Aperture is the size of the lens opening through which light travels to hit the sensor. It is the ratio of the lens’s focal length to the diameter of the lens’s opening. Aperture is represented by an f number, and the standard full stop f number scale would be something like: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32. There’s an inverse relationship between f numbers and the size of the lens opening. So larger f numbers mean a smaller lens opening, and subsequently smaller f numbers mean a larger lens opening. Lenses with large apertures (small f number) are said to be fast lenses because in photography these apertures allow the photographer to use faster shutter speeds.
Aperture and Light
Obviously the larger the hole in the lens, the more light is able to reach the camera’s sensor. Consequently, the larger the aperture the better it is in low light situations.
If you’re in a dark room, one way to get a brighter exposure is to open up the aperture (lower the f number) to allow more light in.
Sunny 16 rule
You may hear of the sunny 16 rule which is a general guideline that says use a f/16 aperture to get a good exposure when it’s sunny.
Looney 11 rule
Additionally, there is the looney 11 rule which says to use an aperture of f/11 when doing lunar shots (shots of the moon).
*In photography the Sunny 18 and Looney 11 rules both go on to suggest using a shutter speed that is the reciprocal of the ISO. For example, on a sunny day at aperture f/16 using a 200 ISO, set the shutter speed to 1/200.
Aperture and Depth of Field
Aperture is also important because it affects the depth of field. Remember that depth of field refers to the amount of the scene that is in focus. The larger the aperture the shallower the depth of field, meaning you can have sharp focus on a subject and have the foreground and background blurred. Use a smaller aperture if you want to have more of the background in focus. So remember: larger f number means larger depth of field. Smaller f number means smaller depth of field.
Aperture and Bokeh
Related to having a shallow depth of field is the Japanese term, bokeh, which refers to the effect created when you have an out of focus point of light. Different lenses and apertures create different bokeh effects from circular bokeh to more polygonal shapes. If you have a shallow depth of field you can have your subject in focus while points of light in the background are out of focus and create a bokeh effect. As in the below picture, circular bokeh is produced from the out of focus lights.
Aperture and Sun Stars
Have you ever noticed photos or videos where the sun looks like a star with rays of light shooting out of it? Well now you can create that effect with the sun, or any other light source. All you have to do is close down your aperture. The number of light rays is determined by the number of diaphragm blades on your lens. Because I’m a girl, I have the motherly urge to remind you not to look directly at the sun! Also be wary of leaving your camera pointed at the sun for long periods of time as the lens can act like a magnifying glass and damage your camera.