A dome port has no effect when viewing through it with the same medium (air) on both sides but once you put the outer surface in contact with water the refraction between that and the air in front of the camera lens comes into play. What happens is that a virtual image is formed ahead of the dome port and the camera lens is allowed to focus on that instead of the real subject some distance ahead. The effect is to produce an image that is more saturated in colour. The problem comes when you realise that this virtual image is curved and the distance in front of the lens is quite close.
Some photographers get disappointed when they find that their expensive wide-angle lenses are no longer giving images that are sharp from side to side and resort to fitting them with high-strength dioptre close-up lenses in order to get them to focus close enough to get this virtual image sharp.
That’s because most expensive wide-angle lenses are rectilinear designs that have a very flat field-of-view, something that is admirable when using them solely in the medium of air. At the same time, few focus close enough.
That’s why you’ll see top underwater photographers using full-frame fish-eye lenses behind dome ports. Often these give very disappointing results in air but in conjunction with a dome port their aberrations actually become an advantage. A curved field-of-focus is a positive bonus where trying to make a sharp record of a curved virtual image.
Dome ports come in different diameters with a different radius to their curve. The bigger the dome port the further in front of it the virtual image is formed and the easier it is to get the camera lens positioned at the right point behind it. That said, big dome ports can be unwieldy to use, hence the popularity of mini-domes. The smaller domes produce their virtual image much closer to the front of the port and it’s really important that the front node of the lens is positioned in the correct place relative to it.
The front node is not something you can see. It’s an optical term. Camera housing manufacturers have done empirical tests with most popular lenses to confirm what spacer ring might be needed to allow the dome to be positioned in the right place relative to the camera. They provide lens/port charts for this purpose and your underwater photography equipment dealer will have that information if you cannot find it on-line.
Dome ports can be made of acrylic material, polycarbonate or glass. Glass is the most expensive and the most hard wearing but if you are unlucky enough to scratch or chip it, there is nothing you can do apart from clone out the unwanted mark in your pictures, later with software on your computer.
Polycarbonate is inexpensive and lightweight but the same applies as glass should you damage it. Acrylic ports have an advantage in that the material has the same refractive index as water so minor scratches become invisible in your shots underwater unless you happen to take a picture into the sun. Acrylic is very easily scratched but in the same way it is very easily polished.
Simply take a piece of fine grade abrasive as used in finishing the paintwork of cars and gently cut back the scratch until they are has become an evenly matte surface. Then polish it back to clear acrylic using some proprietary silver polish wadding. It takes some elbow grease but you will be rewarded with a dome port that is immaculately clear of marks.
Some say that glass ports are optically superior to acrylic ports. I have owned and used both including an optically coated glass port that I imported specially from Japan and can tell you that the pictures taken with both this and a top quality acrylic dome are indistinguishable.
A manufacturer like AOI makes a range of glass and acrylic dome ports for Olympus system compact camera housings so the choice is yours.
You may find that a large glass port is easier to use for those over-and-under shots taken at the surface because droplets of water are less likely to cling to the glass. A large dome port certainly helps get those type of pictures because, remember, the lens has to focus on a nearby virtual image for the under part and the over part is in air, probably at infinity. You need to use very small lens apertures to get the huge depth-of-field needed or a split close-up lens that affects only the bottom part of the lens. These rarely fit on fish-eye lenses.
Some underwater photographers report good results using smaller domes for these over-and-under pictures but invariably they are in bright sunshine that allows them to use the smallest lens aperture with perfectly calm water, but they normally need to make an exposure adjustment in digital post-processing to get both halves of the picture in balance.
The important thing to remember is that when buying a port, you will need the right extension ring to position it correctly relative to the lens. Alas, it’s not something you can confirm by taking a picture when in the equipment sales room and not underwater.