How the Camera Aperture Works
The aperture is the adjustable opening in the lens. The aperture is like the pupil of your eye. The aperture blades make the opening bigger or smaller, just like your iris makes your pupil dilate larger and smaller to allow in more or less light.
Camera apertures range from f/1 to f/64 and are called f stops. Standard apertures are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, and 45. An f/1 aperture allows a lot of light in, f/16 is a very small aperture.
The pupil of the human eye ranges from about 2mm to 8mm in diameter which compares to about f/8 - f/2 for a camera. An example of an aperture allowing in more light is our own eyes. Remember being out in the bright sunlight and going into a dark room and not being able to see much? Your pupil was shut down to 2 mm or f/8 while you were outside in the bright sunlight to protect the light sensitive rod and cone cells in the back of our eyes. Then when you went inside into the dark room it took a little while for your pupil to enlarge to about 8 mm or f/2 so more light could come in, enabling us to see better. After our pupils opened wide up, we go back out into the bright sunlight. Our pupils are wide open and it hurts our eyes until the iris adjusts our pupils to a smaller aperture to allow less light in.
Red eye is caused by the pupil, or aperture of the eye, being wide open indoors or at night in low light settings. The pupil opens up to allow plenty of light so we can see better indoors. The flash reflects off the back red surface of the eye which contains the light sensitive cells and this shows up on the photograph, showing red where our black pupils are. The red is the back of our eye. Pupils are normally black because it is normally dark inside of our eye. Some cameras have a pre-flash to shrink the pupils down so red eye doesn’t show up in the picture when the actual flash goes off.
The sweet spot of a lens is usually at f/8 for an aperture setting. The sweet spot for an aperture setting is the aperture that most camera lenses produce the sharpest focus. For example, most lenses take sharper photographs at f/8 then at f/22 or f/2.8. Using apertures in the middle range around f/8 produces sharper photographs. You will get more Depth of Field with a small aperture like f/22 but the lens won't be at its sharpest performance. Most point and shoot digital cameras don't go smaller in size then f/8.
Circles of Confusion is an expression used to describe how light reflects off of objects in all directions. The camera lens focuses these “Circles of Confusion” into points onto the sensor. The smaller the aperture the more the aperture reduces these circles of confusion smaller and smaller. This is why smaller apertures have more things in “focus”, or have more Depth of Field (discussed in next section). If too small of an aperture is used then the circles of confusion blend together and cause the image to be a little blurry. This is called Diffraction.
Why are aperture f stop numbers in such an odd sequence? It has to do with doubling or dividing the light in half. Each aperture f stop either doubles the light or cuts in half the amount of light reaching the sensor. Aperture f stops are based on the surface area of a circle (3.14159 x r2). The square root of 2 is 1.414. The first aperture is 1. So 1 x 1.414 = 1.414, f/1.4 is the next aperture. The next aperture is 2 because 1.414 x 1.414 = 1.999 which is rounded off to 2. 1.999 x 1.414 = 2.827 (f/2.8). 2.827 x 1.414 = 3.99 (f/4). As you can see the numbers are rounded off. If you want to try and impress your friends say "Hey Bob, I think I'll use an f stop of 7.97". It might work.