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 image plane during the time that it is exposed to the scene.  Exposure is intuitively proportional to Luminance from the scene and exposure time .  It is inversely proportional to lens f-number squared because it determines the relative size of the cone of light captured from the scene.  You can read more about the theory in the article on angles and the Camera Equation.

Exposure in Units of lux-seconds

In the SI system it is defined as Luminous Exposure with symbol and units of lux-seconds (lx-s) so that, simplifying somewhat:

(1)  

where
is Luminance from the scene in cd/m
is exposure time in seconds (aka ‘shutter speed’)
is the effective f-number (aka ‘f/stop’) during exposure
is a lens dependent constant, approximately equal to

with
, lens transmittance factor
, lens vignetting factor as a function of the angle of view of the image point from the optical axis
, light falloff factor as a function of

In practice some of these factors are wavelength dependent.

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 as Photon Density

Exposure can also be expressed as a mean number of photons per unit area () incident on the image plane, where a sensor would be located, while it is exposed to the scene.  In this case:

(2)  

where , and are defined as above and is the incident light spectral power distribution and lens dependent constant approximately defined below for an illuminant with uniform spectral distribution:

with
the lens dependent constant described earlier
683 lumen/W conversion factor
0.267 the integral of the photopic curve over the visible range
mean effective energy of incident photons in joules

For example it is easy to verify with a light meter that most current advanced Bayer Digital Stills Cameras (DSCs) set to ISO100 begin to clip or saturate the raw data at an Exposure of about 1 lx-s under daylight. That would correspond to about 11000 photons/micron within a CFA passband of about 395-718nm.  Assuming the DSC has square pixels 5.9 microns on a side, that would suggest an approximate mean of  382,000 photons incident on such pixels while the shutter was open.  You can read how these figures were derived in a dedicated article.