Excerpt
Aperture and depth of field explained
A key technique that controls how much of your photograph is in focus
Aperture is one of the three main aspects of photography that help control the brightness of your photographs, and also how sharp your photos are throughout the scene. Aperture is responsible for how much of your scene is blurred or sharp; it is also known as an f-stop. This is a reference to the early days of photography when the aperture was actually adjusted by using cards with different sized holes in them. They were slotted into the camera behind the lens and were known as 'stops'. That terminology has stuck and it has been with us ever since.
The f number
These days, cards have been replaced with mechanical diaphragms which use a series of curved blades that can expand or contract to alter the size of the aperture and allow varying amounts of light into the camera to alter the exposure of your image. A very small aperture will let only a very small amount of light into the camera, whereas a large aperture lets much more light fall onto the camera's sensor. The numbers used to indicate the size of the aperture can run from f/1.4, which is a very large aperture, all the way to f/32, which is a very small aperture. That's the thing to remember: a large f-number is equal to a small aperture and vice versa.
Depth of field
Not only does the aperture help control the exposure of a photo, it also adds another quality to your photos, in the way it handles depth of field. Depth of field (DOF) is simply an area of a scene that is in sharp focus. When you hear someone refer to a deep depth of field they mean that, from the foreground of the shot out to the far distance, the image is acceptably sharp all the way through. Deep DOF is achieved by using very small apertures such as f/16 up to f/32. Conversely, shallow depth of field means that only a very small area of the shot is in focus. The near foreground and distant backgrounds will be out of focus. Shallow DOF is created by using very large apertures such as f/1.4 to f/2.8.
Fast glass
Lenses that offer very large maximum apertures in the f/1.4 area usually tend to be fixed focal length prime lenses around the 50mm area. You can get zoom lenses that have f/2.8 maximum apertures but they tend to be quite expensive and are referred to as 'fast glass'. The minimum aperture of a lens tends to be less of an issue, as even basic kit lenses can achieve at least f/16 if you need much deeper DOF. Very large apertures let you shoot in lower light situations with higher shutter speeds, rather than if you were shooting with smaller apertures in the same conditions. Smaller apertures are often used by landscape photographers to get their shots as sharp as possible and capture as much detail as they can.