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Products: ExtEnsion Rings & Ports Adapters

Extension ring with focusing knob for the Canon EF 17-40mm f/4

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This extension ring is especially designed for the Canon EF 17-40mm f/4 zoom lens. It is equipped with a knob on its side that connects to the lens focus ring via a gear. This knob has a retractable mechanism that makes installing and removing the lenses a quick and simple operation. Just rotate the release disk to lift the internal pinion gear out of the way.

While Autofocus has always been recognized as a useful tool when shooting still photography, it is not always so when shooting video, in most DSLR Cameras the autofocus performance when used in the video mode is unacceptably slow and most users would prefer to shoot video with access to manual control for critical focusing, a basic and sometime essential requirement for quality video footage.  This new extension ring integrates a knob for manual focus and its dimension has been optimized for our acclaimed 9.25” Megadome, this BK-7 coated mineral glass dome port is the ultimate in optical quality. 

The all aluminum construction benefitted from our latest technology improvements and of state of the art 5 axis machining, this is how we managed to keep its weight to a minimum without compromising the 300ft/90m depth rating associated with all our products.

Weight: 1lb/454gr
Dimension: Diameter 4.81” / 122.4mm
Width including knob: 6.6” /168mm
Height: 3.00”/76mm



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