Outex Dome Ports: 120mm vs. 180mm — Which Should You Choose?
Mar 10, 2019
Outex makes two optical glass dome ports: the Dome 120mm and the Dome 180mm. Both work with any lens through the appropriate lens-specific adaptor or clamp and fit any Outex cover. This guide explains the differences between them, how focal length affects the benefit, how the attachment system works, and how to choose the right dome for your shooting style. The 120mm and 180mm measurements refer to dome diameter, not lens focal length.
For a full explanation of why dome ports outperform flat ports optically — including refraction correction, split-level performance, and sharpness — see the Dome Ports vs. Flat Ports guide.
120mm vs. 180mm: Quick Comparison
| Feature | Dome 120mm | Dome 180mm |
|---|---|---|
| Diameter | 120mm (~5 inches) | 180mm (~7 inches) |
| Best for | Travel, split-level, half over half underwater, uncontrolled environments | Professional, split-level, demanding conditions, controlled environments |
| Split-level performance | Good — requires more controlled variables (focal length, settings, lens) | Excellent — provides greater margin across shooting variables; lens choice, settings, and conditions still matter |
| Portability | More compact and travel-friendly (600 g, or 1.3 lbs) | Larger and heavier (1,700 g or 3.7 lbs) |
| Price | More affordable | Higher cost |
| Lens compatibility | Prime and zoom lenses via the appropriate 120mm dome adaptor or clamp | Prime and zoom lenses via the appropriate 180mm dome adaptor or clamp |
| Focal-length guidance | Greatest optical benefit with wide-angle lenses; gradually diminishing benefit as focal length increases, with no numerical recommendation or cutoff | Greatest optical benefit with wide-angle lenses; gradually diminishing benefit as focal length increases, with no numerical recommendation or cutoff |
| Dome-introduced distortion | Does not introduce distortion at any focal length | Does not introduce distortion at any focal length |
| Cover compatibility | All Outex covers | All Outex covers |
Which Dome Should You Choose?
Choose the Dome 180mm if:
- You shoot split-level (half above, half below water) frequently
- You want the widest margin for error across different lenses, focal lengths, and conditions
- Image quality consistency is the priority over portability
- You want the larger dome's optical and split-level advantages, balanced against its greater size and weight
Choose the Dome 120mm if:
- You travel frequently and need a compact, lightweight kit
- You shoot general underwater work rather than split-level
- You want a more affordable entry point into dome port shooting
- You are comfortable working with more controlled shooting variables
The 120mm can achieve the same results as the 180mm — it simply requires more precise control over variables like focal length, proximity to subject, lighting, and composition. The 180mm makes it easier to get the shot across a wider range of conditions, especially for split-level work. These differences concern optical benefit and shooting flexibility, not distortion introduced by either Outex dome.
How your lens affects the dome-size decision
Outex domes work with any lens using the appropriate dome adaptors or clamps and do not introduce distortion at any focal length. Their curved shape is designed to complement the broad field of view of wide-angle lenses, which derive the greatest optical benefit from the dome.
As focal length increases and the field of view narrows, the lens looks through a smaller area near the center of the dome, gaining less benefit from its curved surface. The benefit therefore diminishes gradually as focal length increases. There is no recommended focal-length number or cutoff, and diminishing benefit does not mean added distortion.
Water clarity, lighting, shooting distance, and the lens’s own optical characteristics can affect the image. These factors should not be confused with distortion introduced by the port being used.
Both dome sizes support prime (fixed-focal-length) and zoom (variable-focal-length) lenses. The 180mm offers more separation between the lens and waterline and greater flexibility for split-level compositions; the 120mm prioritises compactness and portability. As focal length increases, a larger dome may be preferable, while the optical benefit from dome curvature still gradually diminishes. Dome size does not create a focal-length cutoff or a threshold beyond which an Outex dome introduces distortion.
With a zoom lens, consider the images you want to create and confirm that the fitting and cover accommodate its movement. A longer focal length does not require switching to a flat port to avoid dome-introduced distortion. A flat port remains a practical option when smaller size, lower weight, or portability is your priority.
Does an Outex dome introduce distortion with longer-focal-length lenses?
No. Outex domes do not introduce distortion at any focal length. Wide-angle lenses gain the greatest optical benefit; that benefit gradually diminishes as focal length increases because the lens looks primarily through the center of the dome.
How the Adaptor and Clamp System Works
Both Outex domes are universally compatible with any lens — past, present, or future — via two attachment methods:
- Dome adaptors — for lenses with accessible filter threads. The adaptor matches your lens filter thread diameter (not its focal length) and positions the dome around the lens. A threaded lens can use either a matching flat port or a dome with the appropriate adaptor: domes are not reserved for unthreaded lenses.
- Dome clamps — secure around the lens barrel when front filter threads are unavailable or unsuitable, including certain fisheye, curved-front, and built-in-hood designs. A nonremovable hood matters when it obstructs flat-port attachment; select the clamp for the exact lens rather than assuming all hooded lenses need the same fitting.
The two domes use different adaptors and clamps, each engineered to optimise optics and usability for that dome size. As your lens collection grows, you simply add the relevant adaptor or clamp from the Parts page — the dome itself remains compatible.
Keep the dome; change the lens-specific attachment. The optical-glass dome is the main investment. Adaptors and clamps are the less-expensive components that vary by lens, so changing lenses does not mean buying another dome of the same size. If you switch between the 120mm and 180mm domes, use the attachments designed for that dome size rather than assuming the same fitting transfers between them.
Unlike rigid housings where the lens floats inside a fixed dome (risking vignetting and misalignment), Outex adaptors and clamps anchor the dome precisely to each lens, guaranteeing correct optical positioning every time.
Compatibility
- Both domes are compatible with all Outex cover sizes (Entry 100, Pro Regular 110, Pro Large 130).
- Both domes are interchangeable with all Outex flat ports and accessories.
- Both support prime and zoom lenses with the appropriate lens-specific adaptor or clamp.
- You can own both domes and switch between them depending on your shoot, using the fittings designed for each dome size.
- Domes can be added to any existing Outex kit at any time.
How to Order
Dome kits (dome + adaptor or clamp) are available in the Accessories section. Individual domes, adaptors, and clamps are available separately in Parts.
Each kit includes one adaptor or clamp of your choice. If you are unsure which adaptor or clamp you need, specify your exact lens make and model when ordering — for example, "Canon EF 11-24mm f/4L USM" rather than focal length alone — so we can confirm the correct fit. Include your chosen dome size, since the 120mm and 180mm use different attachments. You can add further adaptors or clamps from Parts as your needs evolve.