But there is a brightness triangle!
Exposure and the rendered rendered image brightness are
not synonymous. Exposure and the rendered image brightness are only identical when a camera's ISO parameter is set to the base (native) sensitivity setting.
For digital cameras, exposure is the amount of energy measured by the sensor when the shutter is open. This is determined by only three things:
- Scene luminance
- Shutter time
- Aperture
Exposure determines the S/N for the data which in turn determines the information-content's uncertainty. Optimizing exposure minimizes uncertainty and maximizes perceived, rendered, image quality.
ISO is irrelevant. ISO has no affect on the sensor sensitivity(1). The raw file data S/N may or may not depend on the camera ISO setting(2). The camera ISO parameter is a simply calibrated exposure index based on one of five industry standards.
However, for any combination of shutter time and aperture, the meter will estimate the appropriate amount of gain (ISO) required to render a bright image when exposure is limited by camera motion, subject motion and, or DOF. This change in gain is not necessarily required to maximize perceived image quality(3).
Over exposure occurs when one or more sensor photo-diode sites generates enough photoelectrons to exceed their maximum electrical charge capacity. All useful information for that pixel is lost.
Under exposure occurs when the opposite happens. Some amount of sensor under exposure is almost always unavoidable. However, unnecessary under exposure always degrades perceived image quality. The most common cause of unnecessary under exposure is not using the lowest practical camera ISO setting.
After the shutter closes the photo-diode sites' electrical charge is emitted as a spatial array of DC voltages. Those voltages are digitized as integers and written to an in-camera raw file.
When the shutter time and aperture are set for optimum exposure, the result is a rendered image with an aesthetically appropriate brightness. Optimum exposure means the shutter and aperture were set to retain highlight region information required to achieve the photographer's goals. This means unimportant highlight regions can be intentionally over exposed.
Often convenience, subject motion, or depth of field mandates the sensor be under exposed. The result would be a rendered image that is too dark. Now, the gain DC voltages' gain can be increased before digitization. Changing the camera ISO parameter changes the gain level. As long as the gain does not exceed the maximum ADC's signal level threshold, the image brightness will be aesthetically appropriate.
The role of ISO is to achieve a useful image brightness.
The triangular relationship between ISO parameter, shutter time and aperture holds as for rendered image brightness. It doe not apply to exposure.
1. Recently sensors with dual conversion gain became available. In bright light the phtotodiodes' conversion gain is set for maximum dynamic range. In low light (higher ISO settings) the gain is changed to increase sensitivity.
2. For newer cameras increasing signal gain (ISO) does not significantly decrease the electronic noise levels. These cameras are described as ISO invariant or ISO-less. For older cameras increasing gain can decrease the electronic noise levels. With the former, image noise is dominated by photon noise. For the latter electronic noise increases as exposure decreases.
3. With raw files increasing signal gain after the shutter closes does not significantly increase perceived image quality. This is not the case for in-camera JPEGS.
