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Part 1 : Introducing the exposure triangle

*Disclaimer: Notes written by Alistair Keddie.


Photography exposure triangle for DSLR notes



Part 1 : The exposure triangle

To photograph quite literally means ‘to draw with light’ and by understanding how light enters our camera and subsequently, how we affect and control that, then we open up to a new world of photography. Taking a photograph is technically speaking, actually the act of making an exposure. It’s called an exposure because we physically expose a light sensitive material, the camera’s digital sensor, to light.

Generally we control the exposure by understanding the combined affect of three main factors and how, individually these affect the visual statement we want to make. These factors are, ISO which controls how sensitive our camera is to light, Shutter which controls the length of time our sensor if exposed to light, and Aperture, which controls how much light enters our camera. ?

Changing any one of these affects the other two and is why we represent them together as the Exposure Triangle.


Better understanding ISO on your camera

Understanding ISO

ISO is a measure of how sensitive the camera is to light. The higher the ISO the higher the sensitivity or the faster its speed and the less light we’ll need to make an exposure. For instance, as day falls into evening, we increase our ISO to a higher or faster setting increasing our camera’s sensitivity to the lower light levels. The lower the ISO, the lower the sensitivity or the slower its speed and the more light we need to make an exposure. For instance, on a sunny or bright day we can use a low or slow ISO like 100 or 200.

This begs the question, why not set the camera to a fast ISO to cope with all lighting conditions?

We don’t have a single ISO setting because the higher or faster we make the sensitivity, the more noise will be introduced into our exposures which can degrade our image quality. This ‘noise’ across the image can get in the way and lead to a loss of fine detail. Colour also suffers at higher ISO’s and begins to desaturate and lose strength. Likewise, high ISO’s tend to produce more contrasty images, losing finer tonal gradients and ranges.

We therefore aim to use as low an ISO as conditions will allow. This produces finer image quality with virtually no noise or loss of fine detail as well as capturing strong, saturated colour and finer tonal ranges and gradients.

This is not to say that using high ISO’s will result in poor photographs, quite the contrary. But it is necessary to be aware of the visual impact of raising ISO.

For instance, in the days of high speed film (high ISO) these tended to be black and white producing high contrast grainy images with a very distinctive look or aesthetic to rock and roll, documentary or even war photography where, working hand held without flash in low light you had no alternative.


Better understanding noise and grain on your camera for taking better photographs


Film grain however is very different to digital noise and is often very beautiful in its own right lending considerably to the visual look and feel of the photography. Digital noise though is a different matter. Untreated, it ugly and can look like a green and red porridge covering our shot. The camera does usually have some built in noise reduction and we can do more on the computer to make noise more attractive, more part of our image.

Typical ISO settings for general use:

100 – 200 ISO:
Generally good for daylight out of doors hand held capture giving very low noise and fine image detail.

400 – 800 ISO:
Use as the light begins to fall or indoors without flash. Usually still gives good detail but more significant noise added as the ISO rises.

1600 ISO and above:
Use in low light conditions where you have no alternative. Significant noise will be added with loss of fine detail at these ISO’s. Provided you can work with this, there is no reason not to use them.

ISO directly affects shutter speed. The higher the ISO, the higher the camera’s sensitivity to light and the faster the possible shutter speeds we can expect. The lower the ISO, the lower the camera’s sensitivity to light and the slower the shutter speeds become. This can be an issue if the shutter become too slow for hand held capture and our images start to blur. On the plus side, raising ISO can allow us to safely shoot hand held in low light.


How ISO affects shutter speed in photographs


As we’ll discover later when we understand the relationship between depth of field and aperture, we’ll realise that to get the best out of a landscape shot we’d use a low ISO to increase the fine detail and colour saturation. This combined with a small aperture leads to slow shutter speeds and the need for a tripod.


Understanding the Shutter

We use the shutter to control the exposure by varying the amount of time the it is open for. This can range from very fast to very slow. Shutter speed are expressed in fractions of a second, and usually range from 1/4000 of a second all the way down to 30 seconds.

Your camera will normally identify the shutter speed as a fraction on its display or may only use the lower number, for instance 30, 100, 500 etc to indicate 1/30, 1/100 and 1/500.

For slower shutter speeds, the display changes, usually after 1/4 sec to show 0”3 to indicate 0.3 seconds, then 0”4, 0”5, 0”6, 0”8, 1” etc. 1” is one second with the quotation mark indicating seconds. From there it drops all the way to 30”. Much lower shutter speeds are also possible but require a remote cable release to operate properly with the camera set to Bulb mode.

We think about shutter speed in relation to motion and the way we want to capture it. For instance, to freeze motion we would use fast shutter speeds to capture a water splash or an insect in flight. We can also use slow shutter speeds to creatively blur motion such as smoothing out the motion of water as it tumbles over a fall or of traffic at night as it moves along, the headlights turning into a river of light.


Highway lights shutter speed, a river of light.


We play with shutter speed to vary how sharp or how much blur we want to introduce into a subject in motion.

A general rule of thumb around shutter speed is when they fall to around 1/60 sec you need to start thinking about better supporting or using a tripod. Some cameras have built in Image Stabilisation, usually indicated by the letters IS or IOS. With this turned on, it is possible to shoot hand held at even slower speeds. Bear in mind that the slower the shutter speed, the more you need to support the camera to reduce shake. Camera shake is more of an issue when using a longer lens to zoom as any shake will also be magnified. Using longer lenses generally means working with faster shutter speeds. The general rule here is worry when the speed begins to fall below the focal length of the lens. For instance, shooting at 200mm would mean trying to use speeds of 1/200 and above.

Shutter speeds affect both ISO and Aperture. In order to achieve faster shutter speeds you might need to increase ISO which introduces more noise, or you might want to open the aperture, which decreases the depth of field.


Understanding the Aperture

We use Aperture to control exposure by adjusting the amount of light that enters the camera. We do this, by varying the size of a hole or diaphram which we can open or close. This is the aperture and is generally housed inside the lens.

The size we set this to is called the aperture value (Av) and is measured in F Stops where the f stands for Focal Ratio. Confusingly, the smaller the f stop, the bigger the aperture whereas, the bigger the f stop, the smaller the aperture.

The F Stop Scale measures how much light enters the camera and each full stop down lets in half as much light as before. Indeed, the aperture is half the size of the stop before it. The values used to describe this scale are F numbers. These are worked out by multiplying by the square root of 2 and give a sequences as follows...

F1 one full stop down is = 1/2 the light at F1 F1.4
F1.4 one full stop down is = 1/4 the light at F1 F2
F2 one full stop down is = 1/16 the light at F1 F2.8
F2.8 one full stop down is = 1/32 the light at F1 F4
F4 one full stop down is = 1/64 the light at F1 F5.6
F5.6 one full stop down is = 1/128 the light at F1 F8
F8 one full stop down is = 1/256 the light at F1 F11
F11 one full stop down is = 1/512 the light at F1 F16
F16 one full stop down is = 1/1024 the light at F1 F22
F22 one full stop down is = 1/2 the light at F1 and so on

Because it is a mechanical device, the aperture can in theory be closed down by any amount however as well as the full stop scale above it is often also measured in thirds of a stop. That is, for instance between F2.8 and F4 we’ll find F3.2 and F3.5 or between F5.6 and F8 the values F6.3 and F7.1. These are fractional f stops.

By controlling the size of the aperture we control the amount of light entering the camera in measured increments and depending on its size, whether open or closed will have a profound affect on our Depth of Field (DoF).


Understanding Depth of Field

Depth of field can be a vital creative tool in our photography which we use, for instance, to blur and soften the background behind a portrait or capture as much detail from foreground to horizon in a landscape as possible. It is a term which describes the amount of our scene that is in acceptable focus. Not what the camera is focussed on, the point of focus but rather how that point comes into focus as we move towards or away from it. It is in fact how focus behaves over distance.

If for instance, our camera is focussed on our subject, say a person, the autofocus measures the distance to the subject and sets the lens sharp at that distance. Our person comes out sharp in the photograph. If I introduce another person standing much closer to the camera, they come out blurred. They only become perfectly sharp when they are standing at the same distance from the camera as our original subject. So focus is shifting across distance to the point we have focussed on. As this transition occurs, there is an area in front of, or behind our subject where the scene will be acceptably sharp and that is the depth of field. Using the aperture we can control how wide or shallow this depth of field will appear. That is, choose whether to capture our background soft and out of focus, perfect for a portrait, or sharp and well defined, ideal for a landscape.

Controlling Depth of Field

We use the aperture to control depth of field. Opening the aperture will create progressively shallower DoF’s whilst closing it will widen or deepen the DoF. That is, the lower the F Stop of the aperture, the shallower the depth of field and the higher the F Stop, the wider the depth of field.


Better controlling depth of field in your photographs



example of how aperture value changes affect photo quality

An example of how depth of field shifts as the aperture value changes. Wide open apertures create much shallower dof’s whilst more of our scene becomes acceptably sharp as we close the aperture down.




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