Shutter speed, aperture size and ISO - together form the three basic components of photographic exposure.  To review, the fundamental core principal is...

"X" amount of light is needed to create a photographic image.

That light reaches the camera's sensor (or film) in two different ways.

First way:  The size of the aperture opening.

Second way:  The amount of time that the shutter stays open.

And ... ISO tells us how sensitive the film or digital sensor is to the light that it receives.

The term ISO stands for International Organization for Standardization, the group that helped establish how the number scheme works.


Remember The Old Bathtub Analogy we used to visualize using different shutter speed/aperture combinations:

Imagine a bathtub:  To fill it up, you need to open the faucet, and let the water run for a while.

You can open the faucet all the way for a short time, or turn it on to a trickle, and allow the water to run for a long time.  Either way, the tub eventually fills up.

Think of the faucet as your aperture setting, how wide you open it is the f-stop.

Think of the length of time you let the water run as the shutter speed.

If you open the faucet wide, the tub will fill in a short period. (Wide aperture/fast shutter speed)

If you open the faucet just a little bit, the tub will take a long time to fill.  (Small aperture/slow shutter speed)

Well, in this analogy, ISO is the size of the bathtub

Small bathtub = fills up fast = very sensitive film/sensor = high ISO number

Big bathtub = takes longer to fill up = less sensitive film/sensor = low ISO number


A fairly typical ISO setting for ordinary daylight photography is ISO 100.  Your camera probably shipped from its manufacturer with this number preset.  If you increase your ISO setting to 200, you have effectively doubled the sensitivity of the sensor.

Let's try a real world example.  Set the ISO to 100.  Set your camera's mode dial to Aperture Priority.  That's "A" on Nikons, "Av" on Canons.  Set the aperture to f/11 - Bryce's favorite.  Take a picture.  You'll notice your camera will probably use a pretty slow shutter speed .. maybe 1/30 of a second ... maybe even slower depending on the lighting in the area.  That's a pretty slow shutter speed to hand hold the camera: Almost impossible not to get a teeny tiny wobble in that amount of time.

If you bump your ISO setting to 200, you camera can make the same exposure with a shorter, faster burst of light.  1/60 of a second instead of 1/30.  Easier to hand hold.

So why wouldn't you want to shoot at the most sensitive ISO all of the time?  The sensitivity has its downside.  The sensor is more sensitive to light; so it is also more sensitive to digital "noise".  At higher ISOs, pictures can exhibit a "grainy" appearance.

There are times that "grainy" appearance will be just what you want for a creative artistic effect.  Think black and white portraits with a "film noir" look, or jazz musicians performing in a dimly lit club.