How different sensitivity settings can influence image quality.
It’s common for today’s cameras to support ISO sensitivities of 25,600 or even higher. Yet, while many photographers see this as advantageous, few understand the real relationship between sensitivity and image quality. And it’s more complex than you might think. ISO sensitivity is one of the key parameters that comprise exposure, the other two being the lens aperture and the shutter speed setting. Correct exposure requires balancing all three to deliver the best picture quality.
It often boils down to deciding what’s the lowest ISO setting you can use, given the aperture and shutter speed settings you’ve selected (or been forced to accept) before you encounter other problems like potential for image noise and the dynamic range recorded, along with management of camera shake and aesthetic issues associated with depth of field.
Image Noise
You can’t avoid image noise in digital photographs. It’s caused by random fluctuations in electronic signals associated with the behaviour of light as photons. Its effect is proportional to the number of photons a sensor receives in a given length of time.
The number of ‘usable’ photons can be reduced by heat (thermal noise) and the sensor’s ability to ‘read’ signals (read noise). Thermal noise is quite easy to control by keeping the ambient temperature low, while read noise has been reduced in the latest image sensors and processors.
The three factors critical to exposure control and the aspects of the image they influence. Lens aperture: controls the area across which light can enter the camera. Shutter speed: controls the duration of the light exposure. ISO sensitivity controls the response of the camera’s sensor to the amount of light delivered by the aperture/shutter speed combination.
Less controllable is ‘shot’ noise, which is proportional to the square root of the number of photons the sensor actually receives.
When the sensor receives 100 photons in the specified time (‘exposure’), the signal-to-noise ratio is 10:1. If it receives 10,000 photons in the same exposure, the signal-to-noise ratio becomes 100:1. This explains why noise levels are higher when you shoot in dim lighting.
In most cameras, the relationship between ISO and image noise is predictable – and fairly linear. As ISO is increased, you are likely to see more ‘shot’ noise due to the increased randomness of the photons hitting the sensor.
All cameras have a default base level – determined by the manufacturer – at which image noise is at its lowest. For most cameras, it’s ISO 100, although some set the base at ISO 125 or even ISO 200. Pushing sensitivity up from the base level doesn’t have much effect on the amount of noise you can see (visible noise) – to a certain point.
For ‘full frame’ DSLRs that point is somewhere between ISO 800 and ISO 3200, although you will probably have to look very closely to find it at settings as high as ISO 6400 with high-end professional cameras. Cameras with APS-C sensors might produce discernible noise at ISO 1600 but they would probably have none at ISO 800. You might find noise in shots from M4/3 cameras at ISO 1600 but shots taken at ISO 400 will be effectively noise-free.
These figures shouldn’t be taken as definitive since most camera manufacturers automatically increase the noise reduction processing as sensitivity is increased beyond the ‘non-visible’ noise level. In the process, fine details may be blurred as noise is ‘smoothed’ away.
Many higher-featured cameras provide settings to control the aggressiveness of the in-camera noise reduction. It’s up to the photographer to determine how much processing is required to suppress noise patterns while preserving the detail they require. And that will depend on whether the noise will be detected when the image is viewed.
This, in turn, will depend on how the image is displayed and it’s essential to compare like with like because the viewing ‘platform’, output size of images and the distance from which they are viewed will all influence how much noise you can detect. Images viewed on screens are more likely to show visible noise than those that are printed, and large images viewed up close will inevitably appear less sharp and detailed than smaller images viewed from a distance.
Dynamic Range
Dynamic range in photography refers to the difference between the brightest whites and the deepest blacks in an image. Ideally, we’re looking for a smooth tonal transition that records tonal detail from shadows to highlights.
The goal is to extend the range to include as many intermediate tones as possible with smooth gradations between them. The wider the dynamic range, the greater degree to which detail will be retained.
Photography requires us to understand two crucial dynamic ranges: the brightness range in the subject; and the dynamic range your camera can record. The way images are viewed should also be taken into account; screens have a much wider dynamic range than paper.
Interestingly, human eyes can perceive a much wider range of tones than cameras can record. Consequently, we are often disappointed to find our photographs have ‘clipped’ highlights and/or shadows and compressed tones between them.
This is particularly true of cameras with small sensors, such as those in smartphones and tablets, which tend to produce pictures of sunny scenes with all the lighter tones ‘blown out’ to stark white and all the darker tones ‘blocked up’ to black. Cameras with larger sensors have larger photosites that can capture more photons and can therefore record a wider tonal range.
Photosites can be visualised as buckets for capturing photons. If only a few photons are captured, the signal is overwhelmed by the shot noise, whereas if more photons arrive than the photosite can accommodate, it is effectively ‘saturated’.
Ideally, each photosite should be able to record the maximum number of photons it can accommodate, and the minimum number required to produce a signal. Most modern cameras include settings for tweaking the recorded dynamic range, usually with algorithms that map the image signals differently in shadow and in highlight areas, boosting exposure in shadows and suppressing it in highlights to distribute the light levels in the subject across the recording range of the sensor. Alternatively, high-dynamic-range (HDR) settings capture and combine multiple exposures to average out the lighting across the image brightness range.
While the dynamic range recorded is generally optimal at the lowest ‘native’ ISO setting, some cameras include an extended low ISO setting which, although it usually produces less noise, will also decrease the dynamic range. This happens because the setting effectively exposes for the same time as it would at the lowest native ISO setting and pulls all values back by the difference between the two ISO values. This almost always causes highlight values to clip and effectively reduces the dynamic range in the shot.
These two pictures illustrate the effect of the ISO setting on the recorded dynamic range. In the top picture, ISO sensitivity was set at the extended ‘Low’ value (equivalent to ISO 64). In the lower picture the lowest ‘native’ sensitivity of ISO 200 was used. Note the difference in highlight details in the two shots.
Both these portraits were taken with the same ‘full frame’ professional DSLR camera. The picture on the left was shot at ISO 6400, while the one on the right was taken at ISO 102,400, the highest sensitivity setting. It’s difficult to discern noise in the shot taken with the lower sensitivity but noise is very obvious in the high-sensitivity shot.
This portrait was taken with a compact camera with a 1-inch type (12.8 x 9.6mm) sensor using a focal length equivalent to 130mm in 35mm format. Without stabilisation, the minimum shutter speed should have been 1/130 second but effective stabilisation and good shooting technique enabled a shutter speed of 1/20 second to be used. Dim indoor lighting required a sensitivity setting of ISO 1600, which resulted in good sharpness with minimal observable noise.
Camera Shake
Camera shake has become less problematic with the almost universal adoption of image stabilisation (IS). It doesn’t matter whether the stabilisation is in the camera or the lens (or both); it can enable you to hand-hold the camera at between 2.5 to 4 f-stops slower shutter speeds that you would otherwise use without IS.
Shooting with the camera hand-held improves with experience; some photographers can use shutter speeds as slow as 1/10 second with a 200mm lens, while others are hard-pressed to keep the same lens steady at 1/100 second.
As a rule of thumb, without IS you should be able to keep a camera steady at the reciprocal of the focal length you’re shooting with. For example, with a 50mm focal length, the slowest shutter speed for hand-held shooting would be 1/50 second. With a 100mm focal length, it’s 1/100 second.
Image stabilisation that provides two stops of shake correction would allow you to use a
shutter speed of 1/12 second with the 50mm lens or 1/25 second with the 100mm lens.
In low light levels you might decide to use an ISO setting of 1600 to achieve the 1/100 second shutter speed with the 100mm lens. With two stops of shake correction you could reduce the ISO to 400 with the same 1/100 second shutter speed to take advantage of lower shot noise levels.
These two close-ups, taken with a macro lens, show the importance of the lens aperture setting for controlling background blurring. The shot on the left was taken with the maximum aperture of f/3.5 for the macro lens used (30mm focal length in M4/3 format), while the shot on the right was taken with an aperture of f/13.
Depth of Field
ISO settings also provide scope for controlling depth of field in shots. Here again, a lower sensitivity will enable you to use the lens aperture wide open for blurring backgrounds, while increasing sensitivity provides scope for stopping down to increase sharpness throughout the scene.
When you can’t use the maximum lens aperture while shooting in very bright conditions, neutral density (ND) filters enable you to reduce the amount of light reaching the sensor if the lowest ISO setting isn’t low enough. Some cameras include built-in ND filters, although they’re an optional addition for most interchangeable-lens cameras.
Article by Margaret Brown – see Margaret’s photography pocket guides
