Disclosed is an interchangeable conversion lens for underwater photography or cinematography able to change from macro to wide angle photographic perspectives and consists of a sequential assembly of optical lens elements. This conversion lens includes an objective lens, a relay lens and a focusing unit which are positioned on an optical axis and are interrelated in their optical performance to one another. The objective lens is to form an intermediate image (reversed and inverted) from wide angle perspective to half the size of the sensor in the camera, this intermediate image is then transmitted through the relay lens and the focusing unit which projects the image through a macro lens into the sensor of a camera. The three parts can be disassembled and reassembled underwater, all three separate parts are waterproof and therefore the optical performance of water is taken into consideration in the optical system. The length of this lens can be extended or reduced, and the three individual parts can be changed according to personal preferences. This provides a flexible underwater optical system that can easily be changed in water between macro and wide angle perspectives, depending on the photographer's needs.