In photography, exposure or brightness of a photograph depends upon three variables;
aperture, shutter-speed and ISO.
These variables together are called the exposure-triangle.
In general, the word "exposure" in photography means the amount of light
that is captured by the sensor or film when a photograph is taken.
It is the overall light that reaches
the sensor or film through the lens during the period while the shutter remains open.
When this gathered light is more than the requirement, the image becomes
over-exposed, and when less than the requirement,
the image is under-exposed, as seen in the following examples :
For getting correct exposure, we set different values of aperture, shutter-speed
and ISO before pressing the shutter-release button. In other words, setting exposure is
the way of gathering less or more light on the sensor using these three variables.
APERTURE
The function of aperture is to control
the amount of light reaching the film/sensor.
Larger the aperture-diameter, more light is allowed
to the sensor, and smaller is it, less light is allowed.
The size of the aperture also determines the depth of field of a photograph.
F/number and aperture-diameter
Larger the aperture-diameter, smaller the
f/number, and smaller the aperture-diameter,
bigger the f/number.
Aperture is a hole in the lens through which light
reaches the sensor. We can widen or narrow the diameter
of this hole to control the quantity of incoming light,
as well as depth of field of the scenario. It is a function of the lens.
The size/diameter of the aperture effects exposure/brightness, depth-of-field, bokeh,
diffraction, starburst/sunburst effects etc. For more details about aperture, check the pages:
In these days, there are are lenses available
with widest apertures like f/1, f/0.95 which can allow a lot of light.
Larger the aperture, higher the price of the lens.
Shutter is a curtain located in front of the film or sensor inside a camera.
When the shutter opens, light reaches the sensor/film
through the aperture-hole.
It opens when shutter-release button is pressed,
and closes automatically.
We can increase or decrease the
opening time of the shutter to control light.
Longer the shutter remains open, more light reaches the sensor or film.
Shutter-speed is the time-duration for which the shutter-curtains remain open.
It effects exposure/brightness of a photograph. Shutter-speed can also
capture or freeze motion of a moving subject.
For more details about shutter-speed, check the pages:
In the above animation, you can observe what happens when a shutter-speed
of ½ second is applied. During this ½ second
period, the sensor is exposed to light, and an image of the scene/object is
recorded. The unit of shutter-speed is expressed in time, generally in seconds,
such as 1⁄10 sec., 1⁄4 sec., 2 sec., 30 sec. etc.
In SLR/DSLR cameras, a mirror is placed in front of the shutter-curtain
which is not shown here. This mirror directs the incoming light towards
the optical viewfinder. When shutter is pressed, this mirror is moved
upwards blocking light towards OVF; at the same time, shutter-curtain opens,
then light reaches the sensor and an
image is recorded.
To get an idea about shutter-mechanism of a camera, please check
this page.
ISO in photography
ISO increases the light sensitivity of a camera-sensor.
It was also known as film-speed when analogue cameras were used.
ISO is denoted by a number without any unit.
More ISO results in brighter images with more noise.
The following image was shot using ISO 6400 to
obtain a shutter-speed of 1/60 second in very low light.
So there is
too much noise that can be seen in the cropped image.
Find details about ISO in a separate page
ISO in photography.
A high ISO image
Cropped version
What is one stop exposure in photography?
When we say "one stop exposure", we mean doubling or halving
the amount of light that is to reach the sensor or film. Thus,
"one stop more exposure" means double the previous light, and
"one stop less exposure" means half of that.
Suppose we shoot an under-exposed photograph (unintentionally) with the settings:
aperture f/5.6, shutter-speed 1⁄60th. sec.,
ISO 200.
After checking the image in camera-screen, we decide to
increase the exposure(brightness) by one
STOP, and re-shoot the scenario. This means we must allow
double the light than the earlier image. In any of the following
3 ways, we can increase the exposure :1234567890
1 - We can apply a wider aperture by 1 stop (f/4 in place of f/5.6).
This will allow double the light than the previous image;
but that will shorten the depth of field too.
2 - Or, we can apply a slower shutter-speed by 1 stop
(1⁄30th. instead of
1⁄60th. ) to let
double the light in. But that might
capture any unwanted motion-blur or
camera-shake.
3 - Or, we can apply double the previous ISO (400 in place of 200)
for more light-sensivity. This will of course brighten the image,
but at the cost of more noise.
Any one of the above 3 methods will increase the exposure by 1 stop.
The method to be applied will be decided by the
photographer. Here is the difference between auto mode and
manual mode. In auto mode, maximum settings are decided by
the camera itself; whereas in the latter case, settings are
decided by the photographer.
In the above scenario , the following studies can be made.
a) If all the three methods are applied together,
the exposure will increase by 3 stops ( +1+1+1 = 3).
b) If we apply 2 of the methods together,
the exposure will increase by 2 stops ( +1+1 = 2).
c) If we increase exposure by 1 stop through shutter-speed
(apply a one stop slower shutter-speed : 1⁄30th. sec.),
and decrease exposure by 1 stop through ISO (iso 100)
at the same time, the exposure will remain the same ( +1-1 = 0).
So, we can say that the word STOP represents aperture ,
shutter-speed and ISO with a common unit. When we say "increase the
ISO by 1 stop(keeping the other two variables unchanged)", we certainly
mean to increase the exposure by 1 stop.
When an ISO value (say 400) is multiplied by 2, the resulting ISO
(400 × 2 = 800) will increase exposure by 1 stop; i.e. brightness
of the image will be doubled. To decrease exposure by 1 stop through ISO,
divide present ISO by 2 (400 ÷ 2 = 200); the brightness of
the image will be halved.
When a shutter-speed value (say 1⁄120 sec.) is multiplied by 2,
the resulting shutter-speed (1⁄120 × 2 = 1⁄60 sec.)
will increase exposure by 1 stop; i.e. double the previous light will be allowed.
To reduce exposure by 1 stop, divide present shutter-speed value by 2
(1⁄120 ÷ 2 = 1⁄240 sec.) to receive half the previous light.
But in case of aperture, there is an exception.
In stead of multiplying by 2, we have to divide the
present aperture by √2
to allow 1 stop more light. If present aperture value is f/8, divide 8 by
√2
(approx. value of
√2
is 1.41) and the resulting aperture value
(8 ÷ 1.41 = 5.67 ≈ 5.6)
will increase exposure by 1 stop; i.e.
double the previous light will be allowed.
To decrease exposure by 1 stop,
multiply present aperture by √2 (8 × 1.41 = 11.2 ≈ 11)
to receive half of the light.
The exception in the case of aperture happens due to the fact that
the light reaching the sensor/film is regulated by the area of the
circular hole of the aperture. The area of a circle includes one
square function (πr² ).
You don't have to perform such calculations in the field.
Those values will be automatically memorised after some practices.
If you are interested, the following table shows calculations of aperture,
shutter-speed and ISO beginning from low exposure to high exposure.
Each aperture number in the table allows 1 stop more light than
the previous aperture number. The same is applicable to shutter-speed
and ISO values too.
Aperture values one stop apart
Let the smallest aperture of a lens be f/22.
This aperture will allow the lowest quantity of light to the sensor.
f/16 is 1 stop wider than f/22 [22 ÷ 1.4 ≈ 16]
f/11 is 1 stop wider than f/16 [16 ÷1.4 ≈ 11]
f/8 is 1 stop wider than f/11 [11 ÷ 1.4 ≈ 8]
f/5.6 is 1 stop wider than f/8 [8 ÷ 1.4 ≈ 5.6]
f/4 is 1 stop wider than f/5.6 [5.6 ÷ 1.4 = 4]
f/2.8 is 1 stop wider than f/4 [4 ÷ 1.4 ≈ 2.8]
f/2 is 1 stop wider than f/2.8 [2.8 ÷ 1.4 = 2]
f/1.4 is 1 stop wider than f/2 [2 ÷ 1.4 ≈ 1.4]
Large and small apertures
Wider/larger aperture allows more light resulting in brighter image and vice versa.
Wider/larger aperture also produces shallower depth of field and vice versa.
and vice versa.
Shutter-speed values one stop apart
Let the fastest shutter-speed of a camera be 1⁄4000 sec.
This speed will capture the lowest quantity of light.
1⁄2000 is 1 stop slower than 1⁄4000 sec.
[ ∵1⁄4000 ×2=1⁄2000 ]
1⁄1000 is 1 stop slower than 1⁄2000 sec.
...
1⁄4 is 1 stop slower than 1⁄8 sec.
1⁄2 is 1 stop slower than 1⁄4 sec.
1 is 1 stop slower than 1⁄2 sec.
Shutter-speed effects
Slower shutter-speed allows more light resulting in a brighter image.
Slower shutter-speed also records motion blur or camera-shake, if any.
ISO values one stop apart
Let the lowest native ISO of camera-sensor be 100.
This ISO will provide lowest sensitivity of light, hence darkest image.
ISO 200 is 1 stop brighter than 100 [100 × 2=200]
ISO 400 is 1 stop brighter than 200 [200 × 2=400]
ISO 51200 is 1 stop brighter than 25600 [25600 × 2=51200]
ISO 102400 is 1 stop brighter than 51200 [51200 × 2=102400]
ISO effects
Higher ISO increases sensor-sensitivity resulting in a brighter image.
Higher ISO also increases noise in an image.
HISTOGRAM
A histogram of an image is the graphical representation of the
brightness of individual pixels present in that image.
Explanation of a histogram
requires a separate article, but for the time being, let us know that
it indicates the overall exposure of a photograph. When the graph of a
histogram tends to move towards the left, it generally indicates
an under-exposed image. And if it tends to move towards right,
that indicates an over-exposed image. The following three
histograms more or less represent an under-exposed(A), properly exposed(B) and
over-exposed(C) images respectively.
Histograms are very helpful, especially in the field, to determine the
actual exposure of an image or scenario. If the LCD of a camera is set to be
sufficiently bright, in the field it (the LCD) may mislead us about the overall exposure of an
image. Later when we open it in PC, we may find it to be enough dark to repent.
So, it is a very good practice to observe the histogram of an image in the
field so that we may re-shoot if required to obtain proper exposure.
Aperture, shutter-speed and ISO are the three
variables which effect the overall exposure of an image.
Whenever we have to change brightness of a photograph, we just
change one of them according to the situation.
