Aperture is the circular opening in a lens through which light
reaches the sensor. We can widen or narrow the diameter
of this opening to control the incoming light.
As human has eyes to see this world, so a camera sees through the aperture
of a lens. Our eyes receive light which is converted to neural signals
and sent to the brain where an image is formed and identified. In a camera, light
passes through the aperture-hole and reaches the sensor/film to form an image.
Not shown here, the maximum aperture of a lens may be larger than f/1.4,
say f/1.2, f/0.95..., or smaller than f/22, say f/32, f/45...
The aperture-hole of a lens is made up of a number of
blades. Those blades create a roughly circular hole by
moving themselves when we change aperture setting in a
camera. Light reaches the film or sensor through this
hole. A large aperture-hole allows more light to reach the sensor;
a small aperture-hole allows less light.
Aperture is a ratio of focal length and diameter
Aperture is one of the three variables to control the overall
exposure or brightness of a photograph. It is an element of lens,
and expressed as f/numbers such as f/2, f/5.6 etc.(the 'slash'
is omitted sometimes ). When aperture-hole
is large in diameter, it is represented by a small f/number like f/1.4, f/2
and allows more light to enter.
On the other hand, a small aperture-hole is represented by a large
f/number, like f/22, f/32 etc., which allows less light.
This happens because aperture is the ratio of
focal length
and the diameter, i.e.,
Focal length is the distance between
the optical center of a lens and the sensor or film,
when the subject is in focus.
Every lens has a maximum (or widest or largest) and a minimum
(or narrowest or smallest) aperture.
For example, the AF-S DX NIKKOR 35mm f/1.8 G lens
has a largest aperture of f/1.8, and a smallest aperture of
f/22. In general, we should
avoid the maximum or the minimum apertures while shooting, as they may show some unwanted
effects in a photograph.
For example, for the above-mentioned lens, at apertures
f/1.8, f/2 some optical aberrations may creep in the image. To avoid that we should
stop-down the aperture 2 to 3 stops than the largest aperture
(stopping down an aperture
is to apply a smaller aperture).
Why small F/number represents large aperture
Why does a small f/number represents a large aperture and allow more light,
and a large f/number represents a small aperture allow less light?
A large aperture is represented by a small f/number,
while a small aperture is represented by a large f/number.
If we attach a 35mm f/1.4 lens to a camera, the distance between the lens
and the sensor will be 35mm when the subject is in focus.
The focal length of this lens is constant(35mm), it being a
prime lens.
The diameter
of the opening of the aperture will vary depending upon the f/number.
At f/1.4, diameter of the aperture-opening will be :
This will be the largest opening (diameter)
at the smallest f/number of f/1.4.
Again, this lens has the smallest aperture of f/16.
So, at f/16, diameter of the aperture-opening will be:
diameter = ( 35 ÷ 16 )mm = 2.18mm
This will be the smallest opeing at the largest f/number of f/16.
With the above format of calculation, we can get the f/numbers and respective
diameters of a 35mm f/1.4 prime lens in the following table.
For a 35mm f/1.4 lens,
aperture = f/16, diameter = 2.18mm
aperture = f/11, diameter = 3.18mm
aperture = f/8, diameter = 4.4mm
aperture = f/5.6, diameter = 6.25mm
aperture = f/4, diameter = 8.75mm
aperture = f/2.8, diameter = 12.5mm
aperture = f/2, diameter = 17.5mm
aperture = f/1.4, diameter = 25mm
With largest diameter 25mm, highest amount of
light will be allowed to reach the sensor. At that time
the aperture is f/1.4 (lowest f/number). The opposite is
happening at f/16 when diameter is 2.18mm.
So, a small f/number represents a large aperture
and allows more light.
Apertures one stop apart
The following aperture numbers are said to be
one stop away from
its previous aperture. This means each of the apertures
allows double the light than its previous value :
Generally, the size of the aperture-diameter effects the
follwing aspects of photography :
Exposure / Brightness
Depth-of-field / use of bokeh
Diffraction (loss of sharpness)
Starburst / Sunburst effect
In addition to the above, there are some
optical aberrations
like chromatic aberration, coma etc. which happen due to
unavoidable imperfection of a lens. Lens making is a complex process;
no lens is perfect. Some of those optical aberrations
may sometimes be avoided by using a narrow aperture.
Please take time to observe the following animation. When aperture is
altered, it effects different aspects of a photograph.
Effect of aperture on exposure
Changing aperture is one of the ways to set exposure before clicking
the shutter finally. Large apertures like f/1.4, f/1.8 etc. allow more light,
while narrow apertures like f/22, f/32 etc. allow less light.
Large apertures allow maximum light, small apertures allow minimum light.
Both the following two images were shot from a heavy tripod
fixed on the ground and time difference was very minimum,
(say 2 seconds). Those are raw images without any post-processing.
Shutter-speed was 30 seconds, ISO was 64 at 24mm equivalent
focal length.
However, the aperture-setting of the first image was
f/8, and the same of the second image was f/16.
The result is in front of you. Since f/8 is a larger aperture than
f/16 (two stops larger), the first image is brighter
than the latter, other variables remaining unchanged.
Vidyasagar Setu, Kolkata, West Bengal, India 2015 * Camera: Nikon d810, lens: Tamron 24-70mm f/2.8
Why did I shoot the second image when I got proper exposure with the first one?
Of course not because I wanted to show you this example! Five years ago , I did
not think of writing articles on photography. The reason will be explained after we learn
about diffraction and starburst aspects of photography
in this page.
Effect of aperture on depth-of-field:
Depth of field is a very important aspect of photography.
It is the acceptably sharp front-to-back area of a photograph.
The deatils about depth-of field may be read in
in this
and
this page.
Effect of aperture on diffraction and starburst/sunburst:
In photography,
diffraction and starburst/sunburst
effects depend on the aperture settings of lens.
Always try to keep diffraction to minimum level by avoiding
very small apertures.
Starburst/sunburst effect also depends on number of blades in a lens.
Here is a table with detailed explanation of the advantages, disadvantages and
applications of different apertures.
Aperture
F/Number
Effect
Large
... f/0.95 ... f/1.4 ... f/2.8 ...
Maximum light received
Very thin depth of field
Creamy bokeh
Suitable for portrait, astro, low-light
Unwanted elements may be blurred out
Less ISO required, so less noise
Lens aberration may be visible
Focus-accuracy is difficult
Unsharp image may result in
Excessive light may need ND filters
Medium
... f/4.0 ... f/5.6 ... f/8.0 ...
Medium light received
Acceptable sharpness
Safe w.r.t. lens aberration, diffraction etc.
Suitable for street, documentary
Sweet spot of some lenses lie in this range
Small
... f/9.0 ... f/11 ... f/13 ...
Less light reaches the sensor
Large depth of field
Acceptable sharpness from front to rare
Suitable for landscape, cityscape & architecture
Sunstar/starburst effects can be achieved
ISO may have to be increased
Diffraction may creep in
Very small
... f/15 ... f/16 ... f/22 ...
Very very less light received
Very large DOF; useful in macro
Sunstar/starburst effects can be achieved
Sharpness falls due to excessive diffraction
High ISO is required
Avoid the range ...f/22... or smaller
And here is another brief table:
Effect
Large aperture
Small aperture
exposure
brighter
darker
depth of field
small
large
bokeh
strong
less
lens aberration
strong
less
sharpness
soft *
sharp **
diffraction
less
more
starburst
less
more
Sharpness w.r.t. apertures
★ Even with a large aperture, an image may achieve sharpness but
for a small plane only. If the focus point is mistakenly placed
somewhere else, an unwanted result will be produced where subject may become blurred.
★ Narrow or small apertures provide sharpness, but at extremely small aperture
a photograph will lose sharpness due to diffraction.
Now we are familiar with the term "aperture"; in the next page
let us see how aperture effects depth of field of a photograph.
