Focusing Mechanism of a Lens
Focusing mechanism of any camera lens is designed to shift the lens elements and focus an object, which we are shooting. This mechanism can be manual or automatic, but in either case, it must move absolutely freely, without any jerk. Modern auto focus lenses either show movement of a ring or they will just move forward or backward to focus an image. If a ring is provided, which is a better option, then it has a distance scale on it, which shows how far the focused object is.
If the lens is not having an auto focus mode, then be sure that the focusing ring is moving freely, but at the same time, it should not be very loose. When ring is rotated for focusing, ideally, the outer barrel of the lens should not rotate. In any well designed lens, outer barrel will not rotate as this type of rotation will also rotate and change the position of the optical filter, attached on the lens barrel.
If the lens has internal focusing mechanism, then it will not even change its length and outer dimensions.
Some cheap lenses, which are not equipped with an auto focus mode, has front outer lens barrel, which is to be used for focusing. This means that there is no separate focusing ring, and there is no provision for good grip.
A zoom lens may have one ring, which is used for focusing as well as zooming. This type of lens is termed as one touch zoom. This type of design has an advantage of fast action. However, as the zooming is done with pumping movement. Over period of time, it becomes loose and when the lens is pointed up or down, the focal length and composition changes because of gravity. Zoom lens with two separate rings for focusing and zooming, which is termed as two touch zoom lens, does not have this fault.
When we are looking for an auto focus lens, test it for its auto focus mechanism. Be sure for the electrical motor, which should respond fast and should be noise free.
Read about this:
Aperture of lens: What we should check in aperture setting of a camera lens
Built, finish and weight of a lens: Why finishing and weight of a lens is important
Lens faults: Various types of faults a lens can have, and how to check it
Other information on the lens: Indication and information given on a lens barrel
Range of focal length: Range of zoom lens in terms of focal length
Sharpness of lens: Resolution and lens contrast - how to test it
Zoom ring: Ring on a lens barrel for zoom and focusing
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