21 Sep Exposure Triangle

1. Exposure Triangle

What is photographic exposure? It is the amount of light that hits image sensor in the camera. If the amount of light was too small – the image captured by the camera will be too dark – underexposed. On a contrary – if we allowed too much light – we will see as a result an overexposed image – washed out, pale, nearly white photo with no detail visible.

In order to create balanced images, as close as possible to the scenes we have witnessed – we have to control the exposure. It can be achieved by adjusting the three fundamental components of exposure: ISO, shutter speed and apertureThe graph presented above is called exposure triangle and it simply illustrates relationship between the three components. Remember that all the three elements are directly linked and manipulating with one of them immediately affects others. If we change one variable, to maintain correct exposure we have to further adjust at least one of the two remaining settings.

While referring to exposure triangle we use term “correct exposure”, however it is important to notice here that there is no rigid recipe for a good exposure, as there is no such a thing as “perfect” histogram – you can mix all the three elements shutter speed, aperture and ISO to suit your needs and personal taste. I will talk in more detail about creative exposure in the next article. 

For any given image there is always only one mathematically calculated “correct exposure”. There is flexibility though when it comes to obtaining it, as every exposure can be achieved by multiple combinations of the three variables.

2. ISO

First let’s have a look at ISO – the basis of the triangle. The ISO is an International Standards for light sensitivity. The lower the ISO, the less sensitive the camera’s sensor is to the light and higher ISO means higher sensitivity.

The general rule when adjusting your exposure is to always keep the ISO setting as low as possible, which typically means 100. 100 is called slow ISO. High ISO values, such as 1600 can produce excessive noise and less detail in your images. The higher the ISO, the worse and more obtrusive the noise. The ISO adjustments are best kept for conditions when they are really necessary, such as for example low light environment In most situations the typical exposure adjustments of shutter speed and aperture are perfectly enough to handle the scene.

The ISO values start from 100 and every next value is twice as big as the previous. So the sequence goes 100, 200, 400, 800 and so on.

3. Shutter Speed

Shutter speed or in other words exposure time. Shutter speed simply indicates how long the shutter remains open and for how long the sensor is exposed to light. The exposure time is measured in fractions of a second, when they are under a second and they are commonly referred to as stops. Notice that each stop is either a half or a double of the value of the adjacent stop, exactly the same as with the ISO values increment.

The darker the scene, the longer it will take to provide enough light to expose the image and we will have to use longer shutter speed. Slow shutter speed is considered the lowest value that you can handle without introducing a camera shake. A fast shutter speed, such as  1/5000 in case of the image below allows you to freeze movement in the image.

Another example of “frozen” movement:

On the opposite – With slow shutter speed, such as for example 1/30 of a second you can deliberately create a “motion blur” effect when photographing moving objects. 

4. Aperture

And finally let’s talk about the third element – aperture. Simplifying we can say that the aperture is a hole inside the lens that let’s the light inside the camera body to hit the sensor. Aperture is expressed in f-numbers, for example f 1.4. 

Note that low f values mean large aperture, when the hole is opened wide and plenty of light is traveling to hit the sensor. And high f values indicate small aperture.

Aperture value has a great impact on your final photo since it influences depth of field – the area of the image that appears sharp. With high aperture values you will have more focus and these are typically used in landscape photography when we aim for as deep focus as possible as in case of the following examples:

Low aperture values on the contrary will produce shallow dept of field effect which is helpful when isolating subject from the background as in the portraits below:

To maintain best quality and sharpness of your photos try to avoid the extreme f-stop values, as they  typically produce image distortion.

5. Stop of Light

Refer to the exposure triangle when working out your exposures – the key is when you increase the exposure for one component, you have to decrease it for one or both of the other components to keep the balance.

The ISO, the shutter speed, the aperture diameter are all measured in different units. That’s where “stops” were invented as a convenient way to compare them. “Stops” let you quickly compare and swap the ISO , shutter speed and aperture values in order to produce desired results.

To add a stop of light we double the amount of light that makes up an exposure and it leads to brighter image. Analogically – to create darker image we have to take away one stop of light.

I hope the article will help you getting great results when setting your exposures!


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