Exposure triangle explanation please.

Started Jul 9, 2013 | Questions thread
Jeff Veteran Member • Posts: 5,122
It's only an explanatory graphic ...

If you understand the three factors -- shutter duration, f-stop, and ISO -- then the so-called 'exposure triangle' has done it's job. It is simply an explanatory device for introducing concepts, don't try to read too much into it.

In the end, what you're trying to do is create a final image with the desired brightness. Most often you're trying to create an image that more or less replicates the brightness you saw in the original scene. There are four factors contributing to image brightness

  • light incident on the scene
  • the light gathered by the lens determined by the f-ratio (e.g, 1/2.8, 1/4, etc.)
  • the length of exposure determined by the shutter duration (e.g, 1/200 sec.)
  • the net sensitivity of the sensor determined by the ISO (e.g., ISO 100)

Increasing any of these increases the brightness of the final image.  Controlling each of these individually allows creative control over your final image, but in the end they have to blend together to create an image with proper brightness.

The light incident on the scene may or may not be controllable with flash or artificial light, depending on circumstances  This is a complex topic, for now assume the incident light is a given.

Note that we're talking about light incident on the scene, not light reflected from the scene.  That's because we want a black cat in a coal mine to produce a dark image, and a white cat on snow to produce a bright image.  By exposing for the incident light we'll correctly capture those different scenes in the final image.

Using larger f-ratio (i.e, smaller f-number which is the number in the denominator of the f-ratio) means more light is gathered from the scene.  It also decreases depth of field so this is a creattive control, too.

Longer shutter duration also gathers more light from the scene.  It also controls the degree of motion blur, so it is also a creative control.

Finally, ISO is an indicator of the combined sensitivity of the camera's sensor and electronics to light.  Higher ISO allows more latitude for creative control of lighting, f-ratio, and shutter, but comes at the cost of increased sensor noise.

While each of these four factors has its own units of measurements, it's very convenient to use a powers of two to express changes in each factor.  Each doubling increases the image brightness by 'one-stop' ;

  • Incident light is often measured in EV. Each EV is equivalent to one-stop.
  • One click in f-ratio (e.g.  f/4 to f/2.8)  is an increase of one-stop.
  • Each doubling of shutter duration (e.g., 1/400 to 1/200) is an increase of one-stop
  • Each doubling of ISO (e.g., ISO 200 to ISO 400) is one-stop

These are additive.  If you increase f-ratio by two-stops to get a shallower depth of field, then you  need to compensate by reduce shutter and ISO by a total of two-stops. Or if you want to reduce blur in a scene and decrease shutter duration by the equivalent of three stops, you can compensate with increasing f-ratio or ISO by a total of three stops.

It's just that easy.

That was quick and off-the-cuff, but an approach that I find far more useful than an ill-defined 'exposure triangle'.

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