Ambient Light Exposure Underwater (with film)     

     The following discussion relates to the use of film, an ancient technology to many new underwater photographers but still useful when considering its advantages.  Slides and print film can always be scanned so even when shooting with film you still can convert to digital formats and do many of the things that digital shooters can accomplish. 

     Most underwater photographs you see in books and magazines use a combination of ambient light (from the sun) and artificial light (from a strobe), or rely entirely on strobe light to make an exposure. Relatively few make use entirely of the ambient sunlight penetrating the surface. Reasons for this include dim conditions due to the absorption of light by the water and the increasing dominance of blue wavelengths with increasing depth. This creates the need to bring more light down to illuminate the subject and bring out its colors. There are situations, however, where you may desire to use only the ambient sunlight to create your image. Photography from kayaks and in tidepools will also rely mainly on the ambient sunlight.

     Why would you want to forgo strobe lighting in favor of ambient? In shallow, clear tropical waters, strobe lighting may be unnecessary, particularly with a bright sand bottom that reflects light upward. Large animals near the surface, such as manatees, whales, dolphins and whale sharks generally will not need any strobe lighting. Getting rid of the strobe makes for a much slimmer package to carry around if you are snorkeling with these animals. In temperate waters of North America, you are less likely to find situations in which ambient light alone is sufficient. Exceptions include photographing kelp with sunlight streaming through the fronds and surface swarms of jellies. You may also decide to turn off your strobes in order to minimize backscatter when photographing jellies or schools of fish. Silhouettes of divers, marine mammals or some other swimming or floating subject present another opportunity to use ambient light exposures.

Exposure Determination

     Light, whether reflected or produced by your intended subject, creates the image on the light-sensitive material (film). Controlling how much light reaches the film, and how rapidly it does, determines the photographic principle of exposure. Establishing the correct exposure for each photographic situation is critical to success in any type of photography, whether above or below water. Even with modern autoexposure cameras, you should have an understanding of what goes into making a proper exposure. You will also need to understand how to balance light from ambient and artificial (strobe) light sources to come up with a correctly exposed image.

     Defining what a correct exposure is can be difficult. Basically it means that the range of tones in the image on the film is spread out sufficiently to match what you saw. For a photographic slide you thus will have an appropriate balance of light, middle range and dark areas. An underexposed slide will be shifted too far toward the dark tones, while an overexposed slide will be shifted too far toward light tones and appear bleached out. There is some leeway in what would be considered a perfect exposure, however, and you may prefer your images slightly darker or lighter than someone else may. Exposing film to light is an interplay among properties of the camera (shutter speed and aperture), film (film speed), and light from the subject and its surroundings. Two images may appear to have identical exposures but may have been derived from vastly different combinations of these variables.

Light Meters

     With both manual and autoexposure cameras, shutter speeds and aperture are determined with the use of a light meter. This device measures the amount of light reflected from the subject and calculates the appropriate aperture for a given shutter speed. Nikonos II and III cameras lack a light meter and require an accessory meter to measure light. The Nikonos V has a built in light meter, as does every SLR camera currently made. The readout in the Nikonos V (visible as red LED’s in the viewfinder) shows the appropriate shutter speed for a given aperture. The metering pattern on the Nikonos V is a bit more bottom weighted than the typical SLR, which tends to be center weighted. This means that the Nikonos gives somewhat more emphasis in determining the exposure to the area just below the center of the image, which is usually darker than the upper area.

     Modern electronic cameras generally have several metering pattern options. In addition to the standard center-weighted mode, many now have advanced segmented metering patterns that analyze light variations within the area of the image to determine appropriate exposure. Partial metering, with only a portion of the image area analyzed (such as a central area of 10 degrees), is frequently another option. Top of the line SLR cameras usually have the option of using what is known as a spot meter, which has a tight angle of view for more precise metering of light from the intended subject. Spot metering areas can be useful when photographing a subject that is small or varies greatly in brightness from the rest of the background.

     An important consideration regarding light meters is that they are set to come up with an overall average image brightness. If you ignore color and look just at the tone of this average, it would be a gray color intermediate between white and black. With normal scene variation in brightness, this property of meters works just fine and the image will appear like you saw it. A scene that is dominated by bright areas, however (such as a snowfield), may result in a dreary gray photograph since the meter thinks the average for the image should be an intermediate gray. You can compensate by opening the aperture a stop or two to increase exposure, or by using a camera with a sophisticated metering system that adjusts for this problem.

Shutter Speed

     Shutter speed controls the amount of time the film is exposed to light. With everything else constant, the longer the shutter is open, the more light proportionately strikes the film. Generally for underwater photography you will be working with relatively short time periods such as 1/60 or 1/125 of a second. If you double the length of time the shutter is open (from 1/250 to 1/125 second for example), you double the amount of light exposing the film. This doubling of light is known as 1 stop of exposure. Most modern 35-mm camera shutters now consist of a pair of curtains, each formed by a set of thin blades that slide over each other. When you trip the shutter release to start your exposure, the first curtain opens up to begin exposing the film to light. The second curtain follows to close the shutter and stop light from reaching the film. The time interval between the opening of the first shutter curtain and the closing of the second is the shutter speed.

Lens Aperture

     The other factor controlling the amount of light reaching the film is lens aperture. For a given shutter speed, the wider the aperture, the more light that reaches the film (makes sense – bigger opening, more light). A set of blades in the lens slides to open or close and change the size of the circular opening. Aperture values are measured by what is known as an f/stop, the ratio between the size of the aperture opening and the lens focal length. Larger f/stops thus indicate smaller openings. A typical range of f/stops for a 20-mm lens would be f/2.8 (wide open) to f/22 (closed down). Telephoto lenses tend to have smaller wide-open apertures, such as f/4.5 or f/5.6 since a wide piece of glass would otherwise be needed. That’s why f/2.8 telephoto lenses are quite expensive, often several thousand dollars. The advantage for these monster lenses is that they are more effective under dim light conditions and have a brighter viewfinder. If you encounter a lens that is labels as "fast", this is due to its large maximum aperture and hence good light gathering capability. For most SLR lenses, the aperture stays wide open until the shutter is tripped, at which point the blades close down to the proper f/stop for the exposure. Nikonos lenses lack this ability and close down to the setting on the lens dial.

     An increase of 1 aperture stop translates to a doubling of the amount of light reaching the film. Starting from smallest aperture, f/stop values proceed like this :

f/32 - f/22 - f/16 - f/11 - f/8 - f/5.6 - f/4.5 - f/3.5 - f/2.8

     Increasing the aperture opening from f/5.6 to f/4.5 thus increases the exposure by 2 times, given everything else constant. Look at the aperture values on your lens barrel and these are the numbers you will see.

     Various combinations of shutter speed and aperture can produce an equivalent exposure. Increasing the shutter speed by 1 stop (such as from 1/60 to 1/30 second) and reducing the aperture by 1 stop (such as from f/8 to f/11) results in an identical exposure. There are several reasons why you may favor a faster or slower shutter speed, or wide rather than small aperture for a particular exposure.

     Fast shutter speeds (1/250, 1/500, 1/1000 second) are used for stopping action and movement. When photographing from a boat or kayak, for instance, you mat want to use a fast shutter speed to counter the movement of the camera. This is particularly important when using a telephoto lens. The downside of a fast shutter speed is that the lens aperture will generally need to be at or near wide open. With dim light a fast shutter speed may not be possible. A slow shutter speed, on the other hand, may be preferred if you desire to blur the effects of motion. You also can have a smaller aperture when selecting a slow shutter speed. For reasons we don’t need to get into, the smaller the aperture, the greater the depth of field. Depth of field is the area in front of and behind the point of actual focus (usually where the subject of the photo is placed) that still appears sharply in focus. A wide-open lens has a distinctly smaller depth of field compared to the same lens closed down to minimum aperture.

The following combinations of f/stops and shutter speeds will result in the same exposure on the film:

f/2.8 and 1/500 second f/4 and 1/250 f/5.6 and 1/125  f/8 and 1/60 f/11 and 1/30 f/16 and 1/15

     Note that although the exposure should appear the same, the images may vary significantly in regard to both depth of field and lack of sharpness due to camera or subject motion. In this particular situation, if you had hoped to shoot at 1/1000 second, you would need more light, a faster lens, or faster speed film.

Camera Exposure Modes

     In the old days of 20 years ago, photographers set the aperture and shutter speed manually based on light meter readings. Sophisticated electronic cameras of today eliminate this. It’s still a good idea to understand how exposures are determined even if the camera is doing all the thinking. Autoexposure comes in several varieties. With aperture priority (like that in the Nikonos V), you set the aperture and the camera determines the appropriate shutter speed. This is handy if you want the lens always wide open for the fastest possible shutter speed, or closed down for depth of field. Problems can occur if you hope to have a relatively fast shutter speed, however, since your aperture setting may limit the exposure to slower speeds. With shutter priority automation, you set the desired shutter speed and the camera sets the appropriate aperture. This is the setting to use if you must have a fast shutter speed to stop movement of the subject. Of course, if your shutter speed selected is too fast, the lens may not be capable of opening wide enough to gather sufficient light. Indicators in the viewfinder will usually warn when a shutter speed or aperture limit is reached. Another exposure style is known as program mode. In this case the camera has a set selection of shutter speed and aperture combinations that are selected based on the light levels detected by the meter. Usually there are several program patterns that can be selected to provide emphasis on shutter speed or aperture. Program mode leaves more decision making with the camera and probably should be avoided unless you don’t have any preference for exposure settings.

Film Selection

     Another variable in determining exposure is the speed of the film. All films have an ISO (formerly ASA) rating that measures the speed with which an exposure is made on the film. Doubling the ISO value (from 50 to 100, for example) reduces by half the amount of time to make an equivalent exposure. As with shutter speeds and apertures, this doubling is the same as a 1 stop increase in exposure. Faster films enable faster shutter speeds for a given aperture and light value. If you need a faster shutter speed (perhaps you’re photographing from a rocking boat) and the lens is wide open, then a faster film may be the best course of action. Why not just use fast film all the time? Unfortunately, to make a film more responsive to light, the grains making up the emulsion must be larger. This results in the general trend that the faster the film, the grainier will be the image. If you require a very fine-grained photograph, then a slow film speed such as 25 or 50 will be necessary. During the last ten years film manufacturers have made remarkable strides in improving grain in both slide and print films.

     The choice of shooting with print or slide film depends on your photographic objectives. If you hope to submit photos for publication or to use images for presentations, then slides are the obvious choice. Print film may more suitable if your prime goal is to make photographic prints for display. Slide films are a direct representation of the image without an intermediate step that occurs when making prints from negative film. Slides generally appear more brilliant and crisp compared to prints. Most people would rather look at prints rather than slides though, so if you just want to show pictures of your dive trip, then print film would be more suitable. Print films also have a far wider exposure latitude, which means you can be way off on the exposure and still have a chance at getting a decent print. With slide film, if your exposure is off by as little as ½ stop, you may not produce a desirable photo. Errors in your camera’s light meter or autoexposure system are thus more easily detectable when using slide film. With slide film, it’s generally better to err on the side of underexposure (not enough light exposing the film). Overexposing slide film results in washed out images with areas of little or no density. Print films can handle overexposure well, even 2 or 3 stops.

     Most advanced underwater photographers use slide films in the ISO 50 to 100 range. Kodak films, such as Kodachrome 25 and 64, Elite Chrome 100 and Ektachrome 100 SW are popular films for underwater, kayak and aquarium photography. Likewise for Fuji slide films which include Velvia 50, Provia 100 and Sensia 100. Whether a film is considered amateur or professional is generally inconsequential for most of us. Besides being more expensive, pro-grade films are designed for stricter color fidelity from batch to batch and in storage, which is usually not a major concern. Other characteristics, like grain and sharpness, are basically the same. Although some of the newer ISO 200 slide films are vastly improved over earlier versions, you should use the finer grain slower films if possible. Slide films vary greatly in their color balance and saturation. Films like Velvia are made to produce stunning, brilliant color that many people like. Others may prefer the more natural appearance of Kodachrome. The choice really depends on individual preferences. When doing wide angle underwater photography in Monterey Bay where the water often has a greenish color, films with a blue cast may help to create a more pleasing image. For print film aficionados, Fuji and Kodak films are available in a range of speeds. The current selection of excellent ISO 400 and 800 print films enable photography with available light in relatively dim conditions.

Bracketing Exposures

     For any scene, there is not necessarily one absolutely correct exposure. By under and overexposing plus or minus ½ to 1 stop from the recommended setting, you can be more assured of getting an exposure you desire. With manual exposures, you can do this by varying the lens aperture. Automatic cameras often have exposure compensation modes that enable bracketing of +/- 3 stops in 1/3 stop increments. You can also change the ISO setting. For example, if using ISO 100 film and you want to underexpose by 1 stop, then set it for ISO 200. Of course, bracketing several exposures means that the subject must stay still for a while, a situation that is not always possible.

Different Situations

     There’s a lot to think about when it comes to controlling exposure on film with ambient light. For the most part with underwater photography, you are metering light levels for the background rather that the subject itself. For close-up photography there are few situations where you would be metering light levels and using ambient light since strobe lighting is typically the dominant source. Ambient light is almost always far too dim make an image with a macro lens. For wide-angle photography, a center or bottom-weighted light meter should be sufficient. With housed systems you can use the SLR camera’s internal meter; the Nikonos V has its own meter. An excellent meter that many advanced underwater photographers use is the Sekonic Marine Meter. This handy light meter has a very clear read-out for determining aperture settings based on film and shutter speed. They are a little hard to find in decent shape, however. Regardless of the meter, you should point it at the area of the background with an intermediate brightness. Metering at or near the position of the sun will result in an underexposed background. Pointing the meter at the darkest part of the image area on the other hand will create a washed-out overexposed photo. For aquarium photography you will probably not be using ambient light for the exposure so a light meter is not necessary. When photographing in tidepools or from a kayak, the situation will influence the exposure mode you select. Kayak photography usually demands fast shutter speeds, and hence you may wish to use shutter priority exposure. With tidepool photography a tripod can be used, and thus long exposures are possible. Aperture priority may be the best mode since you can close down for greater depth of field.

Focusing: Manual and Autofocus

     Another advantage of modern SLR cameras is the capability of using autofocus lenses. In the old days (which weren’t that long ago), a lens was focused by turning the lens barrel until the focus appeared correct through the viewfinder. Most viewfinders for manual focus have one or more visual aids that help to determine when the proper focus point has been reached. Nowadays we have autofocus lenses that quickly snap into focus far quicker than most of us can focus manually. Autofocus systems have improved considerably – they’re faster and capable of focusing in most situations. Telephoto photography benefits the most from autofocus, particularly with subjects in motion. It can be somewhat difficult to achieve proper focus with a 300-mm lens in a rocking kayak. Autofocus ensures that the subject area will be focused correctly (although camera and/or subject motion may degrade sharpness if the shutter speed is too slow). The benefits for wide-angle photography are quite a bit more limited since these lenses have wide depths of field. Autofocus can also be useful for close-up photography in tidepools and aquariums. With a dimly lit aquarium, however, insufficient light may prevent the autofocus from finding the point of focus and the lens may oscillate back and forth. Underwater, the advantages of autofocus are somewhat limited. It has relatively little use with wide-angle lenses and dome ports since the point of focus is a virtual image just in front of the port. More and more underwater photographers are switching to autofocus for close-up photography. It certainly has advantages if you get used to it, particularly for subjects like moving fish. Autofocus is by no means necessary for close-up photography and perfectly focused images are possible most of the time if care is taken. Dimly lit scenes underwater also can present problems for autofocus systems that require sufficient light to establish the focus point.