Depth of field?

Depth of field can be very useful when shooting landscapes here is a description of depth of field taken from: http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

Depth of field is the range of distance within the subject that is acceptably sharp.  The depth of field varies depending on camera type, aperture and

 focusing distance, although print size and viewing distance can influence our perception of it.  This section is designed to give a better intuitive and technical

 understanding for photography, and provides a depth of field calculatorto show how it varies with your camera settings.

The depth of field does not abruptly change from sharp to unsharp, but instead occurs as a gradual transition.  In fact, everything immediately in front of or in back of the focusing distance begins to 

lose sharpness-- even if this is not perceiv

ed by our eyes or by the resolution of the camera.

Since there is no critical point of transition, a more rigorous term called the "circle of confusion" is used to define how much a point needs to be blurred in order to be perceived as unsharp.  When the circle of confusion becomes perceptible to our eyes, this region is said to be outside the depth of field and thus no longer "acceptably sharp."  The circle of confusion above has been exaggerated for clarity; in reality this would be only a tiny fraction of the camera sensor's area.

When does the circle of confusion become perceptible to our eyes?  An acceptably sharp circle of confusion is loosely defined as one which would go unnoticed when enlarged to a standard 8x10 inch print, and observed from a standard viewing distance of about 1 foot.

at this viewing distance and print size, camera manufactures assume a circle of confusion is negligible if no larger than 0.01 inches (when enlarged).  As a result, camera manufacturers use the 0.01 inch standard when providing lens depth of field markers (shown below for f/22 on a 50mm lens).  In reality, a person with 20-20 vision or better can distinguish features 1/3 this size or smaller, and so the circle of confusion has to be even smaller than this to achieve acceptable sharpness throughout.

A different maximum circle of confusion also applies for each print size and viewing distance combination.  In the earlier example of blurred dots, the circle of confusion is actually smaller than the resolution of your screen for the two dots on either side of the focal point, and so these are considered within the depth of field.  Alternatively, the depth of field can be based on when the circle of confusion becomes larger than the size of your digital camera's pixels.

Note that depth of field only sets a maximum value for the circle of confusion, and does not describe what happens to regions once they become out of focus.  These regions also ca

lled "bokeh," from Japanese (pronounced bo-ké).  Two images with identical depth of field may have significantly different bokeh, as this depends on the shape of the lens diaphragm.  In rea

lity, the circle of confusion is usually not actually a circle, but is only approximated as such when it is very small.  When it becomes large, most lenses will render it as a polygonal shape with 5-8 sides.

Controlling depth of field.

Although print size and viewing distance are important factors which influence how large the circle of confusion appears to our eyes, aperture and focal distance are the two main factors that determine how big the circle of confusion will be on your camera's sensor.  Larger apertures (smaller F-stop number) and closer focal distances produce a shallower depth of field.  The following depth of field test was taken with the same focus distance and a 200 mm lens (320 mm field of view on a 35 mm camera), but with various apertures:

F/8.0

F/5.6

F/2.8