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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.
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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.
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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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