| 摘要 |
The invention resides in a positioning device for a reference surface within a lens port having a longitudinal axis. A reference surface element able to be extended substantially transversely across the port and withdrawn from the extended position are provided. A transverse movement element to effect such extension and withdrawal of the reference surface are provided and the transverse movement element is operable from a position external of the lens port. The invention also includes a camera or camera housing with a reference element of this type associated therewith. The invention finds particular use in the field of underwater photography.
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