The Bermuda Triangle: Aperture, Shutter Speed, ISO
I call these three concepts the Bermuda Triangle because this is where a lot of photographers get lost. Together, the aperture, shutter speed and ISO triangle determines how much light is allowed into the camera. ISO controls how sensitive the sensor is to that light. Understanding the interplay of these three concepts is key to mastering the technical dimension of photography. You can compose an amazing shot in the viewfinder but if these three settings aren't right you'll be the only one to ever see it.
The flow of light is hard to visualize, so let's try water. Think about spraying water on a sponge with a hose. The hose is the lens. The sponge is the sensor.
The diameter of the hose is equivalent to aperture. A fire hose lets more water through than a garden hose and a 6.3 aperture lets more light in than an 11. No, that's not a typo. Higher numbers mean less light. The lower the number the wider the lens opening. Lower numbers are the fire hose, higher numbers the garden hose. This can be counter intuitive so just memorize the rule.
How long the hose is turned on is equivalent to shutter speed. The longer you open the hose, the more water flows through. The longer the lens opens up, the more light comes through.
Shutter speed and aperture together determine how much light comes in through the lens to the sensor. The most important concept to understand to truly master shutter speed and aperture is to understand how different combinations actually let in exactly the same amount of light. A garden hose left on for a long time can pour out as much water as a fire hose left on for a short time.
An f/5.6 aperture at a 1/60 second shutter speed lets in exactly the same amount of light as an f/8 at 1/30. 1/30th of a second is twice as long as 1/60th of a second, but an f/8 lets half as much light in as an f/5.6. Half the light for twice as long is the same as double the light for half the time.
How absorbent the sponge is equates to ISO. Higher numbers mean more absorbent. An ISO 100 needs a lot of water, ISO 1600 not so much. ISO is important when you hit the constraints of what aperture and shutter speed can do in a given situation. In a low light situation at ISO 100 opening the aperture all the way might still require opening the shutter for several seconds to get enough light to properly expose the shot. Chances are you or your subject can't hold still for that long and the picture will be blurry. Bump the ISO to 200, now you can shoot twice as fast. Bump to 400 and twice as fast again.
A note on the lingo - fast lenses and stops.
Different lenses have different maximum aperture openings. Lenses that can open wider than most are often called "fast" lenses because they can open up to let more light in and allow using a faster shutter speed.
f-stop is the unit of measure for the aperture size. The f comes from the use of focal length in the formula that determines the number (see the Wikipedia article if you are interested in the specifics). A "full stop" is a change in aperture that doubles, or halves, the amount of light. Because of how the math works out, a full stop is a multiple of 1.4. An f/5.6 allows twice as much light as an f/8 (f/5.6 X 1.4 = f/8). Similarly, a full stop of shutter speed indicates twice or half the light. In this case multiplication by two is all that is necessary. 1/30th a second lets twice the light through as 1/60th. ISO full stops work the same as shutter speed, ISO 200 is a full stop higher than ISO 100.
In fully automatic modes, the camera figures all this stuff out as best it can. The semi-automatic modes let you take control of one setting then automatically changes the remaining two. Aperture priority and shutter speed priority are the two most common semi-automatic modes. In manual mode the camera leaves it all up to the photographer.
To really get it, put your camera in manual mode. Pick ISO 100 outdoors, 200 if cloudy, 400 indoors, 800 if the lighting is low. Set the aperture to f/8. Now change the shutter speed until the camera exposure meter indicates a correct exposure (in the center for most cameras). Take a picture and look at it. Set the shutter speed to twice as fast to let half the light in, shoot. The photo is darker, underexposed. Open the aperture one full stop to let twice the light in (f/5.6 if you started at a f/8), shoot. The picture should look exactly like the first. Double the ISO to make the sensor twice as sensitive, shoot. The photo is too bright, overexposed. Now either set the shutter speed to twice as fast or close the aperture one stop (from f/5.6 back to f/8).
Just when you think you are ready to navigate the triangle, each setting also has artistic consequences. Aperture controls depth of field, shutter speed controls whether motion is frozen or fluid, and ISO influences the noise or graininess in the picture. But those are topics for another day...