A lens focuses precisely on only one plane at a time, but there is a also a certain distance before and after this point
that also appears sharp. This is the depth of field and it varies predominantly depending on
Most DSLRs have a number of preset focus modes that can be used quickly depending different circumstances. When using
these modes the camera will automatically choose what it thinks are the most appropriate settings to achieve the best
results depending on what the context is.
Most DSLRs offer the similar preset modes. On my Canon EOS 450D, the following settings are available.
A half press of the shutter button will set the focus and the camera automatically takes care of
all other settings.
If you are focusing on a moving subject, the Al Servo AF will lock focus on the subject automatically as
long as you keep the AF point on the subject and the shutter button pressed halfway.
This sets the aperture as large as possible under the conditions to narrow the depth of focus so that the face
stands out from a blurred background.
This sets the aperture as small as possible given the conditions to increase the depth of field to sharpen as
much of the scene as possible.
This sets the highest shutter speed to freeze the action. Focus is set to continuous while the shutter is pressed halfway.
This sets a medium aperture so that there is some depth of field while the background is slightly out of focus.
This balances the flash on the subject with the existing light in the background.
There are a number of exposure modes that you can set that allow you to set certain properties while the camera will automatically adjust
the others. My Canon 400D has the following:
The shutter speed and aperture are set automatically to give a good exposure of the subject.
You set the shutter speed and the camera automatically sets the size of the aperture.
It is used predominantly to freeze the action or achieve a motion blur.
You set the size of the aperture and the camera automatically sets the shutter speed to give the best exposure.
It is used predominantly to affect the depth of field.
You set both the shutter speed and the aperture size. You can also set the bulb exposure..
It is useful under some flash photography conditions e.g. When photographing fireworks.
Automatic Depth of Field
The depth of field is set automatically - all AF points will function to detect the object, both in the foreground and the
background, while the aperture will be set to attain the necessary depth of field.