What Is the Focal Point of a Lens?
Lenses allow you to see great distances and very tiny things. They allow you to get close to things you can't touch, and they allow you, with cameras, to record what you see. Understanding how they work can help you choose the right lens for the right circumstances.
Identification
The focal point of a convex lens is the point where light rays parallel to the axis are brought to a point (see the Homepage Mac website in Resources below for details). What this means in practical terms is that, in a camera, the lens focuses to its focal point, which is set at the film plane. This creates the image. Today, with auto-focus lenses for most consumer cameras, we don't think about the focal point much. When the auto-focus fails, however, which it can do from time to time, the photographer must manually bring the image to the focal point.
Function
We don't think that often about the focal point when shooting photos, but it is important. If the lens elements don't line up correctly to form this imaginary converging point, there is no way to focus the camera. The focal point also can give the distance of the lens to the subject because most 35 mm-type single-lens reflex lenses have distance meters on their barrels. When the focal point is reached, the photographer can check the distance on the lens barrel and set lighting appropriately, whether it is a single electronic flash or several slaved flash units and reflectors.
Significance
The focal point is important in any application where lenses are used. For example, if a telescope is out of whack and the focal point does not form, blurry images result. In this case, it could mean spotting a star or comet in the wrong place or seeing double images. The focal point essentially tells the astronomer whether the lenses are aligned properly.
Effects
The focal point also can be manipulated for specific applications. The Homepage Mac website, for example, suggests that using the focal point could allow a traffic light to be aimed at a specific lane of traffic.
Features
Determining the focal point of a lens is done usually by the manufacturer, but if you are building your own camera--such as a view camera with a lens board, bellows, tripod and focusing screen (see Resources)--it is important to know what each particular lens will focus to before finishing the camera. Even after the camera is complete and in use, knowing the focal point of each lens matters because it will determine how sharp the image is on the ground-glass focusing screen. Unlike electronic SLR-type 35 mm-style cameras or medium-format cameras, the view camera doesn't do anything for the user. The photographer's skill and knowledge of each part of the camera--especially where the image focuses--can mean the difference between crisp, clear shots and slightly fuzzy ones.
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