Malaysian Nature Society Photogroup
APERTURE
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Contents
• Review - Exposure Triangle
• Aperture and Exposure
• Aperture and Depth of Field
• Circle of Confusion
• Depth of Field
• Hyperfocal Distance
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Exposure Triangle
ISO
Any photo exposure will
involve all the elements
from the Exposure Triangle
namely, ISO, shutter speed
and aperture.
Changing any one of these
requires changes in one or
both of the other elements
if the correct exposure is to
be maintained.
APERTURE
SHUTTER
SPEED
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Aperture
• Aperture therefore has two principle impacts on
photographs :
Exposure
Depth of Field
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Aperture and exposure
• What Is Aperture?
• The main function of a camera lens is to collect
light.
• The aperture of a lens is the diameter of the lens
opening and is usually controlled by an iris. The
larger the diameter of the aperture, the more light
reaches the film / image sensor.
• Aperture is expressed as F-stop, e.g. F2.8 or f/2.8.
The smaller the F-stop number (or f/value), the
larger the lens opening (aperture).
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Aperture and exposure
The lens aperture is usually
specified as an f-number, the
ratio of focal length to effective
aperture diameter.
Therefore as the diameter
increases, the ratio will decrease
e.g. 100mm lens/25mm diameter
equals f4 or 100 mm lens/50mm
diameter equals f2.
Note how a larger diameter (more
light) gives a smaller f number.
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Aperture and exposure
Each full f-stop down or up
doubles or halfs the amount of
light reaching the sensor.
The total light getting through the
lens depends on the area of the
open circle within the lens
exposed by the aperture.
Area of a circle varies according
2
to the square of the radius : πr
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Aperture and exposure
2.8 x 2.8 = 7.84 (8)
4 x 4 = 16 (16)
5.6 x 5.6 = 31.36 (32)
8 x 8 = 64 (64)
11 x 11 = 121 (128)
16 x 16 = 256 (256)
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Aperture and exposure
Therefore closing down by one
full stop e.g. f4 to f5.6 will reduce
the light reaching the sensor by
half.
Therefore opening up by one full
stop e.g. f11 to f8 will increase
the light reaching the sensor by
two times.
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CIRCLE OF
CONFUSION
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Circle of Confusion
In optics, a circle
of confusion is an
optical spot
caused by a cone
of light rays from a
lens not coming to
a perfect focus
when imaging a
point source.
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Circle of Confusion
In photography, the circle of confusion (“CoC”) is used
to determine the depth of field, the part of an image
that is acceptably sharp.
Real lenses do not focus all rays perfectly, so that
even at best focus, a point is imaged as a spot rather
than a point.
The smallest such spot that a lens can produce is
often referred to as the circle of least confusion.
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Circle of Confusion
Focus on cat and light rays come to a point (focused) on sensor.
For dog (behind cat) light rays come to a point (focused) behind sensor.
For rabbit (front of cat) light rays come to a point (focused) in front of sensor.
If not focused (if in front or behind) light rays form a circle instead of a point.
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Circle of Confusion
A standard value of CoC is often associated with each
image format, but the most appropriate value depends
on visual acuity, viewing conditions, and the amount of
enlargement.
A typical CoC might be 0.02mm for an 8 x 10 inch
enlargement viewed at a distance of 25cms (10
inches).
But an 8 x 10 inch enlargement viewed at a distance of
50 cms (20 inches) would only need a CoC of 0.05mm.
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Circle of Confusion
Do any of these circles look like points?
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DEPTH OF FIELD
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Depth of Field
In optics, particularly as it relates to film and
photography, depth of field (DOF) is the distance
between the nearest and farthest objects in a scene
that appear acceptably sharp in an image.
A lens can precisely focus at only one distance at a
time.
The decrease in sharpness is gradual on each side of
the focused distance, so that within the DOF, the
unsharpness is imperceptible under normal viewing
conditions.
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Aperture and depth of field
200 mm -- f 4.2
200 mm -- f 4.2
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Aperture and depth of field
200 mm -- f 4.2
200 mm -- f 5.6
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Aperture and depth of field
200 mm -- f 4.2
200 mm -- f 8
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Aperture and depth of field
200 mm -- f 4.2
200 mm -- f 11
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Aperture and depth of field
200 mm -- f 4.2
200 mm -- f 16
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Aperture and depth of field
200 mm -- f 4.2
200 mm -- f 22
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Aperture and depth of field
200 mm -- f 4.2
200 mm -- f 32
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Aperture and depth of field
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Aperture and depth of field
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Aperture and depth of field
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Aperture and depth of field
Sometimes it is important to
obtain the maximum depth of
field possible (e.g. landscapes)
More often we want to restrict
the depth of field so that the
main subject is sharp but the
background is out of focus and
does not compete for attention
with the main subject
(200mm @ f11)
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Aperture and depth of field
Sometimes it is important to
obtain the maximum depth of
field possible (e.g. landscapes)
More often we want to restrict
the depth of field so that the
main subject is sharp but the
background is out of focus and
does not compete for attention
with the main subject
(200mm @ f6.7)
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HYPERFOCAL
DISTANCE
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Hyperfocal Distance
• Sometimes it is important to obtain the maximum
depth of field possible (e.g. for landscapes).
• The hyperfocal distance is the nearest focus
distance at which the DOF extends to infinity.
• Focusing the camera at the hyperfocal distance
results in the largest possible depth of field for a
given f-number.
• When the lens is focused at this distance, all objects
at distances from half of the hyperfocal distance out
to infinity will be acceptably sharp.
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Hyperfocal Distance
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Hyperfocal Distance
Focussing on a foreground object may still extend the depth of
field all the way to infinity if the apperture is small enough.
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Review - Aperture
• Aperture and Exposure
• Aperture and Depth of Field
• Circle of Confusion
• Depth of Field
• Hyperfocal Distance
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APERTURE
THANK YOU
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