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. 
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. Using f/2 as an example, the amount of light reaching the film will change according to f/stop as indicated is given in the image below.  
 





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    Mahesh Babu, a newbie to the world of photography from Kannur District in Kerala State of India. His gadget is Canon 550D. trying to   do something with the resources he has to explore the beauty of life and livings..

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