Aperture and Depth of Field
http://www.digicamguides.com/learn/aperture.html
How adjusting your plane of focus can drastically change your photos
Every time you take a photograph, the lens of your camera opens to let in light. The aperture setting determines how much or how little your lens opens when you take a photograph.
Aperture Priority
Many digital cameras can be set to aperture priority mode. In this mode, you set the aperture and the camera automatically selects the shutter speed so that the photograph has the right exposure.
Why Aperture Really Matters
When your lens is open very wide, the background is typically out of focus. Wide-open aperture is good for portrait photographs, when you don't want the background to interfere with your subject.
When your lens is only open a small amount, everything from the foreground to the background is in focus. This aperture is used for landscape photographs, so that all of the scenery in the photo is in focus.
The Aperture / Depth of Field Relationship
When you focus your camera on an object in space, you are also focusing on everything else around the object that is the same distance away from the camera. This can also be described as focusing on a PLANE, rather than a point.
The following illustrations will introduce you to two important concepts: plane of focus and depth of field.
Illustration One
-
The camera is focused on Item B
-
Since the camera is focused on B, this is where the plane of focus is located — you may have heard of a point of focus (or focal point) but in reality everything that is on the plane of focus (even if it is to the side but the same distance from the camera) will be in focus.
-
C is also in focus since it is on the same plane as B
-
Items A, D and E are not in focus
Illustration Two
-
The camera is now focused on Item A
-
Since the camera is focused on A, this is where the plane of focus is located
-
Only item A is in focus
-
Items B, C, D and E are not in focus
Illustration Three
Notice that in the two previous illustrations the plane of focus is very thin — it does not have any depth. This is an artificial illustration to demonstrate the plane of focus.
In reality, when you focus on an object in space, there is also an area in front of and behind the object that is also in focus. This area of focus is called the depth of field.
-
The camera is focused on Item B
-
Since the camera is focused on B, this is where the plane of focus is located
-
C is also in focus since it is on the same plane as B
-
The depth of field extends in front of and behind the plane of focus
-
Since we have added the depth of field, Item D is ALSO in focus
To take better photographs with your camera, you need to be able to manually control the depth of field.