Depth of Field

Depth of field is the amount of distance between the nearest and farthest objects that appear in acceptably sharp focus in a photograph. A preferred selection Depth of field ("DOF") in a focused subject in an image can be quite subjective. Remember this, adequate selection of DOF for one situation, application may be unacceptable for another photographer. It is all a matter of personal preference when trying to determine the appropriate use of DOF to enhance an effect in a photograph. 

In simpler term, we define depth of field as the zone of sharpest focus in front of, behind, and around the subject on which, when lens is focused on a specific subject; with TTL (through the lens) SLR camera, DOF can be previewed in the viewfinder of a camera - the preview is very handy for critical type of work. For an example, when taking a product shot, when you require absolute certain if DOF is adequate to cover the object you intend to photograph Generally, the closer the subject to the camera, the more evenly with the distribution of depth of field in front and behind the subject. As distance of focus extends, DOF usually will be more behind than in front of the focused area.

A few factors may have a direct relationship with depth of field, they are: 

1) the diaphragm opening of the lens (the Aperture), 

2) the focal length of the lens in use, and 

3) image size (it has a direct relation to distance).

The general rule of thumb for selecting the right aperture for a desired depth of field is: give the same object distance and the image size, the bigger lens opening (aperture) used (like f/2.8, f/2, f/1.4 etc.) will have a narrower band of depth of field - meaning critical focusing will be required in this kind of situation because when you use a large aperture (in particularly when focuses at a near to the subject), the zone of sharpness (DOF) can be very limiting; while on the other hand, if extended depth of field is required, you can just choose a smaller lens opening like f/8, f/11, f/16, f/22 to make the plane of sharpness is extended, so everything will be in sharper focus.

The "amount" of light allows to strike the film plus the duration (time) for the light to strike the film forms an exposure. The camera has two mechanisms to control exposure, the lens diaphragm (lens section - aperture) and the timing of the OPEN/CLOSE of the shutter curtain (camera section - shutter speed). If this confuses you, the lens diaphragm (inside a typical SLR camera lens) consists of multiple blades which can be open and closed to certain size openings, the variations in the lens opening is called aperture. The size of the aperture determines the amount of light which will fall on the film. Various sizes of the lens opening are indicated by a set /series of numbers called f/stops or f/numbers. Each f/stop represents a specific quantity of light that pass through the lens. The smaller numbers are called large f/stops while the larger numbers are called small f/stops. This is because the larger numbers represent smaller apertures and allow less light to pass through the picture taking lens. Each time you move from one f/stop to the next smaller f/stop (larger number the amount of light allowed through is exactly halved. In effect, the amount of exposure itself is also halved. 

The larger number such as 11, 16, 22, 32 marked on the lens is called the lens minimum aperture. The smaller f/number indicated on the lens (such as 1.4, 2, 2.8 etc) is called the lens maximum aperture. The maximum and minimum apertures differ according to the lens-types. The maximum lens aperture is important because it indicates the largest amount of light that the lens will transmit through it to strike the film "hiding" behind the camera shutter. 

Depth of field decreases with increasing focal length (given equal subject distance) In other words, if subject distance stays the same, a long lens gives less depth than a short.There are times when you need to affect the amount of depth of field for a particular scene. In a scenario like low light where you need to to shot wide open and wish to have extended depth of field or cases where you are force to live with a fixed-aperture mirror lens, if you suspect that the aperture is too wide to deliver adequate depth of field, simply move back. The depth of field surrounding your subject will expand.