The lens in the digital camera provides a sharp image of the subject only at a fixed definite point along the lens axis. If the lens is moved from this particular point on the lens axis the sharpness of subject’s image deteriorates. The reasons for losing sharpness of the image may be many like atmospheric conditions, shaking of camera while clicking, dirty lens and other optical defects in lens. But here reason for the subject of photograph not being sharp because of subject being out of the focused distance of the lens will be discussed.
The depth of field (DOF) is the distance allowed between the most focused point on the lens axis and points of acceptable sharpness in front & behind of that focused point along the lens axis. Technically speaking DOF is the zone in which sharpness of the image is acceptable. Mostly, all the things in the frame are required to be as sharp as possible. This applies to vacation, family, architecture, landscape and documentary snapshots. However more creative approach is to have main subject sharp with background out of focus.
The main subject very clear and sharp as compared to the surrounding provides more impact. This is possible when you reduce the depth of field. The extent of acceptable un-sharpness decides the range of depth of field. More the acceptable un-sharpness more is the extent of depth of field. A point subject will have a point image only at a single given distance. At any other distance a point subject will have image in the form of a circular spot which is called circle of confusion.
The main controls used in depth of field are aperture, focal length, distance from the subject and magnification. Here are the different combinations that can be used to get variation in depth of field. For the same image frame size,
- Keeping focal length constant, there is increase in depth of field if you close the lens i.e. decrease the aperture.
- Keeping lens aperture and subject distance same; there is increase in depth of field for lens with short focal lengths as compared to the ones with long focal lengths.
- Keeping image magnification and lens aperture same, there is no appreciable change in depth of field (remains almost same) for different focal lengths of the lens used.
- Keeping aperture and focal length same, there is increase in depth of field as the subject distance increases.
In short, the depth of field increases with small aperture, short focal length lens and smaller magnification while the depth of field decreases with increase in aperture, longer focal length lens and moving close to the subject.
Its very important to understand that the quality of a photograph is not only determined by the sharpness you are able to produce in it but also on the way you use un-sharpness and blur in an effective way.
Further, you will know all about quality settings to be done to get extraordinary photographs in the next part of the tutorials. Next part covers how digital noise and ISO settings affect the picture quality?