Exposure Triangle

Table of contents:

Exposure triangle in Photography

Aperture, Shutter-speed and ISO

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.

Table of contents:

Exposure in photography

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 :

Tajmahal properly exposed
Properly exposed image - f/10, 1/125 sec., ISO-100, FL-20mm eqv.,
Nikon d7000, Nikkor 18-105mm, Agra, UP, India 2013.

Tajmahal under-exposed and over-exposed photograph
Under-exposed(left) and over-exposed(right) image

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.

Exposure Triangle EXPOSURE-TRIANGLE ...1/250...1/125...1/60...1/30...1/15...1/8...1/4... Shutter-speed darker brighter ...200... 400... 800... 1600... 3200... 6400... 12800... ISO darker brighter ... f/1.4... f/2... f/2.8... f/4... f/5.6... f/8... f/11... Aperture brighter darker


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.

Different sizes of Aperture Aperture of a lens, large to small f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22

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.

Why a small f/number allows more light than a large f/number?
Click here for explanation.


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:

Shutter-mechanism Shutter of a camera ½ sec. shutter-speed Sensor exposed to light Sensor blocked by curtain

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 110 sec., 14 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.

Exposure triangle, a high ISO photograph

A high ISO image

Exposure triangle, a high ISO photograph

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 160 th. 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 (130 th. instead of 160 th. ) 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 1120 sec.) is multiplied by 2, the resulting shutter-speed (1120 × 2 = 160 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 (1120 ÷ 2 = 1240 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.

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.

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 effects

Higher ISO increases sensor-sensitivity resulting in a brighter image.
Higher ISO also increases noise in an image.


histogram examples

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.

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