The red filter replaces lost red, the first colour to be lost in the colour spectrum. After 70 ft other colours are also lost and you would only be replacing red with your filter but not the other lost colours...
Don't want to be picky but one of the common misconceptions about the red filter is that it "replaces lost red". It can not. Red light that has been absorbed by the water is gone. Can not be replaced by a filter (only a artificial light can do that - and only over very short distances). The red filter looks red because it does not allow blue and green to pass through it.
Why is this important?
Because, if you don't understand what is really happening with the light and colours, you'll make wrong choices.
The red filter is reducing
blue and green.
If there is enough red left in the ambient light, it will reduce blue and green, shifting the balance between the three primary RGB colours, creating the appearance of more red. If there is none or little red left, it wont help at all.
But, by reducing blue and green, it is reducing the total amount of light that the sensor is receiving. So, if the sensor is struggling to get enough light to get a decent image, you are just making it worse. Different cameras react differently. Some try to boost the light by amplifying it (gain). This can cause ugly fuzzies.
So a red filter (should really be called a "blue and green reduction filter") is only useful if there is a lot of ambient light and some red left in the mix of ambient light.
The depth range in which it is actually useful varies a lot from dive to dive and even during the dive as the sun disappears behind clouds. As a rule of thumb, you'll get some improvement between 5 to 15 meters with a red filter. If it very sunny, don't swing it in until 8 meters deep, otherwise some things will look too red. If it is cloudy start using it from 5 meters deep. Definitely have it swung out by 15 to 18 meters deep as you are just losing light to the sensor. There is no R left to "balance" with the GB.
If you turn on video lights, flip out the red filter. if you do a silhouette by pointing the camera towards the sun, flip out the red filter.
Better than a red filter, if you have a camera that can do white balance in low light, I suggest that you forget about the red filter all together and just set a suitable white balance for the depth that you are at. But that's a whole new story...
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Edited by peterbkk, 20 November 2012 - 05:37 AM.