Aperture Explained.

All articles copyright 2007 Jeff Coleman
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Aperture explained.

The aperture blades of a lens, operate like the iris of the human eye. Blades open and close to make a large or small hole that light passes through. Aperture values are tied to a numbered scale. A low aperture number of F2.8 means the aperture blades are open to their widest setting, letting in the most light. An aperture of F22 means the hole in the lens is very small, letting in a small amount of light.

This image shows the aperture blades of a lens.

Depth of field refers to how much of an image is in focus. An image shot at F22 will be in sharp focus everywhere. If you focus on something at F2.8 your subject may be in focus but everything in front of and behind your subject will be out of focus. If you use a lens capable of F1.4 to shoot a portrait, the subjects eye might be in focus while the end of their eyelash is not.

Sample of the same image repeated at three different depth of fields.

To give you an idea of light requirements at different aperture values... I used an 800 watt second studio strobe to light these batteries. For the F1.4 image I set the light to it's lowest setting which was still too much, so I added a diffuser between the light and subject. On the F16 image I removed the diffuser and set the light to full power. Another way to put it is like this. At F1.4 I could easily light a huge gymnasium with this one light. At F16 that same light would barely illuminate anything beyond 5 feet.

When photographing landscapes it is best to use an F stop of F11 to F22 because you want everything to be in focus. Since F22 closes the aperture down to a very small size, effectively reducing the amount of light hitting your cameras sensor, you will have to increase the amount of time your shutter is open. If you're shooting in bright sunlight at mid day you might have enough light to hand hold your camera. Most of the time you will need a sturdy tripod when using F11 or above.
When photographing people and most other subjects, a blurred background is preferred. Use F2.8 or lower for a single person. For two or more people use F5.6 to F8.
Aperture and Shutter speed are tied to each other by the laws of physics. If you change one, you must change the other. Want a fast shutter speed for sports? Open your lens up to it's maximum aperture value of F2.8 or lower. Want everything in focus? Stop down your lens to an aperture of F11 or more, lower your shutter speed and break out the tripod

Aperture values alone do not determine depth of field. The distance between you and your subject plays a part. For instance, I've photographed most of my small product images with a 100mm macro lens. These lenses achieve their high magnifications by focusing at much closer distances than traditional lenses. When I photograph a ring from a few inches away, I can not get the entire ring in sharp focus even at F32. The image of the batteries above was photographed from about 1 foot away. If I shot from 5 feet away, the F1.4 image would look more like the F8 image.