Definition: The “Changing Zone of Sharp Focus”, either side of object distance on which you actually focused, is known as depth of field. It is the distance between nearest and furthest parts of a subject that can be imaged with acceptably sharp details, at one focus setting.
Practical Significance: By choosing a shallow depth of field, you can isolate one item from others at different distances. It is basically used to create emphasis and suggest surroundings..
Factors of Depth of Field1. f-number (or Aperture Number)2. Subject distance from Lens3. Focal Length of Lens4. Permissible area of circle of confusion.
1. APERTURE ( f. Number) The smaller the aperture, the higher the depth of field (the other two factors remaining the same). For example, if the lens focal length and the shooting distance stay the same, the depth of field is much deeper at f/16 than at f/1.4.
2. Focal Length The shorter the lens focal length, the higher the depth of field (the other two factors remaining the same). For example, comparing a 28mm lens with a 50mm lens at the same aperture and shooting distance, depth of field is deeper with the 28mm lens.
Changing Focal LengthPhotos below were taken by 35 mm camera at constant distance from subject. 28 mm lens 50 mm lens 70 mm lens 210 mm lens
3. Shooting distance The greater the shooting distance, the deeper the depth of field. i.e. other two factors remaining the same). For example, if the subject is photographed from three and then from seven meters away, the zone of sharpness in the foreground and background is greater at seven meters.
REMEMBER: Another characteristic of depth of field is that it is generally deeper in the background than in the foreground. Although, people ask for detail throughout the picture in most commercial/record photography, the picture with minimum D of F will have maximum information.
HOW DEPTH OF FIELD WORKS Before understanding the working of D of F one needs to first understand that how exactly a lens critically focuses an image point depending upon how far a lens is from the subject.
C A a B B c C A Depth of Field B is the only point of object which is forming a Point of Focus on the Film. Light from other parts of the subject, nearer or farther from the lens, comes to focus farther away or nearer to the film. Thus discs are formed instead of points of light. These discs are known as Circles of Confusion.
Permissible Circle of Confusion Large discs gives image an un-sharp appearance because our eyes have a limited resolving power. A permissible Circle of Confusion is the one which our eye can resolve as a point instead of a disc.
Depth of focus is often confused with depth of field, but in reality the two are very different. Where DoF is “Changing Zone of Sharp Focus”, either side of object distance on which you actually focused; Depth of focus, is a measurement of how much distance exists behind the lens wherein the film plane will remain sharply in focus. It can be viewed as an action which is occurring on the opposite side of the lens.