When capturing a typical photograph light from one or more sources is reflected from the scene, reaches the lens, goes through it and eventually hits the sensing plane.
In photography Exposure is the quantity of visible light per unit area incident on the sensing medium (typically film or a digital sensor) during the time that the sensing medium is exposed to light from the scene (often referred to as exposure time or shutter speed). It is proportional to Luminance from the scene and exposure time. It is inversely proportional to the square of the f-number (also referred to as f-stop or relative aperture).
Exposure in Units of lux-seconds
In the SI system it is defined as Luminous Exposure with symbol Hv and units of lux-seconds (lx-s) so that
is Luminance from the scene in cd/m
t is exposure time in seconds
N is the effective f-number during exposure
q is a lens dependent constant better defined below:
the lens transmittance factor
the lens vignetting factor as a function of the angle of view of the image point from the optical axis
the light falloff factor as a function of
Therefore for a given lens and Luminance from the scene Exposure can be held constant or varied by suitable choice of shutter speed and f-number pairs. This is the basis of the Exposure Value system.
Exposure in Units of Photons per Unit Area
Exposure can also be thought of as a certain number of photons per unit area () incident on the sensor while the shutter is open. In this case:
where , and are defined as above and is the light spectral power distribution and lens dependent constant approximately defined below for an illuminant with uniform spectral density:
the lens dependent constant defined earlier
683 lumen/W conversion factor
0.267 the integral of the photopic curve over the visible range
the average energy of incident photons in joules
For instance DxOMark.com measures that with a daylight illuminant most current advanced Bayer Digital Stills Cameras (DSCs) clip or
saturate the raw data at an Exposure of around 1 lx-s . That would correspond to about 11260 photons/micron within a CFA passband of about 395-718nm. Assuming the DSC has square pixels 5.9 microns on a side that means that when the raw data from a pixel just clips there were approximately 392,000 photons incident on it while the shutter was open. You can find the detail of how these figures were derived in a dedicated article.