Why is the target luminance adjusted, rather than exposure compensation? What is the difference between the two?
1. Target Luminance
(1) Definition:
The theoretically ideal brightness value calculated by the ISP through sensor data and scene analysis (such as metering statistics, histogram distribution). It serves as the core output target of the Auto Exposure (AE) algorithm. It is typically based on neutral grey (18% grey) or brightness statistics of specific scenes (e.g., face priority, highlight protection). It represents the brightness benchmark that the ISP considers "correct exposure" should achieve in the current scene.
(2) Characteristics:
Automatically Generated:Calculated in real-time by the ISP's AE module, requiring no user intervention.
Dynamically Adjusted:Changes with scene lighting, subject reflectivity, and metering area (e.g., evaluative/spot metering).
Technical Implementation:Achieved by adjusting the three exposure parameters (exposure time, gain, aperture) to approach this luminance value.
2. Exposure Compensation
(1) Definition:
An instruction from the user or a higher-level algorithm to actively shift the target luminance automatically calculated by the ISP (e.g., +1 EV, -1 EV). It is a subjective correction to the auto exposure result. Its essence is to make the final image brightness brighter or darker relative to the target luminance.
(2) Characteristics:
Human Intervention:Manually adjusted by the user based on specific needs (e.g., preserving highlights/shadows) or scene particularities (e.g., snow scenes, night scenes), or adjusted via a higher-level algorithm.
Relative Adjustment:An offset based on the target luminance (e.g., +1 EV = making the final brightness one stop higher than the target value).
Implementation Method: The ISP recalculates and adjusts exposure parameters after the AE calculation, applying the compensation value.
(3) Core Differences
| Aspect |
Target Luminance |
Exposure Compensation (EV±) |
| Origin |
Result of the ISP's auto-exposure algorithm calculation |
An offset instruction actively input by the user/higher-level algorithm |
| Target Role |
Serves as the baseline brightness for auto-exposure |
A subjective correction applied to the target luminance |
| Adjustment Logic |
Based on physical metering principles (e.g., 18% grey, histogram distribution) |
Based on user intent or empirical rules for specific scenes |
| Dynamicity* |
Dynamically changes with the scene |
A fixed offset (unless manually changed by the user) |
| Technical Implementation |
Directly influences the initial selection of the three exposure parameters |
Applies an offset *on top of* the calculated target luminance |
