Filters and Ambient Light Photography

One of the most important and least understood aspects of underwater photography is the interaction between visible light and water. Water acts a variable filter of visible light allowing some colors to pass while blocking others quite effectively. In fact, it's this very property that defines visible light and influenced the development of our eyesight in the first place. It benefits us all as photographers to understand how light and water interact so that we can better achieve the results we desire. The purpose of this article is to explain how to filters can be used to enhance ambient light photography.

Sinai, Egypt. Image courtesy of Alex Mustard.
Nikon D100, 16mm lens, ISO 200, aperture priority, matrix metering
Red filter and custom white balance using grey card

What does water do?

In order to select the perfect filter to use with a given shot we need to first understand the effects that water have on ambient light. The primary factors to consider are the amount of water, the water quality, and the nature of the ambient light itself. The amount of water in ambient light photography is the sum of the depth of the subject and the distance of the subject to your lens. The quality of the water determines its filtering properties. The greater the dissolved and suspended matter, the greener (or browner) the water becomes. The time of day and cloudiness of the sky also have a great effect on the nature of the light available.

Water behaves as a light filter that has two basic components. First, it shifts the light toward higher color temperatures. In other words, it selectively favors blues and greens and filters out yellows and reds. The second component is that of a bandpass filter that's centered typically on cyan. As water gets increasingly turbid, the center of the band shifts toward green and eventually to yellow. This explains the green and even brown appearance of turbid water. Water also diffuses light in a color-specific way. This results in direct and diffuse light that potentially have different spectral content.

What can we do about it?

Ideally we would know the precise effect of the water on our light and compensate for it using filters. Unfortunately many things get in our way. First, we can't really ever know the effects of water since they're constantly changing. Second, we're restricted on the choices of filters we can use and probably can only choose one per dive. Third, filters can only work by subtraction and cannot compensate for colors that are alreaady lost. Once at sufficient depth, no amount of filtration can effectively restore lost reds. For this there is no substitute for artificial light.

With the advent of digital photography and electronic white balance many say that filters are unnecessary. That is simply not true. Electronic adjustments occur after the picture is taken and only serve to modify it's presentation. Filters work before the image is taken and fundamentally alter its content. If the light entering the camera lens is predominantly green, then the camera is obligated to set exposure based on green and the quality of the red and blue portions suffer. If the green light can be brought into proper balance using filters, the subsequent image will be of higher quality. The use of a green removal filter also helps control diffused light resulting in improved contrast. The combination of filters and electronic adjustments can be quite powerful.

Two classes of filters are interesting for underwater use, color compensating filters and color conversion filters. A third class is interesting as well and will be covered later. The color compensating filters are easiest to describe and will be covered first.

Grand Cayman. Image courtesy of Alex Mustard.
Nikon D100, 16mm lens, ISO 200, aperture priority, matrix metering
Red filter and custom white balance using grey card

Color compensating filters.

The color compensating (CC) filters are used in photography to select or favor a subset of the color spectrum. Of these, the magenta CC filter is useful for counteracting the bandpass effects of water. CC filters are specified by their strength and are typically available in increments of 10 units. The most common magenta CC filters will range from 10 to 50 units. Water varies in its behavior, but will typically benefit from 2-5 units of magenta filtration for each foot of water in the light path. More turbid water needs more magenta filtration.

Kodak Wratten gel filters (photo courtesy of Eric Cheng)

Color Conversion filters.

Color conversion filters come in two varieties, warming and cooling. For this discussion the warming filters are valuable since they can be used to counteract the cooling effects of the water. White light is characterized by its color temperature, specified in Kelvin, where warm light has low values around 3000K and cool light has higher values around 5500K. Although Kelvin is the measure photographers are accustomed to, it is more useful to speak of light in terms of mireds, or micro-reciprocal degrees. You can convert Kelvin to mireds by inverting and mutliplying by 1000000.

There are two series of warming filters, the 81 series and the 85 series. They are characterized as follows:

There are two analogous series of cooling filters, the 80 and 82 series, and they have matching but negative mired values. For our application they are uninteresting but they can be used to adjust the color of strobes or halogen lights.

When applying a filter to light simply add the filter value to the light value to get the result. Water generally needs 2-5 mireds per foot to compensate for its cooling effects with turbid water requiring less compensation.

Flourescent Filters.

Up until now we've talked about filters that are primarily available as gels that you have to combine and mount yourself. As it turns out, photographers have had to deal with another light source that is overly green and that's flourescent lighting. Because of that we have at our disposal a limited range of thread-on and rectangular filters that are combinations of the filters that we desire. Additionally, many underwater photographers are familiar with UR Pro, a company that specializes in filters for underwater use. The UR Pro filters are functionally like flourescent filters. Below is a table that I've compiled characterizing these filters.

Grand Cayman. Image courtesy of Alex Mustard.
Nikon D100, 16mm lens, ISO 200, aperture priority, matrix metering
Red filter and custom white balance using grey card

Recommendations.

First, your choice of filter depends on whether you are deep or shallow and whether the water is blue or green. The stronger FL-B filters are only suitable for blue water and moderate to deep depths (20-70 feet). Use the FL-D type filters for green water or shallow depths.

For any given lens or housing, you may be able to use thread-on filters, rectangular filters (externally) or gels. Some combinations may be supported only by a UR Pro filter in which case your choice is fairly obvious. For those that can use thread-ons I prefer the Singh-Ray FL-B, the Hoya FL-D and the B+W FL-D. I have not used the Hoya FL-W but it may be desirable for very green water. For those who can mount a 4 inch/100mm Lee filter holder externally, the Lee FL-B filters provide essentially a shallow/medium/deep solution. I have also adapted a UR Pro CY resin filter to the Cokin X-Pro filter system. I personally avoid the Tiffen filters and the UR Pro glass filters due to their sandwich glass construction. Sandwich filters have many more light boundries that make them more susceptable to flare.

For those lenses that have rear gel holders your choices are more interesting since you can roll your own. First choose your magenta gel. I would use 20 to 50 units depending on depth and clarity of the water. You can then optionally add a warming gel. I would bring an 81C, and 81EF, and a 85 for example. The benefit of the warming gel is that it brings the balance of the light closer to matching the optimal balance of the imaging sensor. The downside is that you give up light sensitivity. On bright days and deep dives use a strong warming gel. Otherwise use a weaker gel or leave it out entirely.

I hope this has been a useful and informative explanation of what is available to underwater digital photographers. I encourage those interested to give filters a try and please provide your feedback for others to learn from. Digital is a revolutionary learning tool for underwater photography and we should use it to develop new ways to gain new perspectives on the subjects which we love.

Note from the editor - According to Craig, a good starting point is to purchase two strengths of magenta (say, 30 and 50 or 20 and 40), and then add two strengths of warming (81EF and 85). For shallow shots use the weak pair. For deep the strong pair. For green water use the strong magenta with or without the 81EF. You can combine the weak pair or the CC30M alone with strobes if you carefully control strobe power. Reduce strobe power by 1-2 stops to start. Filtering strobes is ideal, but we have some more experimentation to do before anything conclusive can be said.

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There is currently an active thread about available light photography in our forums.

Digideep also has an article about ambient light photography.

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