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Photo Secrets: Working with Available Light
Taking a photograph requires you to work with light. The existing light around you (no artificial light of any kind) is known as “available light.” For successful underwater photography you need to understand how to adapt to and manipulate whatever light is available and capture just the right amount you desire. We’ve got three basic tools at our disposal to manipulate available light. Those are ISO, aperture, and shutter speed.
One of the first choices we’re going to make is the film speed, or ISO. This applies to digital cameras well. Generally, ISO speeds start around 50 and then go, 64, to 100, 200, 400, 800, 1200 even 1600 and, with digital, many numbers in between, depending on the camera. Low numbers are called “slow” and high number are called “fast.” Slow ISO speeds require a lot of light; fast speeds require much less light. In fact, 50 speed ISO requires 16 times the amount of light that 800 ISO does. That means that if you’re shooting in very low light (like just after sunrise), you’re more likely to get an acceptable image with 800 ISO than you will with 50 ISO.
But there’s a trade-off in all of this. While the higher speed ISOs let you shoot with less light, you may notice a deterioration in picture quality. In film, it’s referred to as “grain” because of the silver oxide particles in the film that actually capture light. With digital, it involves how the data is processed. But the end result may be the same: Higher ISOs produces less-crisp pictures when they are blown up to larger sizes. But they may also allow you to take pictures you can’t take at low-ISOs, like manta rays at night in Kona.
With film, you’re going to shoot the same ISO over the entire dive since you can’t change a roll of film underwater. With digital you CAN make ISO changes on the fly (through the camera’s menu) and that’s a big advantage.
Once you choose your ISO, you’re ready to proceed to the next step which is determining your shutter speed and lens opening (or aperture). These two go hand-in-hand because a specific amount of light is going to be required to get a correctly-exposed picture. The amount of light to reach the film or the digital sensor is based on the size of the aperture (lens opening) and the length of time that the aperture is open. If you’re letting in a lot of light, you may not need to keep the lens open very long. But if you reduce the amount of light coming in, then you’d have to leave the lens open longer, so the same total amount of light hits the film or sensor.
So the short version of this is (and this presumes we’re not using a flash but just using the ambient light): Big opening, small amount of time. Small opening, big amount of time.
How this translates to your camera defines the relationship between your aperture and your shutter speed. Large apertures allow a lot of light through, which means shutter times can be shorter. Smaller apertures require longer shutter speeds. Of course, there are more trade-offs.
The size of your lens opening (aperture) affects your depth of field (the range between something being in focus or out of focus). Large apertures (small numbers - f2.8, 4.0., 4.5) have limited depth of field. Small apertures (large number - f16, 22, 32) have much deeper depth of field. So if you’re striving for depth-of-field in your picture (a diver swimming through a forest of kelp, for instance) you want to shoot with as small an aperture as possible.
But that may require a longer time for the shutter to be open. That means that you may incur some camera shake (depending on the lens you’re using) or you may get some blurring of your subject in the case of an animal on the move.
So something you’ll need to start experimenting with are the combinations of shutter speed and aperture that you think work for a given situation. No matter what you do, there’s going to be a trade-off. And it certainly pays to shoot subjects at a combination of apertures and shutter speeds so you can get something you like. (This is a big advantage with digital cameras because you get instant feedback and can make changes on the fly.)
And we haven’t even begun to touch on using a flash... which is what we’ll do in our next installment.
Ken Kurtis is co-owner of Reef Seekers Dive Co. in Beverly Hills, CA. He is a recipient of the California Scuba Service Award and is a frequent contributor to California Diving News.
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Return to Cover Page/Contents for January/February 2006 issue