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Dome vs Regular, Flat Port Lenses

Dome vs. Flat Lenses – General

In general, flat lenses are a great choice for capturing images that start and/or end above water. Today’s array of excellent variable zoom lenses also works well with flat Outex lenses since it gives the user a great range of choices in capturing above, split level, and underwater photos with a single lens, without having to exchange gear. Long lenses (greater than 24mm) also work well with flat lenses underwater in most conditions.  The example below taken with a regular Outex flat lenses illustrates this point.

Flat lenses are less expensive, smaller, and lighter than dome ports, and they work well with several lenses underwater, many of which are very popular:

Nikon 16-35mm, 17-35mm, 17-55mm, 18-35mm, 24-70mm

Canon 10-22mm, 16-35mm, 17-40, 17-55mm, 24-70mm

Sigma 10-20mm, 17-35mm, 17-50mm,

Sony 11-18mm, 16-35mm, 16-50mm, 24-70mm

Pentax 12-24mm, 16-50mm,

Tamron 10-24mm, 11-18mm

Tokina 11-16mm, 11-20mm, 12-24mm,


See a comparison below taken within seconds of each other, in the same condition, using a Canon 6D on Auto, using Canon 24-105mm f/4L.  Image 1 was taken using an Outex Flat Port 77mm lens.  Image #2 was taken with a Dome 100 + Adaptor Type 5-77mm.

Image 1 using Outex Flat Port 77mm lens

Image 2 using a Dome 100 + Adaptor Type 5-77mm

The perception that dome ports are better for underwater photography stems from unique optical qualities that flat lenses are unable to correct when shooting in water. There’s a great deal of detailed information about the differences between dome and flat lenses, but our goal here is to pragmatically clarify the advantages and disadvantages of both so you can make an informed decision as to what’s better for your objectives, budget, and gear.

There’s no question, however, that dome ports are superior for dedicated in-water use and split-level capture.  Despite the higher price, size, and weight, dome ports offer significant advantages, specially for wide angle lenses.  And dome ports are the only Outex solution for many fisheye lenses that do not offer a filter thread.  Thus, Outex designed dome ports that modularly work with most (if not all) fisheye lenses thru our custom brackets and adaptors, in addition to working with rectilinear (non-fisheye) lenses.  That means you can use the same Outex dome port with multiple makes and models of lenses without having to buy a new/different dome port each time – just like the rest of the Outex system.   And the Outex dome ports, like all of our lenses, are made of high quality glass (not acrylic/plastic), and are compatible with all Outex kits and covers.  Outex offers an automated Dome Kit selector that shows you the suggested and available options based on your lens here.  You can find it under BUY -> Shop by parts.   Select the make and model lens, and the system will suggest a solution for you.  See example below for the Canon 11-24mm f/4L USM.  Outex is constantly adding, updating lenses, so contact us if you can’t find the one you want.

Dome lenses are ideally suited for wide angle lenses (less than 35mm), which in turn are ideal for underwater and split-level photography for several reasons.  We’ll explain each in more detail below:

  1. Refraction; Image is not as magnified due to refraction of light in the water. Refraction is the visual bending that results from the change in density and therefore change in light speed as it crosses from air to water.  Refraction occurs when light changes speed when it enters a body of water, causing it change direction.  The subject becomes ¼ closer and 1/3 bigger. See pencil image below.

(Canon 16-35mm lens using Outex regular lens 82mm by Satiro Sodre)

(Canon 15mm lens using Outex Dome 100 mm by Julie Thies)

Note again the excellent results with both the regular lens as well as the dome port photos above.


  1. Sharpness; Improved color and sharpness retention since you’re closer to the subject (less distortion). Flat lenses create a progressive distortion away from the center of the image as they do not address the progressive refraction near the edges.  That also means a dome port makes it easier to find focus throughout the frame.


  1. Aberration; Less chromatic aberration (color fringing), as the same refraction impacts not only light, but as the light separates into the color spectrum’s component colors. Dome ports help correct light dissipation over distance, helping ensure a more evenly sharp, focused, and color-correct image.


Why the Dome?

Dome ports help correct for these problems.  The angle of the lens helps compensate for the refraction of the water, as well as the color and sharpness distortions that go along with it.  The domes also increase the distance between the lens and surface of the water, therefore increasing the camera’s ability to find focus.

There are a few draw backs to dome ports, however. They cost more, as a result of the increased materials and much more complex manufacturing process.  High quality domes also have differing glass thickness in the center compared to the extremities.  Domes are also a little bigger and heavier than flat lenses, so they are not as seamless to transport on your backpack as a regular optical lens.  Outex domes are made of high optical quality glass, which also has inherent advantages over plastic/acrylic.  Glass delivers superior clarity and will not scratch as easily.  Outex dome ports come pre-packaged in a neoprene protective case to help store and protect them.


Virtual Image

Dome ports also create a phenomenon called virtual image.  Simply put, virtual image is a reflection of the real image that’s created on the surface of the dome glass onto which the camera must focus.  In other words, when using a dome port, the camera is no longer focusing on the actual object outside the lens, but rather on the object’s reflection on the dome port itself.  The virtual image phenomenon requires your camera/lens’ ability to find focus within the distance from the camera’s sensor to the dome port’s virtual image.  That means the focal length must be taken into account when using dome ports with certain lenses.  And obviously that also varies for variable zoom lenses.  Again, Outex makes it easier for you by suggesting appropriate pairing in its automated dome lens selector at the automated online store  or thru the kit-building process in the lens selection step.


Calculating Focal Length

If your love of math and desire to calculate each of your lenses’ focal length intrigues you, you’re in luck.  A camera’s focal length can be calculated as the distance it requires to find focus thru a particular lens from the camera’s sensor.  All cameras have a marking indicating the sensor’s physical position on the camera, as illustrated below.

Each lens has its own Minimum Focal Distance (MinFD) which is specified by the maker. The MinFD is the minimum distance an object must be from the camera sensor to be in focus. All DSLR cameras (and some mirrorless cameras too) have a marking indicating where the camera’s sensor is located.    For a dome port to work underwater, the virtual image (dome surface placement) must lay outside/beyond that MinFD.  The MinFD can be calculated by the following formula:


D = close-up diopter, can be +1, +2, +3, +4,…, +10, and it is a dimentionless number.

X = MinFD, specified by the lens maker, in meters.

Xmin = minimum focus distance you want to obtain in meters.


Lens Barrel Markings

  1. Focus Window – Shows the current focus distance. Subjects which are this far from your lens will be sharply in focus. On the lens shown, the top line (green) shows measurements in feet, and the bottom (white) shows them in meters.
  2. Focal Lengths – This is a zoom lens, and this marking shows its range of focal lengths. In this case, 24mm to 105mm.
  3. Zoom Ring – The focal length changes as you turn this zoom ring, and the vertical line indicates the current setting.


Examples of MinFD for various lenses


Canon 8-15 ==> MinFD = 15cm ==> lays inside the Dome ==> it will focus

Canon 16-35 ==> MinFD = 28cm ==> lays inside the Dome ==> it will focus

Canon 17-40 ==> MinFD = 28cm ==> DOES NOT lay inside the Dome ==> will NOT focus


In this case the MinFD lays beyond/outside of the virtual image (dome placement), and the camera will not be able to find focus with this lens and the Dome 100.  But wait, there’s more.  One can use a close-up filter to “shorten” the MinFD. The diopter of the close-up filter can be calculated using the same formula.


Close up Filters. 

In photography, a closeup filter, closeup lens or macro filter is a simple secondary lens used to enable macro photography without requiring a specialized primary lens.  They work identically to reading glasses, allowing any primary lens to focus more closely.  A close-up lens, while technically a lens, looks more like a filter and threads onto the front of your lens the same way an Outex optical lens would. For this reason, they are also called close-up filters or supplementary filters.  Close-up lenses work by reducing the MinFD (minimum focal distance) of your lens and allowing it to focus more closely to your subject, which gives you greater magnification. (Source;

From the example above…



D = close-up diopter, can be +1, +2, +3, +4,…, +10, it is a dimensionless number.

X = MinFD, specified by the lens maker, in meters.

Xmin = minimum focus distance you WANT to obtain in meters.

For example, for Canon 17-40:

X = MinFD = 28 cm = 0.28m.

Xmin = 18cm = 0.18m (this is a “guess”; one can use something around it, like 17cm or 20cm).


D = 1.98 ==> Use the smallest integer greater than this ==> D = 2

So, the close-up diopter must be +2. Now, to calculate the real Xmin, we use the same formula:

Solving for Xmin:

Xmin = 0,179m = 17,9cm ≈ 18cm

Canon 17-40 + Close-up +2 ==> MinFD = 18cm ==> it lays inside the Dome ==> will focus


The lens’ magnification is denoted accordingly; +1, +2, +4, etc

If you want to use dome ports with your lens, but find it difficult to find focus underwater, you can obtain a closeup filter online or at your favorite local photo store.

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