Seacam OPP: Rethinking the Underwater Dome Port

One persistent compromise in underwater photography is that an excellent lens on land does not necessarily remain excellent underwater.

The problem is particularly apparent with rectilinear wide-angle lenses. Place one behind a conventional dome port and the interaction between water, glass and air can introduce corner softness, chromatic aberration and field curvature. Photographers have learnt to work around these limitations with large domes, carefully selected extension rings and smaller apertures.

The Seacam OPP, or Optical Precision Port, approaches the problem differently. Rather than simply placing a terrestrial lens behind a dome, it forms a corrective optical system designed around the change in refractive index between water and air.

The concept draws on Ivanoff-Rebikoff optical principles and parallels the renowned Nikonos RS lenses, which were designed specifically for underwater use. The difference is that the OPP enables selected conventional rectilinear lenses to form part of a corrected underwater optical system.

Putting the concept to the test

Seacam has now published extensive controlled testing of the OPP conducted by photographer Christoph Gerigk. A purpose-built test tank at the company’s facility in Austria was equipped with grids, Siemens stars and resolution targets to assess sharpness, distortion, chromatic aberration, field curvature and depth of field.

In all, 105 camera, lens, port and focal-length configurations were examined across their available aperture ranges. Dry reference photographs were followed by underwater images made from identical camera positions.

The principal reference combination was a Nikon 16-35mm f/4 with the OPP, compared particularly with Seacam’s conventional SD Superdome and CP Compact Port.

Differences in the centre of the frame were often relatively small, but became considerably more apparent towards the edges. Across the underwater focal lengths tested, from 16 to 35mm, the OPP produced better overall results than the Superdome, particularly in edge sharpness, chromatic aberration and distortion.

Depth of field was another interesting finding. The OPP produced greater practical depth of field in several configurations, apparently through a flatter and better-corrected plane of focus. This could be particularly useful for close-focus wide-angle photography, where both a nearby foreground subject and more distant background may need to remain acceptably sharp.

Comparisons with purpose-built Nikonos RS optics provide a useful reality check. The celebrated RS 13mm retained advantages, including centre detail and chromatic aberration. The OPP therefore does not overturn the benchmark established by dedicated underwater optics, but its ability to approach that performance in some respects while using a conventional zoom lens is noteworthy.

From test tank to open water

Controlled testing establishes optical performance, but says less about how useful a system proves on actual assignments.

Here, long-standing X-Ray Mag contributor Don Silcock provides another perspective. Silcock has written regularly for X-Ray Mag for many years, a relationship that substantially predates his more recent role as a Seacam ambassador. That ambassadorship should nonetheless be borne in mind when considering his comments, and is a relationship he openly discloses. He has also accumulated considerable practical experience with the OPP.

During a nine-week assignment in northern Indonesia, Silcock completed almost 180 dives and took around 10,000 underwater photographs. Of approximately 9,000 images made with his Nikon Z8 system, roughly 7,000 were taken using the OPP.

Silcock describes the OPP with his 16-35mm lens as effectively his underwater “standard lens”. At 16mm, he can photograph reef scenes and other wide-angle subjects, while at 35mm, the combination can handle considerably smaller subjects. It is not a macro system, but the range provides useful flexibility when the subject encountered underwater differs from what was anticipated before the dive.

For travelling photographers, that versatility may be almost as relevant as improvements in optical performance.

Practical compromises

The OPP is not particularly light. Silcock measured his complete system, including protective caps, at just under 3.2kg. Its travel advantage lies instead in its modular construction: the optical components separate for packing, occupying less awkward luggage space than a large fixed dome or water-contact optic.

There are also optical compromises. With the Nikon 16-35mm, both Gerigk’s testing and Silcock’s field experience found substantial vignetting at the wider focal lengths when shooting split-level images, disappearing only at around 21mm. Conventional dome ports may therefore remain preferable for some applications.

Nor is the OPP a universal port. Correct optical components, extensions and positioning must be matched to compatible lenses.

A different optical solution

These qualifications matter because the OPP should not simply be regarded as a superior replacement for every dome port. Fisheye, split-level photography and other specialised applications may favour different solutions.

Its significance, instead, lies in addressing the underlying optical problem in a different way. Gerigk’s controlled testing indicates substantial improvements in areas that have long challenged photographers using rectilinear wide-angle lenses underwater. In contrast, Silcock’s prolonged field use suggests that the resulting system can also be practical and versatile.

Neither constitutes an independent X-Ray Mag review. Nevertheless, the combination of published controlled testing, downloadable comparative images and extensive field experience provides considerably more evidence than is usually available when a new underwater optical system reaches the market.

For photographers accustomed to managing the compromises of rectilinear lenses behind dome ports, the OPP offers an intriguing alternative: Rather than trying to minimise those compromises with an ever-larger dome, it corrects the optics themselves.