Auto Exposure algorithms

INTRODUCTION


What is Exposure?

In photography, exposure is the total density of light allowed to fall on the photographic medium (photographic film or image sensor) during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance over a specified area.

One of the main problems affecting image quality, leading to unpleasant pictures, comes from improper exposure to light.Digital consumer devices make use of ad-hoc strategies and heuristics to derive exposure setting parameters.It is possible to abstract a generalization and to define the best exposure that enables one to reproduce the most important regions (according to contextual or perceptive criteria) with a level of gray or brightness, more or less in the middle of the possible range.

One of the main issue of is devoted to give an effective overview of the technical details involved in:

 Exposure settings of imaging devices just before acquisition phase (i.e. Pre-Capture phase)

Content – dependent enhancement strategies applied as post – processing

Advanced solution where multi – picture acquisition of the same scene with different exposure time allows to reproduce the Radiance map of the real world. 

Exposure metering Techniques :

What is metering?

Metering means to ensure that u properly expose ur scene

   Metering techniques built into the camera are getting much better with the introduction of computer technology but limitations still remain. In many devices, the selected exposure value is the main processing step for adjusting the overall image intensity that the consumer will see.Integrating exposure metering function into the main sensor (through-the-lens, or TTL, metering) may reduce system cost.The imaging community uses a measure called exposure value (EV) to specify the relationship between the f-number, F, and exposure duration, T:

 Classical Approaches:

The metering system in a digital camera measures the amount of light in the scene and calculates the bets-fit exposure value based on the metering mode.Automatic exposure is a standard feature in all digital cameras.Cameras generally allow the user to select between spot, center-weighted average, or multi-zone metering modes.

 Spot metering:

Spot metering allows user to meter the subject in the center of the frame. Only a small area of the whole frame (between 1-5% of the viewfinder area) is metered while the rest of the frame is ignored).Spot metering is very accurate and is not influenced by other areas in the frame. It is commonly used to shoot very high contract scenes.

Partial Area Metering:

This mode meters a larger area than spot metering (around 10-15% of the entire frame), and is generally used when very bright or very dark areas on the edges of the frame would otherwise influence the metering unduly.This method is probably the most common metering method implemented in nearly every digital camera: it is also the default for those digital cameras which don’t offer metering mode selection.

Center – Weighted Average Metering

 This method is probably the most common metering method implemented in nearly every digital Camera: it is also the default for those digital cameras which don’t offer metering mode selection.

Average Metering

 In this mode the camera will use the light information coming from the entire scene and averages for the final exposure setting, giving no weighting to any particular portion of the metered area.

Topics discussed related to this area are

Advanced approaches for Metering

Skin detection

Exposure correction results

High dynamic range image construction