The Challenges
 Compared to taking photos underwater, taking pictures on land is a walk in the park. When we are on land we often find that the photographer or the subject (or both) are not moving. When we are taking a photo of the Grand Canyon or the Rocky Mountains we would not expect that walking forward or backwards twenty feet (6.1 meters) would dramatically alter the amount of light reaching our camera. When taking photos from the observation deck of the Empire State Building we would not expect to gain or lose color simply due to our change in altitude from the sidewalk. But when we go diving many of the rules and assumptions we operated with on land disappear. When diving we often find that either we or the subject of our photograph (or both) are moving. As we go deeper we have less light and less color. All of these challenges must be addressed in order to obtain quality photographs. In order to understand how to address these challenges it is important to understand how photography works.
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What is Photography?
Photography is the process of recording an image on a "sensitive material." In the days before digital photography, the "sensitive material" was film or a photographic "plate". Today it is a digitized manifestation of film referred to as the camera's "sensor." Although we are in a digital age, many of the terms and concepts of digital photography find their roots in film and, therefore, we still use many of the same terms and concepts in a digital environment.
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Film Speed / Sensor Sensitivity
 In a film environment the sensitivity of the film was noted by its ASA number.
ASA numbers ranged from a low (slow sensitivity) of 25 to a fast (high sensitivity) of 400 or more. Photographers would use a slow ASA film when taking photos in brightly lit environments such as a downhill ski slope or beach scene and would use a fast ASA film when taking photos in a dimly lit area such as an indoor concert. We do not use ASA for digital cameras but instead use ISO (International Organization of Standards). The ISO standardized values for digital photography are 64, 100, 160, 200, 400, 640, 800 and 1600 although many digital cameras have ISO settings below 64, at levels between the standard ISO levels, and levels far above 1600. Some cameras have ISO levels that exceed 12,000!
Changes in an ISO setting can affect a photograph in two ways. First, the lower the ISO number the less grainy will be your photograph. The implication is that if you want to enlarge your photo to a poster then a lower ISO setting will allow you to do so with less risk of the picture becoming grainy. One generally will not find a huge difference in graininess in photos shot at ISO 64 and ISO 160 but they will notice the difference between ISO 64 and ISO 1600. For most of us who share our photos digitally and generally do not make prints larger than 8" x 10" (203mm x 254mm) any differences in ISO below 800 will not have a negligible effect upon our pictures.
 The second impact that ISO has on photographs concerns two other variables: shutter speed and aperture—both of which are discussed later in this article. In a nutshell, the slower the ISO (the less "sensitive" is the camera) the more open (wider) the aperture needs to be OR the longer the shutter needs to be open in order to allow sufficient light to reach the sensor so that a good image is recorded. That is, the three elements that affect how much light reaches the sensor are (1) the ISO setting (the sensitivity of the camera to light), (2) the aperture / lens opening (how much light is allowed to enter the camera) and (3) the shutter speed (how long the shutter is open and allowing light to enter.) As you may have gathered, if the ISO is slow (ISO 64) then either the aperture needs to be open wider (f/11) or the aperture needs to be open for a longer period of time (a slower shutter speed) in order to capture a good image. Likewise, if the ISO is fast (ISO 1600), then a more narrow aperture opening (f /1.4) or a faster shutter speed can be used to obtain a quality photo. Both the aperture setting and the shutter speed have an impact on the composition of the photo. In summary, photography is the process of recording an image on a sensitive material and the three elements that affect the recording are (1) the sensitivity of the material (determined by the ISO setting), (2) the aperture opening and (3) the shutter speed.
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Shutter Speed
Shutter Speed refers to the length of time that the shutter on the camera is open allowing light to reach the sensor. Think of the shutter as your eyelid. We all blink thousands of times a day and the blink is so fast that it barely interrupts our vision and the light reaching our eyes and brain seems almost uninterrupted by the blink. But when we are tired we may close our eyes for several seconds and then no light reaches our eyes or brain. Shutter speeds on the camera act the same way. Standard shutter speeds are (in seconds of duration) 1, ½, ¼, 1/8, 1/15, 1/30, 1/60. 1/125, 1/250, 1/500, 1,000.  As can readily be seen, most of these settings are double the setting to the right and half the setting to the left. That is, if you set your shutter speed at 1/30th (of a second) then it will be open twice as long as the setting to the right (1/60th) and half as long as the setting to the left (1/15th). Remember earlier when discussing f-stops you learned that the standard f-stops also have the 50%+/- feature? Now we can put these two features together. Assuming that the ISO (film speed) stays the same, you can have the exact same amount of light reach your sensor if your f-stop is set at f /1.4 and your shutter speed is set at 1/500 as you would if the photo was shot at f /2.8 and your shutter speed was set at 1/250! The second option (f /2.8) lets in half as much light as the f /1.4 setting but we make up for that lost light by keeping the shutter open twice as long (from 1/500th of a second to 1/250th of a second.) The difference, however, will appear in the depth of field because the shot taken at f /2.8 will have more of the image in focus than the shot taken at f /1.4.
 Photos taken at the "Gardens of the Queen" in Cuba will help to show this phenomenon. Both photos have equal exposure of the fish due to adjustments made in the ISO and shutter speed. The first photo, shot at f /5.6 shows the all three fish (two Stoplight Parrotfish and one Blue Tang) in focus as are most of the surrounding corals and rocks. The second photo, (Blue Tang) was shot at f/2.5 and only has the fish is in focus and all the surrounding corals and plants are out of focus. The different settings of the f-stop resulted in the changes in the depth of field that resulted in different images. When composing a photo it is important to keep in mind what you want the desired image to look like. Some find it easier to shoot in Aperture Priority (more on that in a minute) so that they only need to address the depth of field issue and they will let the camera handle everything else. Others like to shoot Manual so they can tweak every setting. Each approach has its benefits.
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Aperture
Think of the aperture as the pupil in your eye. When we go into a darkened room the pupil opens wider (dilates) to let in more light. When we go outdoors into bright light the pupil contracts and lets in less light. In both instances we generally see everything that is visible even though in the brightly lit environment the pupil may be open only 25% of the amount it was open in the darker room. In both instances the "sensitive material " (our brain) "records" what is seen regardless of the opening size of the pupil. The same is true with a camera.
 Aperture openings also are standardized and range from 1.4 (a very wide opening) to 16 or 22 (very narrow openings.) The standard openings are 1.4, 2.8, 4, 5.6, 8, 11, 16 and 22 although you may find cameras with .4 and 32 openings. (Aperture numbers are like the numbering for electrical wire—the smaller the number the bigger the wire / opening and the bigger the number the smaller the wire / opening.) These openings are referred to as f-stops. (An f-stop is an optical term for the ratio of the lens' focal length to the diameter of the pupil (in humans) or aperture opening (in cameras.) Two things are important to keep in mind regarding f-stops. First, for any given f-stop, the f-stop below will allow 50% more light to enter and the f-stop above will allow 50% less light to enter. That is, if you have your camera set on f /5.6 and your photo is too dark you can increase the amount of light reaching the sensor by 50% if you change your f-stop to 4. Likewise, if your photo is over-exposed you can reduce the amount of light reaching the sensor by 50% by changing the f-stop to 8. Hopefully now you are beginning to see how tweaking the ISO setting and aperture opening can work together. When shooting with a slow ISO (ISO 64) it may be necessary to use a wide aperture setting (such as f/1.4) whereas when shooting with a faster ISO (ISO 800) it may be possible to use a smaller aperture setting (such as f/16). This brings us to the second issue regarding aperture openings: depth of field.
 Have you wondered how a photographer captured a beautiful flower that was perfectly in focus but the leaves on the branch, less than six inches (15 cm) away were blurred? The area that is within the range of focus is referred to as the depth of field and the depth of field is determined by the f-stop. The larger the aperture opening (the smaller the f-stop) the smaller the depth of field. That is, if you want just the face of the Moray Eel to be in focus then you can shoot with an f-stop of f /1.4 or f /2.8 but if you want to have the coral behind the eel also in focus then you will need to consider larger (small opening) f-stop options. Later on we will learn about Aperture Priority and Shutter Priority which can make this task a bit easier to address.
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Exposure Modes
Exposure is the amount of light that reaches the camera's sensor (or in the old days, the film). There are four different exposure options from which you can choose. The easiest exposure mode is Auto-Exposure. When you choose Auto-Exposure your camera will take care of everything and do the best job it can it obtaining a quality picture. The camera will set the f-stop, ISO and shutter speed. Generally you will find that the camera will try to shoot at a shutter speed of at least 125th so that any movement can be stopped. From there it will adjust the f-stop and ISO. I highly recommend that beginning underwater photographers start off shooting in the Auto-Exposure mode. Once you have shot a few hundred photos you can then start experimenting with the other, more sophisticated exposure modes.
 Aperture Priority and Shutter Priority give the photographer the ability to set one variable and the camera will handle the other two in order to try to capture a high quality photograph. With Aperture Priority the photographer sets the f-stop (usually so they can adjust the field of view to obtain their desired image) and the camera will set the shutter speed and ISO. Shutter Priority is when the photographer sets the shutter speed (faster to stop movement or slower to obtain intentional blurring) and the camera will set the f-stop and ISO. Between these two options, I personally prefer Aperture Priority when diving simply because photographing a fast moving fish is exceedingly difficult and, therefore, setting my shutter speed at a particularly fast speed usually isn't necessary. Also, if I set my shutter speed below 1/60th then the picture often is blurred because I too am often moving while trying to take photos. I have generally found that Shutter Priority, although very useful on land, is not overly useful when taking underwater photographs. Finally there is the Manual setting. In Manual, the photographer sets all the exposure parameters. After trying the other options I almost always revert to Manual. While we are traveling to the dive location I note the amount of sunlight and will set my ISO accordingly. Usually an ISO setting of 100-200 is sufficient for most day dives and will not result in an overly grainy picture if set at 200. I next usually set my shutter speed to 1/125th since that will stop most movement. If we are shallow and there is a lot of natural light then I may set my ISO at 100 and my shutter speed at 1/250, especially if there are lots of small and colorful fish darting in and out of the coral reef and I want to "stop" them. Finally, I will set my f-stop based upon the photo composition I am seeking.
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 Bracketing
Bracketing is a technique that is used to help ensure that you get a good photo, especially in low light conditions. Bracketing is when you take the same photo with three different f-stops: one as shot, one as +1 f-stop and one as -1 f-stop. (Remember that each f-stop increases or decreases the light reaching the sensor by 50%.) On many of the newer digital and compact digital cameras there are settings for automatic bracketing. If you choose this setting then the camera will automatically take all three photos. This is great when you are on land or perhaps when taking a picture of coral or a Goliath Grouper that is sitting on the bottom of the ocean. But if you use bracketing when trying to take a photo of a fast-moving or shy fish you may find that your bracketed photos may have the right amount of light but that the object of the picture moved between the original shot and the bracketed shots. This often happens when shooting with strobe lights because the flash of the strobe spooks the fish. Also, unless your strobe recharges exceedingly fast you may find that the strobe fails to flash for one or both of the bracketed shots. If you choose automatic bracketing it will generally take three times longer for the photo to be processed by your camera (so you can move on to the next photography) because three pictures are being taken and stored. Finally, if you choose automatic bracketing you need to make sure that your memory card is large enough to handle the increased number of photographs that will be stored. When underwater I tend to avoid automatic bracketing and will simply try to do the best I can with my initial shot.
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After Diving
 Diving and taking photographs is only half the fun. The rest of the fun is viewing your photographs and using a post-processing software package such as Adobe Photoshop®, Adobe Lightroom® or Apple Aperture to really produce a quality image. You can do wonderful tweaking of your photos if you shot them in the Jpeg format but if you really want to have flexibility in post-processing then you will shoot in the RAW format. But post-processing is an entirely different theme than what is covered in this Primer and warrants its own treatise.
In the end, you will find that taking your own pictures and being able to share them with others, both divers and non-divers, adds greatly to your trip. As with purchasing a new regulator or dive computer, talk to people you trust and who are knowledgeable about underwater photography before you buy. When on a trip talk to some of the photographers who seem to know what they are doing. Most are very friendly and will gladly help a new photographer learn some of the ropes. New cameras, housings and lights are introduced to the market on almost a daily basis and some great used equipment can produce wonderful photographs and be the perfect first camera for a beginning photographer.
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