Focus

Chapter Eight

Adjustment of the distance between subject to film (Camera) is called focus. There are Two types of  focusing, manual focus and auto focus

Manual focus

In the field of photography , a manual focus camera is one in which the operator has to adjust the focus of the lens by hand. Before the advent of auto focus , all cameras had manually adjusted focusing thus, the term is a retronym.

The focus itself may be adjusted in a variety of ways, larger view cameras and the like slide the lens closer or further from the film plan on rails; on smaller cameras, a focus ring on the lens is often rotated to move the lens elements by means of a helical screw. Other systems include levels on the lens or on the camera body.

There are number of ways in which focus may be determined. Simplest is simple using a distance scale and measuring or estimating distance to the subject. Other methods include the rangefinder, which uses triangulation to determine the distance. On other cameras, the photographer examines the focus directly  by means of focusing screen. On the view camera, this ground glass is placed where the film will ultimately go, and is replaced by a sheet of film once focus is correct. Twin Lens Reflex Cameras use two lenses that re mechanically linked, one for focusing and the other to take the photograph. Single Lens reflex cameras, meanwhile, use the same objective lens for both purpose, with a mirror to direct the light  to either the focusing screen or the film.

Auto Focus

  

Auto focus (or AF) is a feature of some optical that allows them to obtain (and in some systems to continuously maintain) correct focus on a subject, instead of requiring the operator to adjust focus manually. Auto focus system can capture a subject in a variety of ways .

Auto focus system rely on a single sensor, while other use an array of sensors. Most modern DSLR cameras use through the lens optical AF sensor, with a separate sensor array providing light metering , although the latter can be programmed to priorities its metering to the same area as one or move of the AF (auto focus) sensor.

Through the lens optical auto focusing is now often speedier and more precise than what can be achieved manually with an ordinary viewfinder. More precise manual focus can, of course, be achieved with special accessories such as focusing magnifies. Auto focus accuracy within 1/3 of the depth of field (DOF) at the widest aperture of the lens is not uncommon is professional AF DSLR cameras.

Most Multi Sensor AF cameras allow manual selection of the active screen and many offer automatic selection of the sensor using algorithms, which attempt to discern the location of the subject. Some AF cameras are able to detect whether the subject is moving towards or away from the camera, including speed and acceleration data, and keep focus on the subject a function used mainly in sports and other action photographs.

The speed of the auto focus system is highly dependent on the maximum aperture offered by the lens. F-stops of around f/2 to f/2.8 are generally considered optimal in terms of focusing speed and accuracy. Faster lenses than this typically have low depth of filed, meaning that it takes longer to achieve correct focus, despite the increased amount of light. Most consumer camera system will only focus reliably with lenses that have a maximum  aperture of at least f/5.6 , while professional Models can often cope with lenses that have a maximum aperture  of f/8 , which is particularly useful for lenses used in conjunction with  teleconverters.      

Next Chapter is on Aperture. If you have any question regarding  Photography  just feel free to ask . So stay with us and like us on facebook