availability of aperture and shutter values, and is therefore an important part of the ‘exposure triangle’ – the correlation between the three settings. All modern digital cameras have an ‘Auto ISO’ function that increases the setting when light levels are measured as insufficient. Each step up in ISO increases the sensitivity of the sensor, reducing the light requirement by half each time. Therefore, higher ISO settings are very useful when shooting in low light and fast shutter speeds are required.
When to use high ISO
As previously mentioned, the option to increase the sensitivity of the sensor adds flexibility to your exposure control. When shooting in low light, shutter speed will be limited to force a longer exposure.
This increases the chance of blur caused by your subject moving, or through camera shake – a tripod is advisable whenever shutter speed is too slow. However, increasing the ISO will allow faster shutter speeds to be used, thereby reducing the danger of blur.
Take a look at these pictures of a clock pendulum.
The first image shows motion blur due to the low ISO and shutter speed – an effect you may prefer, in some circumstances.
The second image, however, is sharp; blur was avoided because a higher ISO meant that a faster shutter could be used.
How sensitivity effects picture quality
The best picture quality is possible when your camera’s sensitivity is set to normal low levels (100 – 200 ISO), and there is plenty of light landing on the sensor, but as the sensor works harder at higher ISO settings (when light levels are low), picture ‘noise’ begins to affect areas of the image. It is a bit like listening to music; the more you turn up the volume, the more distortion can begin to affect the sound.
In the days of ‘film’, increasing the sensitivity meant the image would appear ‘grainy’ (small dots covering the picture), whereas today’s ‘digital noise’ manifests itself as specks of discolouration.
Most Interchangeable lens system cameras can easily cope with up to 800 ISO due to their larger sensor and pixel size (compared to compacts) without too much noticeable picture noise.
Click on, and enlarge these four images in turn to see how picture noise becomes more evident as ISO increases – they were taken with 800 ISO, 1600 ISO, 3200 and then 6400, respectively:
As noise increases, the image processor is programmed to apply ‘noise reduction’ to try to smooth it out. But too much reduction can result in images becoming too ‘soft’ and lacking in detail. The best processors work on specific areas of an image, applying different levels of noise reduction accordingly.
The amount of noise that appears in your final images will depend on your camera – image processors and programming vary greatly between manufacturers – but one thing that is consistently true: the smaller the sensor size, the greater the risk of picture noise at low light levels.
With the increased risk of picture noise when choosing high ISO settings, why do camera manufacturers offer them? Because it is better to have the option to get the shot you want, albeit with unavoidable noise, than not to get the shot at all.
The best advice is to get used to the limitations of your camera by trying shots in low light at different ISO settings. This way you will learn what noise levels are acceptable to you for the specific purposes you intend to use your photographs for.
Ultra-high and Extended ISO
Modern system cameras with the latest image sensors and processors often offer sensitivity control up to 12800 ISO or higher. With the ‘digital gain’ option – usually labelled ‘Extended ISO’ – it is even possible to achieve 25600 ISO or above.
These ultra-high-sensitivity levels enable shooting in almost total darkness, but are generally only used in exceptional circumstances because, accordingly, noise levels can be extreme.
The LUMIX Advantage
LUMIX G cameras include not only ‘Auto ISO’ – a mode that sets ISO according to detected brightness – but also ‘Intelligent ISO’, which can detect the movement of a subject and automatically increase the ISO accordingly, to help eliminate blur. In addition, you have the option to cap the ISO setting so that, when shooting in Auto mode, the ISO limit you set will never be exceeded. This is a great option if you want to stop too much picture noise from affecting your shot.
Panasonic’s Venus Engine processor lies at the heart of all LUMIX models. Its latest noise reduction techniques utilise dedicated signal read-out circuitry from the sensor and ‘multi-tap sampling’, which extends the noise detection area by approximately 64 times. Noise level is finely controlled by applying ‘multi-process’ noise reduction, meaning areas of the image are individually analysed and varying levels of smoothing are applied accordingly. The result is dramatically suppressed low-frequency noise without losing sharpness, so images remain pleasingly detailed.
Before moving on to the next tutorial in this module, why not try out the different ISO settings on your camera? You can compare for yourself the levels of noise that are generated in your images as you increase the sensitivity while remaining in the same shooting conditions. Note how, if left on auto exposure, the shutter speed increases the higher the ISO setting becomes.
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