This focus gear enables manual focus adjustment for the Canon EF 17-40mm f/4L USM lens via the Extension Ring 46 with Focus Knob #30117 when used with Sea&Sea MDX-series housings.
The focus gear is useful in the following situations:
* When you need to manually adjust focus as your subject is not situated within the AF frame or in order to maximize the effect of the lens’s minimal shooting distance of 28cm/11 inches.
* When you shoot fast moving subjects, like dolphins under limited ambient light (dark environment), so auto focus does not function effectively.
* When you shoot “half-in half-out” photos and would like to manually adjust focus on subjects on land and underwater.
* The focus gear can be used in conjunction with the zoom gear (product #31109).
