This Port Extension Ring with Focusing Knob from Aquatica features a manual focusing knob for the Canon EF 17-40mm f/4 zoom lens. The ring extends the length of the port by 2.50". The knob connects to the lens focus ring via a gear. This knob has a retractable mechanism that makes installing and removing the lens a quick and simple operation. Optimized for the Aquatica 9.25" (23.50 cm) Megadome port, the ring will also work with the 8" dome port. It features anodized aluminum construction and a depth rating of 300' (91 m).
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