Exposure Triangle: How To Make Those Nutty Numbers Work

The Exposure Triangle is an important concept to understand especially if you do any fully manual shooting.  It defines the interactions between the 3 determining factors of exposure.

In the previous lesson Stops: What Are All Those Numbers we learned about stops, and the numbering systems of Shutter Speed, ISO, and the Aperture.  Now we are going learn one other concept to help see how they interact together.

This concept is Exposure Value or EV.  The easiest definition of of EV is a value of light.  EV15 defines the light level of a typical sunny day.  Every EV value is separated by, you guessed it, a stop.  So EV16 is one stop more light (double the light) than EV15, EV14 is one stop less (half the light) than EV15.  So you can use EV to describe a light level.  To give us a reference point, EV0 = ISO 100 @ 1 sec @ f/1.  But EV0 does not have to be ISO 100 @ 1 sec @ f/1, but any combination that produces the same exposure.  If we increase the ISO to 200 we would now be at EV1, to get back to EV0 we need to change either the shutter speed to 1/2 sec or the aperture to f/1.4.

Camera: Nikon D80      

Lens: 24-50mm f/3.3-4.5G

Focal Length: 34mm

Shutter: 1/250

Aperture: f/4

ISO: 1600

Flash: None

Now lets see this in action. It isn't hard to see that even though the subject is the same, these two photos are drastically different. These shots were taken one after the other in the exact same conditions, but using the Exposure Triangle allows the manipulation of the shutter speed to either stop the water, or to have a nice smooth flow.

Camera: Nikon D80      

Lens: 24-50mm f/3.3-4.5G

Focal Length: 34mm

Shutter: 1/15

Aperture: f/4

ISO: 100

Flash: None


So the first shot is taken at ISO1600, 1/250sec, f/4 which is EV8.  To get to 1/15 of a second we need to slow the shutter speed by 4 stops (1/125, 1/60, 1/30, 1/15).  But just slowing the shutter speed won't do the job.  Since we slowed the shutter speed by 4 stops we are now 4 stops over exposed and are now adjusted for EV4.  What I chose to do was slow down the sensor by 4 stops (ISO: 800, 400, 200, 100) to get the exposure correct again and back to EV8.  I could have also reduced the aperture by 4 stops (f/5.6, f/8, f/11, f/16), or a combination of the two.

In this case our light level being EV8 gives us numerous options, these are some of the options for EV8:

• ISO100, 1/15sec, f/4
• ISO200, 1/30sec, f/4
• ISO200, 1/15sec, f/5.6
• ISO800, 1/30sec, f/5.6

All these provide the same exposure level but can dramatically effect the picture.  Look for the next article Aperture: Using The Aperture To Focus Your Viewer.

Thanks for reading!