15/05/2024
Dome Ports – Should You Choose Glass or Acrylic?
Any time there’s a choice of equipment that seems to do the same thing, there’s a dilemma. Regulators are a good example. Mechanically there are piston and diaphragm first stages and downstream, pneumatically balanced and pilot second stages. Which is best? Objectively, if you look at machine testing, no engineering solution offers a clear winner. Choosing on ease of breathing alone might not even be the only criteria you need to figure in. Other factors might need to be considered, such as how cold the water you will be diving in is or, for a photographer, whether the second stage prevents you getting your eye close enough to your eyepiece to see the whole frame or exhaust bubbles will spoil your view.
Dome ports are another contentious subject where to get the one that is right for you, you may need to consider more than just the basic image quality it produces. As you probably know already, domes are most commonly used to correct refraction, so land lenses used inside housings retain their original angle of view. Even that statement is an oversimplification, but that’s another subject – let’s go with it for today!
INON produces conventional dome ports for their X series housings for land cameras. They also make domes for their wet lenses. Some of these are integrated into the lens, such as those used on the UFL -G140SD semi-fisheye for action cams. Others are options that can be fitted to wet wide -angle lenses, such as the UWL-95S series to expand the underwater field of view from 95 to 140 degrees. This type of dome that is designed to integrate with an INON wet lens is called a Dome Lens Unit to distinguish it from a dome port for an X series housing.
So, which is the better material to construct domes from? It depends a little on the application. INON uses glass for some domes, acrylic for others. In the case of dome lens units for the 95S, you get to choose either. Optically, it’s impossible to really see any difference between images shot through a well-designed dome whether it is made from glass or acrylic. Although polycarbonate is a go to material for building housings, it’s rarely used for ports because it can discolour over time.
INON uses glass for dome ports for X series housings. Glass has some advantages and some disadvantages compared to acrylic. The three main disadvantages are higher cost, greater weight and deep scratches are usually uneconomic to polish out. The advantages are it’s much more resistant to being scratched in the first place and it’s naturally more water repellent, so better for taking split-level images as water runs off more easily and faster. Another and often overlooked advantage for housing owners is it creates less static than plastic. Static attracts dust. This creates a cleaning issue. Obviously, the wet side of the dome is going to get washed off as soon as you submerge, so cleaning the outside isn’t a concern. But cleaning the inside of an acrylic dome can abrade it over a long period of time. This can eventually degrade its performance, both in terms of sharpness and light transmission.
Glass not only requires less cleaning, its toughness makes it unlikely to be degraded if cleaned gently like a lens. And on that point, it’s worth mentioning, camera lenses are very rarely made of plastic!
Acrylic has two major advantages over glass, in addition to being lighter. Firstly, If you put a scratch in it, with a bit of patience and some polisher, like T-Cut, you can often make repairs on location. But if the damage is too great to fix, it’s less expensive to replace. However, there is an important caveat to this.
Dome ports usually incorporate multi- coating chemical layers to prevent light reflecting off the dome. These are needed to maximise light transmission and to inhibit flare. Coatings also help improve colour rendition and contrast – just as they do with land lenses. If the coating is placed on the outside of an acrylic dome, then these layers are damaged if you try and polish out scratches. INON domes have the coating on the inner surface of the dome, so it’s not an issue.
Action cams equipped with the INON UFL-G140SD are workhorse rigs in the broadcast industry. They are often used in filming situations where the risk of impact damage is substantial. As these are monoblock lenses, with the dome incorporated into the lens, there’s no reason ordinarily to remove the dome, so internal dust accumulation and the need to clean inside aren’t a consideration. On the other hand, being able to polish out the dome on a distant location is an asset. The dome is also inexpensive to replace if damaged beyond field repairing. This makes acrylic a better choice than glass for this niche type of user.
For owners of INON lenses like the UWL-95S, they get to choose for themselves. Trust INON to present their clients with a nice problem to have!
Official web site of INON INC. Manufacture of underwater imaging gears, strobes, conversion lenses, arms, housings and more.
