Exposure is the very heart of photography, drawing with light.  Volumes have been written about it.  Here’s an attempt to present exposure in a nutshell, or bucket.

When controlling exposure you have three main aspects available as tools.  A balanced exposure needs to have these aspects remain in balance.  If one control aspect changes at least one other also needs to change in order to maintain the balance.

ISO:  controls sensitivity to light

Expressed as, for example,  100 speed,    200    400    800

Aperture:  controls depth of field

Expressed as, for example,  f/1.4    f/2.0    f/2.8    4    5.6    8    11    16    22

Shutter speed:  controls subject motion

Expressed as, for example,  1/4 of a second,   1/8 sec.,   1/15   1/30                     1/60   1/125   180    250

Among the contributors the math can be confusing.  Here’s a helpful, simplified explanation of their effects:

ISO:  As the number increases the sensitivity to light increases or the number of electrons activated on the sensor.

Aperture:  As the number increases the depth of field increases:

YouTube video from MichaelTheMentor.com

Shutter speed:  As the number increases the ability to freeze motion increases.

Michael Coles has an extensive explanation of apertures, f/stops.
His “favorite analogy for exposure is filling a bucket of water.  A bucket is of fixed size and needs a certain amount of water to fill it” in a given period of time.   This is like a digital camera sensor recording an image.

“To fill your bucket, you can pour a small stream of water for a long time or a fast (BROAD) stream of water for a short time.  Either way, you end up with the same amount of water.  In photography, the size of the stream of the water is analogous to the f/stop, the length of time you pour is analogous to the shutter speed, and the size of the bucket is analogous to the film speed (ISO).  Broadly speaking, from the bucket’s point of view, it doesn’t matter which combination of stream size and length of time you choose as long as the right amount of water ends up coming in.  Film and digital sensors are the same; within limits, they are indifferent to the combination of time and amount of light as long as the right amount of light eventually arrives.”

Another exposure analogy some people may prefer is to a teeter totter:

YouTube video from Dan Feildman at ClickHerePhotographyTips.com

To try to understand the effect of aperture and shutter speed we’ll maintain the ISO at 200.

Sunny 16 example:   The “sunny 16 rule” means that a normal outdoor exposure on a bright, sunny day would use an aperture of f/16 and a shutter speed to match the ISO, or in this case 1/180 sec. which is close to the 200 ISO.  If you wish to have more of the scene in focus, to increase the depth of field, you may increase the number of the f/stop to, say, 22.  To balance that, you’ll need to slow down the shutter speed to 1/120 sec. or 1/90.  If you’re in Iowa it may very well be windy.  You may wish to record a moderately moving subject like trees or tall grasses, to increase the ability to freeze motion.  In that case you could change that original shutter speed of 1/180 to 250 but you will need to then change the aperture to f/11.

To return to Matthew Cole’s bucket analogy, to fill the bucket in the same amount of time (the balanced photographic exposure) you’ll need to keep the size of the stream of water in balance with the speed of the flow of water (the relationship of the aperture to the shutter speed).  The Internet offers several handy exposure calculators.

Here are a couple of samples.  These images were taken at the Iowa Great Lakes Maritime Museum in Arnolds Park IA (USA).

The camera was on a tripod with 28 mm lens (45 mm 35mm equivalent), ISO set at 100, and focus set on the gold-trimmed motor.   For the first image the camera chose a shutter speed of 1/6 sec. and aperture of 3.5.   For the second image I manually set the shutter speed at 3 sec. and an aperture of 13 (six stops different for each).

shallow depth of field from aperture of 3.5

shallow depth of field from aperture of 3.5

deeper depth of field from aperture of 13

deeper depth of field from aperture of 13

If  you’re new to photographic exposure control I hope this is beneficial.

To those of you with experience, do you have an explanation that works for you?

Happy snapping!