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.
EVB is Brightness, scene luminance not to be confused with the brightness of the final image.
EVX is eXposure compensation, an optional adjustment to EVB.
EVS 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.)
EVA is Aperture, F-Number.
EVT 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 EVB EVX + EVS = EV = EVA + EVT
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 EVX term from frame to frame over a series of frames.
Flash bracketing in any mode, has the side effect of varying EVB 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.