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Film speed underwater

Factors affecting your choice of film speed or digital sensitivity setting


Film with high contrast and good color saturation is needed to accurately capture colorful deep sea subjects, such as this commensal (clown) shrimp on a crimson anemone. Photo by Tom Sheldon.
Film with high contrast and good color saturation is needed to accurately capture colorful deep sea subjects, such as this commensal (clown) shrimp on a crimson anemone. Photo by Tom Sheldon.

Although photographers have their individual preferences, there is no one film or film speed (or Sensitivity or ISO equivalency setting in a digital camera) that is suitable for all underwater photography, but ISO 400 film (color or black and white) or a digital camera's Sensitivity setting of 400, comes pretty close to being right when you will be facing a variety of situations. This does not mean that slower-speeds cannot be used, especially when employing flash, and many underwater photographers shoot with film or Sensitivity settings as fast as ISO 800 and faster.

As in photography on land, shooting conditions and your objectives determine film choice. Films with high contrast and good color saturation help combat the reduced contrast and saturation underwater. (Be sure to use rolls of 36-exposures if you are shooting 35 mm film so you won't have to come up as frequently to change film.) Shutter speeds of 1/60 sec and 1/125 sec are the most commonly-used, with 1/125 sec providing greater reduction of blur from camera movement.

Quality film and proper exposure are essential for good images of underwater subjects. Photo of translucent nudibranch by Tom Sheldon.
Quality film and proper exposure are essential for good images of underwater subjects. Photo of translucent nudibranch by Tom Sheldon.

NATURAL LIGHT PHOTOGRAPHY

The factors that affect your selection of film speed or your digital camera's sensitivity for natural light underwater photography include:

  • the amount of sunlight striking the subject,
  • surface conditions (calm or choppy),
  • subject depth,
  • subject distance and
  • water clarity.

Let’s assume you have the best possible conditions at first – lots of sunlight on a calm, unruffled surface, crystal clear water and shallow depth – after which we’ll look at film needs as conditions change. A bright, sunny day with the sun more or less directly overhead provides ideal illumination for underwater, natural-light photography, permitting a wide range of exposure settings for subjects near the surface, even with slow speed daylight film or a low digital camera's sensitivity setting - in the range of ISO 64 to ISO 100.

If you are using ISO 64 film, a meter reading taken in sunlight just above the water’s surface would provide you with an exposure of about 1/60 sec at ƒ16, in accordance with the Sunny 16 rule.

This means you would be able to shoot just below the surface with the same shutter speed but at an aperture setting of ƒ11, which is one stop less. (There is a one to one-and-a-half stop reduction of light due to surface reflection. At six-feet of depth, there is a two-stop reduction in light.)

This provides you with a choice of exposure settings from among the following range of combinations:

  • 1/60 sec at ƒ11,
  • 1/125 sec at ƒ8,
  • 1/250 sec at ƒ5.6 or
  • 1/500 sec at ƒ4.
Shallow underwater photographs taken with one of these combinations should provide you with properly-exposed images. (See Exposure to learn why you would choose one combination over another.)

If you switch to slightly-faster ISO 100 or ISO 125 film, your exposure settings will change by about one stop, permitting you, for instance, to shoot just below the surface with a shutter speed of 1/125 sec at ƒ11 instead of 1/60 sec at ƒ11.

Calm, shallow water and a bright sunny day provide the best conditions for natural-light underwater photography.
Calm, shallow water and a bright sunny day provide the best conditions for natural-light underwater photography.

DEEPER, CLEAR WATER

If your subject is in deeper water (three to twenty feet) that is calm at the surface and crystal clear, you will need a faster film or sensitivity setting to be able to use the same settings, because water absorbs one stop of light for every ten feet of depth. Therefore, use ISO 200 instead of ISO 100 film for subjects that are around ten feet below the surface, and switch to ISO 400 film for deeper subjects.

However, water absorbs proportionately more of light’s red wavelengths as depth increases, requiring you to use a light-red color-restoring filter for subjects that are around four to ten-feet deep if you wish to keep colors looking natural, and a medium-red filter to restore proper color rendition from the 11 or 12-foot level. Light’s red wavelengths are completely gone at about the twenty-foot level.

Artificial light is essential for photography in deep water.
Artificial light is essential for photography in deep water.

USING A RED FILTER REQUIRES EXTRA EXPOSURE

You must increase exposure by the filter factor of whichever red filter you employ. The factor for a light-red filter is around 1.2 to 1.3 - meaning you need an increase in exposure of about 1/3 of a stop - and is around 8 for a medium-red filter, requiring you to increase exposure by up to three stops - a big increase. A camera with automatic exposure control will correct the exposure automatically when a filter is attached.

Let’s summarize the exposure changes needed for underwater photography using natural light:
1. an increase of one stop for the fall-off in the intensity of light from surface reflection;
2. plus an increase of one stop for every ten-feet of depth to compensate for light’s absorption by water;
3. plus an increase in exposure based on the filter factor for whichever red filter you use.

What you could photograph at the surface using a setting of 1/60 sec at ƒ16 with ISO 100 film would need an exposure setting of 1/60 sec at ƒ4 if the subject was around 10 feet deep and you have a medium-red filter attached. Your range of useful exposure settings is too limited at this depth with ISO 100 film. Switching to ISO 400 film, which is faster by two stops, would mean you could shoot subjects at a depth of 10-feet or so at, for instance, 1/60 sec at ƒ8, or 1/125 sec at ƒ5.6.

As your subjects get deeper – beyond 12-feet – decreased light levels overall make photography by natural light more difficult. As you get beyond 20-feet of depth, photography by natural light becomes even more restricted, although natural light can still impact your exposure down to about 30-feet, at which depth red and orange light are gone (completely absorbed) and yellow is quite diminished.

Slow speed color film can be used with flash underwater.
Slow speed color film can be used with flash underwater.

OTHER FACTORS BESIDES DEPTH AFFECT YOUR CHOICE OF FILM SPEED OR DIGITAL SENSITIVITY

Subject depth is not the only factor in your choice of film speed or the sensitivity setting for your digital camera. If the sky is overcast or the surface is rough, causing less light to penetrate the water, you need a fairly fast film for shots very near the surface. Depending upon the amount of light, you may need ISO 200 or ISO 400 film within a few feet of the surface, and ISO 800 film as you go a bit deeper.

The worst-case scenario for natural-light underwater photography is a combination of murky water, a dark sky, a choppy surface that can further reduce light penetration by one stop or more, a dark bottom and a subject beyond ten to twelve-feet of depth. Under these conditions, there may be no film that is able to provide proper exposure with natural light as your only source of illumination.

WHAT ABOUT USING VERY HIGH SPEED BLACK AND WHITE FILMS?

Since the increased speed of super-fast black and white film is largely due to extra sensitivity to red light waves, very fast black and white films do not retain their high speed in deep water, because water absorbs red light waves first. So, unfortunately, they are not an option.

This subject is much too colorful for black and white film.
This subject is much too colorful for black and white film.

FLASH ILLUMINATION

As you can see, natural light photography underwater is restricted to depths relatively close to the surface, but artificial light photography can be accomplished at any depth, and is only limited by flash-to-subject distance, just as it is on land – well, almost the same. Flash distance is limited to about 10 to 12-feet underwater, whereas on land it can be effective at much greater distances.

If you will be shooting with flash, slow films or low sensitivity settings in your digital camera will give the best results for close-up photography, with speeds from ISO 25 to 100 for color films and ISO 100 to 125 for black and white.

For more distant flash pictures up to about ten-feet, use faster film, ISO 400 or faster for black and white or color, and for your digital camera's sensitivity setting.

Natural light photography underwater is restricted to depths relatively close to the surface.
Natural light photography underwater is restricted to depths relatively close to the surface.

 
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