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one of the most common camera lens parts on any camera is the focusing ring. This is a ring on the lens body, which moves the lens elements inside the lens. By adjusting the position of the glass elements, the image produced by the lens can be brought into clear focus. Although most modern lenses feature auto-focus, they will usually have a focusing ring to allow for manual control if desired. Generally, a switch needs to be flipped from AF (auto-focus) to M (manual) to engage the focusing ring and allow for manual focus.

Modern Classic SLR Series :
Canon EOS-1N Series AF SLR camera
Focusing with EOS-1N - Part III
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Manual Focusing Use manual focusing when the subject is difficult to focus with the camera’s autofocus system, or when you need to control the focus for alternative focus effects.

Operation:- 1 Set the lens’ focus mode switch to “M”. The AF mode display is extinguished in the LCD panel. 2. While pressing the shutter button halfway and looking through the viewfinder, turn the lens’ manual focusing ring until the subject appears sharp. When the focus mode is switched from “AF” to “M”, the focusing point and in-focus indicator in the viewfinder light when the subject is in focus.
Full-time manual focusing with Canon's USM (Ultrasonic Motor) EF lenses
(This method of focus adjustment cannot be used if your USM lens does not have a distance scale). Autofocusing can be overridden by manual focusing at all times. Most USM lenses have a full-time, manual~focusing feature allowing you to subtly adjust the focus with the focusing ring after autofocusing is achieved in the One-Shot AF mode. You can, therefore, focus manually even in an AF mode, which is one of the major features of the EOS system. Lastly, when using a USM lens equipped with an electronic manual focusing system, setting this custom function will allow manual focusing only when the lens’ focus mode switch is set to “M”.To disable AF altogether and switch