Aperture and Depth of Field

ApertureThe aperture stop (diagram on the right) is what controls the amount of light let into the camera’s image sensor. The aperture adjusts the size of the hole that lets light into the sensor. The aperture size is measured by the f-number. The higher the f-number, the smaller the aperture and the larger the depth of field (depth of field is the area of your photograph that is in focus). However, the smaller the aperture, the longer the shutter speed that is required. Normally every step that you raise/lower the aperture, you will either have to double/half the shutter speed. For example, let’s say I had an aperture of f-5.6 and a shutter speed of ¼. If I changed the aperture to f-6.8, I would need a shutter speed of about ½ to get the same exposure. Each step that you enlarge your f-number, it will cut your aperture in half.  See the example above. So, how do you use aperture to get the desired results? If you want a shorter shutter speed, you will want a larger aperture (or smaller f-number). However, if you want a longer shutter speed (e.g. to blur clouds or water), you will want a smaller aperture (or larger f-number). If you want just your subject in focus, you will want a large aperture (or small f-number). If you want as much of the scene in focus as possible, you will want the smallest aperture possible (or largest f-number). To form the light into a starburst, set your camera at a higher aperture (such as f-8 through f-22).
Light Example2

An example showing a point and shoot taking a picture. The arrows represent some of all the dirrections light is coming from.

For those who are wondering how depth of field works, it is slightly confusing. Light is reflecting off of almost everything on earth in many directions. The camera captures this light that is straight in front of it; however, light is also coming from all directions. This light is called circles of confusion. When the aperture is small, it shrinks these circles of confusion allowing the camera sensor to “see” more of the actual image. That is why smaller apertures have more in focus (or a larger depth of field) while the opposite is true for larger apertures. Although, if the f-number is too large (or the aperture is too small), it causes the circles of confusion to blend causing the image to be blurry. Note, that a smaller aperture does use a smaller area of the image sensor. If you are wondering where the aperture stop is, it is in front of the actual lenses in the inside of the lens casing. One commonly discussed idea is the focal length of your lens affects your depth of field. This is true. However, it does it so slightly that it probably won’t be noticed (see - http://www.luminous-landscape.com/tutorials/dof2.shtml). To see some additional basic, easy to understand information on this subject, check out this link - http://www.slrphotographyguide.com/blog/resources/aperture-explained.html.

Trackbacks & Pings

Leave a Reply

Your email address will not be published. Required fields are marked *