The Exposure Waterfall - A Simple Graphical Explanation of Exposure
Prepared 2015-10-13 by Bill Claff

Exposure, determining how much light to collect when taking a picture, is not a simple matter.
Certainly not simple enough to reduce to a triangle as some have attempted.
But the concepts are simple if you approach them in a methodical way.

In this article I cover how the camera determines exposure based on photographer settings and internal metering.
Once these concepts are mastered then influencing exposure is easier to grasp.

This article uses a nomenclature identical to that in my more technical article entitled Additive Photographic Exposure System (APES).
EV stands for Exposure Value. It is also commonly used for any photographic values stated as a logarithm of 2.
One stop is also one EV.
If this doesn't make sense to you, don't worry; it's not central to understanding this article.
I use a subscript to distinguish five different EV values that affect exposure.
EV_B is Brightness, scene luminance not to be confused with the brightness of the final image.
EV_X is eXposure compensation, an optional adjustment to EV_B.
EV_S is Speed, how sensitive the camera is to light. (This is a sensitivity as observed in the raw data and not simply in the pixel.)
EV_A is Aperture, F-Number.
EV_T is Time, shutter time.

How these values come into play depends on your shooting mode.
This is one of the reasons that triangle explanations don't work correctly.
Regardless of the shooting mode, the camera will attempt to make EV_B – EV_X + EV_S = EV = EV_A + EV_T
If the left hand equation does not match the right hand equation this is called the Exposure Difference.

We can represent this process graphically as a waterfall chart that I call the Exposure Waterfall.

Before discussing the chart let's review what happens depending on the shooting mode:

Shooting Mode	Photographer Sets	Camera Determines
Program	EVS and EVX	EVA and EVT
Shutter Priority	EVS, EVT and EVX	EVA
Aperture Priority	EVS, EVA and EVX	EVT
Manual Mode	EVS, EVT, EVA and EVX	None
Program with ISO Auto	Initial EVS and EVX	EVA and EVT then EVS
Shutter Priority with ISO Auto	Initial EVS, EVT and EVX	EVA then EVS
Aperture Priority with ISO Auto	Initial EVS, EVA and EVX	EVT then EVS
Manual Mode with ISO Auto	Initial EVS, EVT, EVA and EVX	EVS

Exposure bracketing, in any mode, varies the EV_X term from frame to frame over a series of frames.
Flash bracketing in any mode, has the side effect of varying EV_B from frame to frame over a series of frames.

Note that for most cameras any "scene" or "auto" mode generally behaves like "Program".

I have constructed an interactive Exposure Waterfall chart to aid in understanding these interactions.

Here is an example of the Exposure Waterfall showing how Shutter Priority works:

The green shows that the photographer set ISO to 100, shutter to 1/125s, and exposure compensation to 0 EV.
The gray shows the meter reading, 8660 lux (which is full sunlight).
The camera computed an aperture of f/8 to bring EV back to zero for an exposure difference of 0 EV.

All of the shooting modes hinge in some way on metering (scene Brightness).
Once you fully understand the Exposure Waterfall principles then you're equipped to understand how metering modes and exposure compensation can be used to achieve the correct exposure for your situation.