What Is Exposure

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.

Exposure

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

(1)   \begin{equation*} H_v = q  L_v  \frac{t}{N^2} \; \: \text{lx-s} \end{equation*}

where
L_v is Luminance from the scene in cd/m^2
t is exposure time in seconds
N is the effective f-number during exposure
q is a lens dependent constant better defined below:

 q = \cfrac{\pi}{4} \cdot T \cdot V(\theta) \cdot cos^4(\theta)

with
T the lens transmittance factor
V(\theta) the lens vignetting factor as a function of the angle of view \theta of the image point from the optical axis
cos^4(\theta)  the light falloff factor as a function of \theta

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 (N_{ph}) incident on the sensor while the shutter is open.  In this case:

(2)   \begin{equation*} N_{ph} = q_1 L_v \frac{t}{N^2} \; \: \text{photons/micron}^2 \end{equation*}

where L_v, t and N are defined as above and q_1 is the light spectral power distribution and lens dependent constant approximately defined below for an illuminant with uniform spectral density:

q_1 = \cfrac{q}{683 \cdot 0.267 \cdot E_{ph} \cdot 10^{12}}

with
q the lens dependent constant defined earlier
683 lumen/W conversion factor
0.267 the integral of the photopic curve over the visible range
E_{ph} 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 ^2 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.