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Basic Shutter Speed, Aperture and Depth of Field Underwater

Although shutter speed, aperture, and depth of field are at times difficult and confusing to understand. Mastering these basic concepts and how they relate to one another will greatly improve your underwater images. 

Shutter Speed
Shutter speed is a time function.  It refers to the length of time in which the shutter of a camera is open and the amount of time light enters the camera and exposes the film.  The faster the shutter speed, the less time there is to expose the film to light.  The slower the shutter speed, the greater the time that the film is exposed to light.

Shutter speeds are expressed in fractions of a second that the shutter is open: 1/30, 1/60, 1/125, 1/250, 1/500, and so on.  1/30 means the shutter is open for one thirtieth of a second.  Notice how each fraction is doubled from one shutter speed to the next, or decreases by half, respectively; indicating that the shutter remains open twice as long, or closes in half the time. 

Changing the shutter speed does two things. 
1. It controls Ambient Light:  As you manipulate the shutter speed you decrease or increase the amount of natural/ambient light entering the camera. For most marine life a setting 1/60 is usually adequate.

If your shutter speed is 1/125 and you change the speed to 1/250 you have just decreased the amount of light entering your camera by half.  If you went down to 1/60 you would be increasing the time the shutter is open by double and allow more light into your camera.

2. You can use shutter speed to “stop” action: The quicker the action, the higher the shutter speed needs to be in order to freeze that action.  Most fish and marine life can be freeze framed with a shutter speed of 1/125, but sharks, seals and dolphins who are known for their speed, are best shot at faster shutter speeds; 1/250 or better.   

Synchronizing shutter speed with flash
It is important that your camera and strobe (underwater flash) work together. They need to be synchronized with each other.  The older Nikonos had a sync speed of 1/60.The Nikonos V, a very popular underwater photographic system, syncs at 1/90. 

Today’s advanced camera systems will sync at much faster speeds, for instance Nikon’s F801/8008, F90x series when used in an underwater housing can sync with most strobes at speeds of up to 1/250th.


Aperture / F-stop
Aperture refers to the size of the lens opening.  A camera lens works very much like the iris of an eye.  The aperture in a camera is a mechanical sizing device; it opens and closes much like the iris; regulating the quantity of light that enters a camera and exposes the film.   

Aperture sizing or openings are expressed in values known as f-stops; common f-stops are: f2.8; f-4; f5.6; f8; f11; f11; f16; f-22.  As with shutter speed, f-stop openings decrease by ˝ with each step. If an aperture setting is f8 and you change to f11 you have just decreased the aperture opening size by half.  If you were to go from f8 to f5.6 you would have increased the opening by twice the size – admitting twice as much light. 

Where people get confused by f-stops is the concept of big and small. It seems like f22 should be a larger opening than f8. But the opposite is true. That’s because f22 is shorthand for f 1/22nd and f8 is shorthand for f 1/8th.

Aperture Has Three Functions:

1. It controls the amount of ambient or natural light entering the camera.

2. It determines the depth of field.

3. It determines flash to subject distance. 

Depth of Field
Depth of field is the minimum to maximum distance in a photographic image that is acceptably sharp and in focus. 

Aperture settings determine what your depth of field in any given photograph will be.  The widest aperture, f2.8, will give you very shallow depth of field.  Conversely, the smallest f-stop, f22, will create the greatest depth of field in a photograph.

Keep in mind that depth of field is also affected by the focal length of the lens; the shorter focal length of the lens the greater the apparent depth of field.  For example, a 19 mm lens will have a much greater over all depth of field than a 100 mm lens.

Getting it in Focus
The point at which you focus your camera on a subject, is said to be the “point of sharpest focus”.  The depth of field in any photograph is always greater beyond that point as opposed to before.  So, objects closer than that point may appear soft, while those farther away will appear sharper.  As a matter of fact if you look at the area that is in focus in an image 1/3rd of the sharp area will fall in front of your focus point and 2/3rds will fall beyond your focus point. 

Another important point is that the greater the distance from the camera to the subject the greater the depth of field.  On the other hand, the closer the subject is to the camera’s lens, the lesser the depth of field.  That’s why depth of field is so shallow in macro photography. 

One Final Rule of Thumb
It is often best to place your point of sharpest focus, approximately one third into to your planned composition (frame).  This makes sense if you think about it. Since the area of greatest sharpness is divided 1/3rd in front of your point of focus and 2/3rds behind, focusing 1/3rd into your frame will give you maximum depth of field. This is called the hyperfocal distance and will provide you balanced depth of field in your final image. 


To be continued…
This column laid down the basics. In our next column we’ll look at how shutter speed, aperture and depth of field work together. I’ll also pass on some “magical” settings to ensure better underwater photographic results.


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