APERTURES

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APERTURES SHUTTER SPEEDS
Presentation and photographs by David Blumenkrantz
EQUIVALENT EXPOSURES DEPTH OF FIELD
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Photography is an art form (or medium) based on
scientific methods and techniques. The optical
workings of the camera lens can be used by the
thoughtful photographer to create specific
effects.
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APERTURE SHUTTER EXPOSURE
The aperture is an opening in the lens that
determines how much light will be allowed in.
The shutter, whether a leaf or curtain,
determines the length of time that light will be
allowed to enter the camera.
The combination of these two factors, in concert
with the light sensitivity rating (ASA/ISO),
determine the exposure.
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FILM SPEED/CCD LIGHT SENSITIVITY
ASA (American Standards Association) Doubling the
number means doubling the film speed doubling
one ASA step is one f-stop (aperture opening).
Most common film speed rating in the U.S. until
the conversion to ISO. Only the name has changed.
ISO (International Standards Organization) Most
common film speed rating in the U.S. Doubling the
value doubles the film speed.
The higher the ASA/ISO rating, the more light
sensitivity. (Emulsion for film, CCD for
digital) In bright sunlight, ratings of 100 or
200 are generally sufficient. In low light
situations, ratings of 400 and above are more
effective.
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The higher the ASA/ISO, the more contrast and
grain, or pixelation. This is often the case when
shooting stage-lit performances or live sporting
events indoors or at night.
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Higher ASA/ISO ratings allow the photographer to
shoot indoors without using a flash.
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High ASA/ISO ratings allow the photographer to
capture the natural light.
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DEPTH OF FIELD There is only one plane of focus,
at the distance where the lens is focused. Any
other part of the image is increasingly blurred,
the further away it is from the plane of focus.
Strictly speaking the plane of focus is not a
plane, but part of the surface of a sphere whose
radius is the distance of the focus point from
the camera.
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WHETHER USING A DIGITAL OR FILM CAMERA WITH
PROGRAM MODES To control depth of field, switch
to aperture priority mode and select a small
aperture for great depth of field or a large
aperture for shallow depth of field.
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f/22
f/5.6
These two images illustrate how changing your
aperture affects depth of field.
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SHALLOW DEPTH OF FIELD
The ability to control the depth of field is one
of the photographers best visual tools. A
shallow depth of field creates an illusion of
three-dimensions to an image, and allows the
photographer to direct the viewers attention.
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The main subject stands out against the
out-of-focus background . . .
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SELECTIVE FOCUS
The shallow depth of field caused by a large
aperture opening can be manipulated by the
photographer, who can choose to focus the camera
on either the foreground or the background.
Its also possible to focus on something in the
middle, and have both the foreground and the
background out of focus.
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MAXIMUM DEPTH OF FIELD There are times when the
photographer prefers to have both the foreground
and the background in focus, to reveal details to
the viewer that are important to the story. This
is achieved by using a small aperture opening.
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SHUTTER SPEEDS Shutter speeds are measured in
fractions of a second . . . The faster the
shutter, the more motion is frozen. It is
generally found that anything slower than 1/60th
of a second is too slow to hold the camera by
hand, and a tripod should be used.
1/125th
1/60th
1/30th
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Faster shutter speeds are desirable when the
priority is to freeze action or motion.
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Eadweard J. Muybridge (1830 1904) was an
English-born photographer, known for his early
use of multiple cameras to capture motion.
Muybridge had successfully photographed a horse
in fast motion using a series of twenty-four
cameras. The cameras were arranged along a track
parallel to the horses, and each of the camera
shutters was controlled by a trip wire which was
triggered by the horses hooves.
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Parallels between shutter and aperture - This
diagram attempts to show the parallel graphically
- the idea here being to equate halving or
doubling of exposures both with time and
aperture. In other words ... if we move from the
left with time, OR with exposure, there is a
halving.
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Exposure equivalence - As aperture sizes get
smaller (numerically bigger) from left to right
..... we show shutter speeds getting larger -
longer. Thus, on the left we show a combination
this time, of full aperture f1 and a short
shutter speed of 1/000 second. At the other end
far right we have progressed through to the
smallest aperture f32 but with a shutter speed of
1 second. EVERY single position on this diagram
is the SAME exact exposure - in other words the
amount of light landing on film or CCD (charge
couple device) is identical.
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The shutter speed/aperture combination on the far
left shows an exposure of 1/000 second at f1 ...
great for stopping movement and camera shake but
extremely shallow depth of field. On the far
right we go to the other extreme and use a 1
second exposure - at f32! We see a considerable
increase in the depth of field (zone of
sharpness), but we are now at a shutter speed so
slow that we must use a tripod to avoid camera
shake. If possible try and use in-between stops
as much as practicable.
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Is Depth of Field arbitrary? Essentially yes
Depth of Field is affected by the focal length of
the lens being used, and the distance from the
camera to the object(s) being photographed.
http//www.dofmaster.com/dofjs.html (depth of
field calculator for digital lenses)
In this photograph, the woman with the cart is so
far away from the lens that she is essentially in
the background of the field of focus.
Therefore, depth of field is not evident. (If
there were objects between the subject and the
camera they might appear out of focus).
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WIDE ANGLE LENSES
Using a wide angle lens tends to give you greater
depth of field, partly because everything in the
picture is pushed farther away from the camera .
. .
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. . . unless you are very close to the subject.
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TELEPHOTO LENSES Longer focal length lenses tend
to result in images with shallower depth of field.
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CIRCLES OF CONFUSION
There are conventions which define how blurred an
object can become before it is perceived as
blurred by the viewer. These conventions are all
based on a rather complex phenomenon of optics
called Circles of Confusion http//www.northnet
.org/jimbullard/CoC.htmCircles of Confusion
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