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Question:

I've just started doing underwater video. My first attempt was disappointing due to an overall blue cast. I now know that a filter would probably help. What should I buy and when should I use it?

 

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Questions and Answers (Q & A's)

From Alan Broder (from Ocean Realm Magazine - September 1993)

Answer:

You’ve just pointed up an interesting difference between the human eye and brain and the video camera lens and electronics. You’re probably disappointed because the video didn’t record the color as you saw it on your dive. Video cameras have the ability to shift the color of the recorded image toward what is programmed into the camera’s computer as a more normal representative range, that is, to "white balance." White balancing refers to electronically shifting the color spectrum to make "clean" whites without any tint. Underwater, the white balancing process should operate towards minimizing if not eliminating the blue tint. Your eye and brain do the same, and they do it a gosh darn heck of a lot better. As you go deeper, and the color shifts toward overall blue due to an absorption of light at the red end of the visible spectrum, your mind makes continuous corrections to your perceived color. You do this very quickly and to a greater degree than the camera can.

Setting consideration of color, or chrominance as it is referred to in video, aside for a moment, let’s take a look at luminance, which is the other major component of the video image. Luminance refers to the overall brightness of the recorded image and is analogous to exposure or density in a photographic image. Simply put, luminance is rarely a problem underwater. Let’s see if we can turn this around a little and come up with an acceptable way of stating this fact with appropriate emphasis. Hmmmm—you could stuff a modern video camera four feet down a cow’s throat and do video! Don’t try this at home. Just take my word for it. Video is that sensitive to light—much, much more sensitive than photographic film—and video can adapt to lowering light levels much faster than the human eye can. As a matter of fact, video is so sensitive to low light levels that you can point the camera under a ledge where it’s far too dark for you to see any detail, press the record button. Go home and play the tape, and be utterly amazed at the bright images you managed to record. You’ll probably get very little inn the way of color, but the recorded images will reveal virtually everything that was going on in that dark environment. Had you stuck your head under the ledge, it would have taken minutes to dark adapt to the point that detail was discernible, whereas the camera did it almost instantaneously.

So what’s the point of all this? Why, the point is that you should use a filter, of course! Since chrominance is a problem, and luminance simply abounds, you can make a deal with the devil to trade off some luminance for some chrominance. The filter you’ve heard about is almost undoubtedly the U R PRO filter manufactured by Underwater Research Products of Naperville, Illinois. The standard and by far the most common of the three basic U R PRO types is the tropical filter. This filter is sort of orangey in color and has been scrupulously balanced to correct away the "blues" in tropical waters between about ten and fifty to sixty feet. Use of this filter will cost you one stop of light, but then you’ve got stops to spare! The results are astounding. Video taken at ten or twenty feet on a sunny day can look as if it were shot in a studio! However, somewhere around sixty feet or so, the gradual loss in effectiveness of the filter with increasing depth reaches the pint at which the filter becomes more of a liability than an asset. At just about the exact point that the filter fails to do much to shift the color of the video image, the specter of luminance-deficiency heads its ugly rear.

Probably 99 percent of the filters in use thread directly into the lens of the camera and can’t be removed during the dive. You’d have to be awfully fast!

A couple of manufacturers provide a filter which can be moved into and out of the image path by means of a control on the housing. This is a nice feature that they acquired by making a deal with you know who. Space must be provided between the primary lens on the camera and the almost always desired secondary wide-angle lens to allow for the filter to swing in and out. The further you separate these two optics, the more degraded the image—theoretically, at least.

You could mount the filter on the outside of the port. U R PRO makes such a model, about four inches in diameter by about two inches. It is, however, considered by most people who have used it to be clumsy, unstable, and potentially disastrous to videotaping should it slip out of alignment and become part of the scene. And what can you do with this big thing when it’s not in use? It could hang by a cord—and get into all sorts of mischief on the reef, in front of the lens, or with your gear. You could take your regulator out and hold the damned thing in your teeth—probably not an option for most of us. You could hold it between your knees and swim using the "modified ruptured dolphin kick." Of course, it’s possible to affix the filter to the housing with a hook or Velcro when not in use, and if you’re prepared to devote some serious attention toward its positioning on the port, this filter is a viable option. In general, however, you’re best off to belay using it.

If you expect to do your taping at over sixty feet for the grater part off your dive, it’s better to leave the filter off. Pick up color by using lights. If you’re planning a shallow reef dive—mostly above fifty feet—the filter is indispensable. Above ten feet, especially on upward angle shots, the tropical filter will add an ugly reddish cast. For a very shallow dive, such as a shark dive in a cage, leave the filter off. If you’re greedy like the rest of us and want both deep shots and shallow reef shots, consider the approach most pros might take in addressing this problem: make some of your dives with the primary objective of getting the deeper shots, and some for the shallower, and set up appropriately. This way you make no compromise in quality. It will also introduce you to the discipline of planning your scenes, resulting in more effective and more interesting videos, for you and your audience.

U R PRO makes two other filter types. A tropical type with only a half-stop density is made for use with lights in situations where ambient and artificial light are in about equal intensities when falling on the subject. Use of the normal tropical filter with lights will virtually always result in overly red and very displeasing video. A magenta-flavored "green water" filter is offered for—you guessed it—green water! This was supposed to include California water, but most divers use the tropical type in California waters with excellent results.

Keep in mind that the closer you are to your subject, the better the filter works. Keep in mind, also, that leaving the filter on in very low light conditions degrades the image quality since the electronic image enhancement utilized by the camera in its high sensitivity mode produces coarser detail in a manner similar to that of high speed film in still photography. Take all of this with a grain of salt, and so your own testing and experimenting. You may wish to bend or break any of the above general rules at any given time.

 

 

 

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