UNDERSTANDING ISO
DEFINITION
In digital photography, ISO is the light sensitivity of the image sensor. The term was originally used to describe film speeds, 100, 200, 400, and 800. Higher film speed means greater light sensitivity and consequently more grain or noise present.
The advantage of shooting at a higher ISO speed is the ability to shoot at a faster shutter speed, which means it is less likely to blur a shot in low light circumstances. The disadvantage of shooting at a higher ISO is increased noise and less dynamic range.
NOISE REDUCTION
Most cameras use internal software to reduce the amount of noise present at higher ISO speeds. When shooting in JPEG, the camera converts RAW image data into JPEG (14 bits into 8 bits per color channel per pixel). It evaluates the camera settings and if higher ISO speeds are used, the camera implements noise reduction algorithms to eliminate noise artifacts. This allows camera manufacturers to market higher ISO capabilities. However, noise reduction algorithms soften image edges and ultimately reduce sharpness. Noise reduction is always implemented in smaller point and shoot and cell phone cameras because they are always inherently noise because of their small sensors.
True ISO performance is measured without noise reduction and evaluated in RAW image sensor data. Nikon used this misunderstanding to market their Nikon D7000, DX sensor camera. They claimed it could match the performance of their much older D700, FX (larger) sensor camera.
SIGNAL TO NOISE RATIO (SNR)
Signal to noise ratio in digital photography describes the ratio between image quality and noise artifacts. It should be measured in various exposure levels. Typically images that are underexposed possess more noise because of the logarithmic characteristics of the sensor. Fewer bits are allocated for shadow detail than are for highlight detail.
TRUE ISO
ISO speed is a standard setup by the International Standards Organization. The specification is ISO 12232:2006. This helps keep the exposure and noise consistent between camera manufacturers. However, the specification provides 5 different techniques for camera companies to determine ISO speeds, which leaves some ambiguity. I have seen some variations between camera manufacturers, which claimed ISO speeds which did not achieve the correct light sensitivity.
Noise is measureable, regardless of what ISO specs are marketed by camera manufacturers. Third party websites, like DXO optics, provide details on how well cameras perform ISO based on signal to noise ratios. Although I am still trying to determine the method in which they measure signal to noise ratio, the statistics on the website appear to be unbiased and consistent. DXO optics rate cameras based on a signal to noise ratio of 30dB.
CONCLUSION
ISO, in digital photography, describes a sensor’s sensitivity towards light.
Higher ISO speeds allow use of faster shutter speeds, which reduce the likelihood of blurred images.
Consequently, higher ISO speeds decrease dynamic range and increase noise. Bigger sensors (FX) usually provide better ISO performance because their larger sensor area can capture more light.
ISO performance is measureable. Although camera manufacturers market greater ISO performance, the image quality can be measured using signal to noise ratios.