Underwater Photography - Exposure

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Underwater Photography - Perfect Exposure

There is really only one perfect exposure to any photo: the one that looks good to your eyes. However, your camera photometer plays a big role in helping you get a starting point for your image.

The fact is that cameras have not changed much in 20 years. Still, an exposed image is the one that combines the correct amount of light that hits your film (or digital sensor) based on your current ISO. You can have a wide variety of f-stops and shutter speed combinations, but only one of those combinations will create an exposed image. Using one f-stop or shutter speed over another changes either depth of field or freezes the action. So, if I say that my photo is exposed at f/8 using 1/125th with ISO 100, it is basically the same as saying that my photo would be exposed at all these combinations in one stop increments:

- f/2.8 at 1/1000th
- f/4 at 1/500th
- f/5.6 at 1/250th
- f/8 at 1/125th
- f/16 at 1/60th
- f/22 at 1/30th

This brings up the question: "What's the difference between shooting the photo using f/22 or f/2.8?" The answer is depth of field. The bigger the number, the more depth of field you are going to get in your photo, but at the same time, you are compromising speed. Although the number is larger and you get more depth of field, your lens is actually closing and letting less light in. This requires a longer exposure or longer shutter speed. Using the examples above, we can see the difference between f/28 with 1/1000th of a second and at f/22 where we have only have 1/30th of second.

When should you use one setting over the other? It depends of what you are trying to achieve in your photo and how fast your subject is moving. All of the combinations above will give you an exposed photo, but, if you are shooting a fast moving fish and you choose f/22, then most likely you are going to end up with a blurry fish. Consequently, if you are shooting a fast moving fish and choose f/2.8, then you are going to have virtually no depth of field. This would require your focus to be very accurate because if you narrowly miss your target, it will be blurry; not because you didn't freeze the action, but because your depth of field is very narrow.

Your distance to your subject will play a huge role in depth of field as well. If you are very close to your subject (as you should be underwater), it will have a big effect on depth of field than if you are further away. As a rule of thumb, if your lens is focusing to infinity when you are targeting a subject, you really don't need to worry about depth of field. Pick the sharpest f-stop for the lens that you are using (every lens has one, read your lens reviews and specs as they are all different) and if you have enough light, then shoot on that aperture.

If you are close to the subject (eg: underwater), then you need to make compromises:

- More depth of field needed: go with a higher f-stop.
- Less depth of field needed: go with a lower f-stop.
- Fast moving fish: go with a lower f-stop (faster shutter speed).
- Slow moving fish or stationary objects: go with a bigger f-stop (slower shutter speed).
- If you are using strobes (as most of us do underwater), then you have more options since you can adjust their intensity to meet your lighting needs. In this case, we are making another compromise: the faster your shutter speed, the less ambient light you are letting into your photos. Remember that only one exposure is correct and still remains correct, even if you are using strobes.

For example:

Imagine I'm diving at 40 feet. I measure the green water and it gives me f/8 at 1/125th. I would start by shooting at that, and then:

- If I wanted more depth of field, I would leave the shutter speed at 1/125th and change my f-stop to f/11 or f/16 and compensate the lack of light with my strobes by increasing the power until my subject is well lit.

- If I wanted less depth of field, I would leave the shutter speed at 1/125th and change my f-stop to f/5.6 or f/4 and reduce the power of the strobes.

- If I'm happy with the depth of field but would like more ambient light (lighter background), then I would leave my selected f-stop and decrease my shutter speed until I get the ambient light I wanted.

- If I'm happy with the depth of field but I would like a darker background, then I would increase my shutter speed and increase the maximum flash sync speed of the camera (check your camera's manual). My Nikon D300s has a flash sync speed of 1/320th.

Lastly, keep in mind that if you are shooting with strobes, you are going to be limited by your strobe's sync speed. So even if I wanted to select f/2.8 and 1/1000th, I couldn't since my flash is not fast enough to light the frame for me. The reality is that when we are underwater, we always deal with loss of light starting at very shallow depths, so this last point is not really a problem in 99.99% of the cases.

Underwater photography in the Pacific North West waters comes with some challenges. We not only have to deal with the liquid medium, but also have to deal with a lot of more gear to be able to dive this temperate waters. Drysuits, heavy equipment, lots of weight, thick gloves are some of the extra challenges we need to deal in our waters. In addition darker more nutrient-rich water create interesting lighting challenges. This waters are the most rich waters in the world. Life diversity is hard to describe.

Will Ferrero http://underwaterphoto.baires.ca

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