Read Part 6 here.
A part serialisation of a book by Jeff Goodman
PART 7: Flat & Dome Ports
Having paid good money for your camera and housing, it would be a shame to now spoil your image quality with a cheap port.
The optical properties of your port are as important as the lens of your camera. A low quality port would be like putting the wrong pair of spectacles on a person, so although they could still see well enough, the images would be slightly soft or degraded.
Once again you usually get what you pay for. Often the problem of choosing your first port is solved for you by the housing manufacturer in that a particular housing and supplied port will be specially designed for optimum results with a particular camera and lens. It is always worth grilling the sales person that this is the case for the set-up you are interested in (In the semi-professional range of cameras and above, you can of course change camera lenses. It then becomes very important to use a compatible port).
No single port will cover all the types of filming you may like to do.
There are 3 main port types:
- The flat port which is mainly useful for macro work and good close ups.
- The standard dome port which covers most applications.
- The wide angle dome port for videoing ultra wide angle.
The flat or macro ports are great for filming close ups and macro but because of the refractive index effect as previously mentioned, this port will decrease the natural angle of coverage of the lens and so increase the apparent image size. These ports would not be the best for videoing large subjects such as whales or wrecks, especially if the water visibility is minimal and you have to get really close to see anything at all.
The standard dome port is a good option for covering a wide range of subjects and is probably the most widely used. These standard dome ports will compensate for the 1/3rd angle of coverage loss and restore your lens to normal coverage.
The wide angle port provides the maximum angle of coverage when used with a wide angle lens and is good for getting close to the subject but maintaining a wide perspective. These are particularly useful in temperate waters where the visibility may generally not be so clear. Many manufacturers of underwater housings put in a standard port by default. More expensive housings will allow you to change ports which in effect is the same as changing lenses. Another option for some housings is to use a ‘wet’ wide angle adaptor which can be added to, or taken away from the flat port while still underwater. These ‘wet’ ports are very versatile but there is a small drop in quality when compared to a prime wide angle port.
Fisheye lens & port
A fisheye lens and port give an even greater angle of coverage, but does so with a high degree of distortion. This will vary in degree from lens to lens. The effects can be quite dramatic and are usually better used with stills photography than video.
Zooming through a dome port
Not all dome ports, especially the cheaper ones, will facilitate a zoom and at the same time, keep focus. Auto focus can sometimes compensate for this but it is not guaranteed. And remember, if you do zoom in, then the depth of field will decrease. Generally zooming while actually filming can be very off-putting for an audience and so unless there is a very good reason for doing this then I would suggest you mainly use the function for re-sizing and framing your shot.
A scratch or mark on your port can be a real pain. Scratches are more noticeable on glass ports than on Perspex ones. This is because Perspex or acrylic is roughly the same refractive index as sea water and so scratches are effectively filled by the water and visually removed. But with glass the refractive index is different and as light passes through a scratch it behaves differently from the rest of the port glass and becomes noticeable. Under normal front, side or top-lit situations, a scratch may not be noticed but may very quickly become apparent when back-lit. With glass I’m afraid it is time to change your port. With Perspex there is the opportunity to buff the port with ‘wet & dry’ to remove light scratches. This will take hours of patient work.
In short, take care of your ports. Glass is tougher than Perspex but is far more expensive. Your choice. If you can afford it, go for glass.
Condensation and fogging in a housing can lead to missed shots and great frustration and, of course, it doesn’t become noticeable until you are underwater. The problem can occur when there is a slight amount of moisture in your housing and then that housing is put in the heat of direct sunlight. The moist air in the housing, upon being put into relatively cold water, will immediately condense on the thinnest and coldest part of that housing, which is going to be at the centre of the dome port. It rarely happens with a flat port but it is not impossible. So, you jump in the water hoping for a great video session and all you get is foggy day shots. You can see the condensation right away, but just occasionally, if you are not paying attention and the amount of moisture is very slight, the effect will be to soften the focus of your image. You may think all this is obvious but be vigilant.
The solution is to make sure your camera and housing interior are thoroughly dry and you do not leave your rig out in the hot and direct sun at any time, especially while you are kitting up. Keep the housing shaded until you get in the water.
Next time we look at Colour Bars and Monitors.