As promised in part 4 of underwater photography, we said we'll discuss how the underwater cam system works in more detail . So today were gonna talk about housings .
Flexible housings:
One solution to the pressure problem is to make the housing flexible. The housing can then 
simply contract (reduce in volume) to keep the pressure on the inside and the outside the same. This is a very reliable system, because the lack of any pressure difference means that there is no real tendency for the housing to leak. This system is also very simple, because the controls of the equipment inside the housing can mostly be operated through the flexible plastic. There are limitations to this approach however, because as you go deeper, there will come a point when the housing clings tightly to whatever is inside it, the housing won't be able to shrink anymore, and a pressure difference will begin to appear. Consequently, flexible housings still need to have a pressure-resistant sealing system. The ewa-marine housings in the pictures above all use a metal clamp, held together by hand-tightening screw-fasteners ('hand-wheel nuts'). Closing the housing is a matter of making sure that the two surfaces which will be clamped together are clean, since dirt, and particularly hairs, can make microscopic channels through which the water can creep. Cleaning the surfaces is simply a matter of wiping them with a lint-free cloth or paper towel, and inspecting them carefully before fitting the clamp. When assembled properly, the sealing system, and the tough flexible plastic material, can withstand large pressures, 30 or 40 metres of water, and the real limitation is that of how much punishment the thing inside the housing can take. With cameras, there comes a point when the water starts to press buttons which you don't want pressed, and so the recommended depth limit for flexible housings is usually (but not always) about 10 metres.
simply contract (reduce in volume) to keep the pressure on the inside and the outside the same. This is a very reliable system, because the lack of any pressure difference means that there is no real tendency for the housing to leak. This system is also very simple, because the controls of the equipment inside the housing can mostly be operated through the flexible plastic. There are limitations to this approach however, because as you go deeper, there will come a point when the housing clings tightly to whatever is inside it, the housing won't be able to shrink anymore, and a pressure difference will begin to appear. Consequently, flexible housings still need to have a pressure-resistant sealing system. The ewa-marine housings in the pictures above all use a metal clamp, held together by hand-tightening screw-fasteners ('hand-wheel nuts'). Closing the housing is a matter of making sure that the two surfaces which will be clamped together are clean, since dirt, and particularly hairs, can make microscopic channels through which the water can creep. Cleaning the surfaces is simply a matter of wiping them with a lint-free cloth or paper towel, and inspecting them carefully before fitting the clamp. When assembled properly, the sealing system, and the tough flexible plastic material, can withstand large pressures, 30 or 40 metres of water, and the real limitation is that of how much punishment the thing inside the housing can take. With cameras, there comes a point when the water starts to press buttons which you don't want pressed, and so the recommended depth limit for flexible housings is usually (but not always) about 10 metres.Hard, Rigid, or Pressure-Resistant Housings:
Hard housings, whether part of a camera, or a separate box into which the camera must be installed, are nearly always sealed by means of a device called an O-ring. In fact, there will be an O-ring seal for every control, and every plug, socket, or window, in the housing, but the user is mainly concerned with O-rings (such as the one around the door which gives access to the camera) which have to be disturbed in some way while the equipment is being prepared for use.
The O-ring itself is simply a rubber band, usually of circular cross-section, but manufactured to high tolerances so that it is free from nicks, holes, and other defects which might let the water into the housing. The O-ring is also made of a particular type of rubber (there are lots of types), chosen for its chemical resistance and hardness, and it is not a good idea to use a substitute unless you know what you are doing.
The 'O-ring seal' is the combination of the rubber ring and a specially shaped groove, which is designed so that, when the housing is placed under pressure, the ring is forced into the gap through which water is trying to pass.
The cute thing about this system is that the seal actually
gets tighter as the pressure increases; which means that the seal is inherently reliable, provided that the O-ring and the O-ring groove are kept in good condition and are kept thoroughly clean while the housing is being closed. This is the most important thing you need to know about underwater photography with hard housings, because bad husbandry in this department can lead to serious disappointment, whereas bad photographic technique can be rectified with practice. There is nothing difficult about assembling an O-ring seal however; it is simply a matter of cleaning the O-ring and the groove with a paper towel, 
inspecting the surfaces for dirt and hairs, and, for the type of seal shown above, applying a very thin coating of grease to the O-ring. The grease used on O-rings incidentally, does not form part of the seal; its job is simply to make the O-ring slippery so that it can slide into position without being damaged. Most manufacturers opt for the hard-housing approach to underwater equipment design, and a wide range of underwater cameras, camera housings, camcorder housings, and torches are available, with depth ratings from 20 metres to over 100 metres.
The O-ring itself is simply a rubber band, usually of circular cross-section, but manufactured to high tolerances so that it is free from nicks, holes, and other defects which might let the water into the housing. The O-ring is also made of a particular type of rubber (there are lots of types), chosen for its chemical resistance and hardness, and it is not a good idea to use a substitute unless you know what you are doing.
The 'O-ring seal' is the combination of the rubber ring and a specially shaped groove, which is designed so that, when the housing is placed under pressure, the ring is forced into the gap through which water is trying to pass.
The cute thing about this system is that the seal actually
gets tighter as the pressure increases; which means that the seal is inherently reliable, provided that the O-ring and the O-ring groove are kept in good condition and are kept thoroughly clean while the housing is being closed. This is the most important thing you need to know about underwater photography with hard housings, because bad husbandry in this department can lead to serious disappointment, whereas bad photographic technique can be rectified with practice. There is nothing difficult about assembling an O-ring seal however; it is simply a matter of cleaning the O-ring and the groove with a paper towel, 
inspecting the surfaces for dirt and hairs, and, for the type of seal shown above, applying a very thin coating of grease to the O-ring. The grease used on O-rings incidentally, does not form part of the seal; its job is simply to make the O-ring slippery so that it can slide into position without being damaged. Most manufacturers opt for the hard-housing approach to underwater equipment design, and a wide range of underwater cameras, camera housings, camcorder housings, and torches are available, with depth ratings from 20 metres to over 100 metres.
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