ANYONE WHO HAS DIVED IN WATER clear enough to see anything through will appreciate that light is absorbed selectively. Not only does it get darker as you go deeper but colours change, and it gets bluer as you get further from the surface. That's because the red light in daylight is quickly absorbed by a few feet of water, yellow light is absorbed within a few metres of the surface, and green light is gone by about 15m. Once you are deeper than that, you are left with an almost monochromatic blue scene. The solution is to take an independent supply of white light in the form of an underwater lamp. This will light up details in their natural colours, provided the light does not have to travel very far through the water. Visibly black objects are often revealed as red, for example, once lit up. Another, recently rediscovered solution for digital photographers, with cameras they can switch to high light-sensitivity (high ISO), is to use a filter over the camera lens to hold back the excess of blue light and let through whatever red light makes its way down there. Why not do the same for your eyes? What about a mask with a similar filter built in? The SeaVision mask has lenses coloured in a pale pinky-red or magenta. It's like looking at the underwater world through rose-tinted glasses. The lens is also finished with a patented anti-fog coating that is said to prevent steaming up. You are told not to polish the inside of the lenses with toothpaste or other abrasives, but simply to use the liquid supplied to wipe over the insides before each dive. This is because the lenses are actually made of very clear plastic instead of much heavier glass. I found that the mask had a soft skirt that was very comfortable but only too easy to fold under, with a resultant leak. I did a whole dive with my mask mysteriously flooding before another diver recognised the problem, even though I had felt around the seal several times myself. The filtered lenses also need sunshine. If there is no red light, they simply act as a neutral-density filter and make an otherwise dark dive even darker. Don't expect miracles. This mask works well at putting the colour back into what you see, but in the sunlit shallows only. I was in the Maldives during a particularly dark and stormy week, and found that the reduced amount of light passing through the SeaVision lenses made it almost impossible to line up shots through the eyepiece of my digital single-lens-reflex accurately. I had to be content with checking the shot on the LCD after I took it. A local dive-guide told me that her SeaVision mask had become an indispensable part of her kit. But she was used to predominantly bright sunshine! SeaVision masks are also now supplied with your optical prescription if you wish it. The importer can replace the original lenses for those that will suit your individual eyesight defects. Naturally that pushes the price up a lot. However, there are some standard-strength lens options available with plastic lenses. This keeps the weight of the mask down, and that is something to consider if you need a lens akin to the bottom of a Coke bottle. Overall, the US manufacturer claims that this is the best mask you can buy - but it would, wouldn't it? The SeaVision UltraVision costs £32. Cost of prescription lenses vary but masks so equipped cost from around £90 extra.