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Exposure Triangle - Shutter speed, Aperture and ISO

Updated on May 7, 2013


Def: is the amount of light that is allowed to reach the film/image sensor so as to form an image.

Therefore you need the right settings to have the right exposure to have the right image get formed. Too much light means over-exposed images whereas too little light means an image that can not be clearly seen

There are three main settings in your camera that determines how the image is exposed.

These are:
A) Shutter Speed

B) Aperture

C) ISO/Film Speed

But remember you not only change these three settings to adjust the exposure but also to achieve certain effects. Thats why you need to know which setting should be set at what value and why

exposure triangle

The diagram above illustrates that they all work together to achieve the right exposure. If you change any of them, you will also need to change one or both of the other settings. e.g if you change the ISO to a different value, then you also need to change the aperture or the shutter or both to compensate for the change in ISO. so, let discuss these settings and why you need to play around with them

A). Shutter Speed

Shutter speed is the amount of time the image sensor is exposed to light. When you press the shutter release button, the door or the shutter, through which light enters your camera is opened, exposing the film/image sensor to light.

Shutter speed is measured in seconds, The bigger the denominator the faster the speed, e.g. 1/500 is much faster than 1/60.

freezing effect using faster shutter speed

There are different factors that will determine the kind of exposure you select for your photography.For instance, if you want to freeze a moving object or you want to show a subject was moving. Slower shutter speeds will leave your shutter open for a longer time hence accomodating any movement that may take place which is ideal to show movement, whereas you can set a faster shutter speed which will not give time for the camera to sense any movement, hence freezing.

B). Aperture

This is how wide the shutter opens, ie, the wider the opening the more light that goes through and vise versa. for instance, if you are in a dark room and the door was partly open, the shadow formed behind you may not be as clear as when the door is widely open. If you want you image to be exposed more then you need to increase the aperture. i.e, open the door more to allow more light

how the aperture opens to allow light through

The aperture determines how much light travels through the lens to the film/sensor. It is the diameter of the opening where light is allowed to pass through to reach the image sensor. This means the larger the opening(aperture) the more the light that will pass through.
The size of the aperture is measured in f-stops. The f-stops values are inverse to the aperture diameter such that a small f-number (e.g f/2) translates to a wide aperture whereas a larger f-number (e.g f/22) means a narrow aperture. So if you want to expose your image to more light in case you encounter inadequate lighting conditions, then you need to chose a smaller f-stop on your camera, this will ensure your aperture opens more which will allow more light to pass through.

C). ISO/Film speed

This is the sensitivity of the image sensor/film to light, ie, the higher the ISO setting the more sensitive the image sensor. A high ISO means forming an image with minimal light and vise versa.

Its worth noting that, if you use/set high ISO which means high sensitivity it means even the slightest entry of light will be sensed by the image sensor so you need to allow minimal amount of light to get through, this reduction in exposure time results with having a compromised exposure or grainy image because it has to be fast.

a very high ISO setting giving a grainy image


Low ISO's numbers means you have a fine image, so why would you set you ISO at a maximum and end up getting a grainy image? Firstly, as we have discussed above, change in any of the three main setting will affect the other settings, for instance is you aperture is very narrow and your shutter speed is very fast then you might find it irresistable to set the ISO to a high value. Secondly, if you do no want to interfere with the other two settings(you have set the preffered aperture value and the right shutter speed value) and still find you image is under-exposed then you might have to increase the ISO


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