Depth of Field
This to me is one of the most important principles in photography!
Photography has some limitations. Depth of field is one of them. Simply put, depth of field (DOF) is the area in your photograph that is in focus. By adjusting the aperture (the lens opening) on your camera you can vary the area of focus in your image. Below are two images shot at two very different apertures. The first one is shot with the lens wide open (letting in a lot of light) which produces a very narrow DOF. The second one shot with the lens stopped down (which lets in very little light) which produces a deeper DOF. Notice in the first picture the clock in the background is very out of focus. In the second image it's more clear.
If you are not a photographer you might ask why is this important? Here's another example of a portrait image where a shallow DOF is used to make a better image.
In portraiture you want the subject in focus but nothing else. In the above image the foreground and the background are out of focus which draws your attention to the subject! Which is my grandbaby Ari!
By using the aperture priority setting (A) on your camera, you control the lens opening which determines the DOF in your photographs. Lens opening is expressed in f-stops. The lower the f-stop (i.e. 1.8, 2.0, 2.8) the wider the lens opening. The higher the f-stop (i.e. 16,22,32) the smaller the lens opening. When using the aperture priority setting (A) on your camera, you set the aperture and the camera sets the shutter speed automatically. Your camera will give you a correctly exposed image.
But, be aware that anytime you stop down the lens (higher f-stop) you need a slower shutter speed because you are letting in less light with each higher f-stop. As long as you keep your shutter speed above 1/60 th of a second you can hand hold it.
Speaking of higher f-stop there are times when you want everything in focus then use a high f-stop like in the image below. Notice everything is in focus in the image.
If you have any questions please leave me a comment, and I will try to answer it!