Basics - Understanding Exposure

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Perfectly exposed image. Ashley Beolens

Perfectly exposed image. Ashley Beolens

As part of our basic series is trying to give some understanding into the basics of photography, and it has to be said that understanding exposure is one of if not THE most important aspect of photography (I guess with automatic settings on cameras it could be argued that composition is most important, but let?s not start a debate now).

My aim with this article is to give an understanding of exposure, there is far too much to detail in full on one page, in fact there are plenty of books written just on exposure so I hope you will forgive me for keeping it short but hopefully to the point. It is worth noting at this point that the advice on this page is aimed at those who are comfortable altering their camera settings, those that have only mastered automatic settings so far SHOULD learn this, but until you learn to be comfortable using your cameras other settings you won?t really be able to change much. So get out there and learn your camera.


Photography is all about capturing a scene on film or on your digital sensor right? Well, yes and no, what it is actually about is capturing the light reflected from surfaces, without light there is no photo, too much or too little light and there is no photo, so photography is really all about learning to capture light. (Learning to control that light is a step you will learn over time and in different tutorials to this).

Exposure is the term used to describe showing enough light to your sensor or film to create the desired image that you want to capture (sorry if this sounds very basic or patronising but it is an important element to learn), too much light in areas and you will have elements of your photo with ?blown? unrecoverable highlights (objects you can see no detail in, think a white object in an otherwise dark scene), too little light in areas and you have the opposite, no details at all. It is important to learn that while images that are over exposed or under exposed may be recoverable within post processing software learning to get it right in your camera may stop heartache at the missed photo opportunity.

Dynamic Range

Our eyes are fantastic organs, they are able to see the detail in deep, dark shadows or crisp, bright light, they can adjust in milliseconds to the altering light; cameras on the other hand cannot (yet) do this. Our eyes can see in a single moment a range of light equal to around double (or more) of that which a camera can pick up. Cameras pick up around 10+ stops of light (depending on the camera) at one time, anything outside that range will be black or white (under or over exposed).

It is the skill of the photographer to know which way to place the ends of your dynamic range, lack of exposure will give a more moody look, more light will give a more peaceful serine look, neither is correct and neither is wrong it is all interpretation.

There are ways to increase the dynamic range of your camera, filters such as graduated Neutral Density filters will allow the camera to be tricked into giving more range, or you can use software to create HDR (High Dynamic Range) images with layering of differing exposures creating one final image. But high range is not everything.


In order to get a really well exposed image you need to find out what sort of light you have available, this used to be achieved with a hand held light meter (and still is by many pros and studios) this was a simple tool that you placed where the subject was, it then measured the light and told you which settings would give the technically correct exposure, allowing you the ability to alter your cameras settings to suit the light of the scene. These days most cameras have built in light meters that are pretty accurate. These work with the same principle, so you can use this to set your camera. See our understanding your light meter for more information.

Under Exposed Image

Under Exposed Image

Sample of Original shot under exposed İAshley Beolens

Altering Exposure

There are three in camera elements with which you can control the light that enters your camera (external elements like flashes or reflectors are not covered here), and thus alter exposure (These are known as the Exposure Triangle):

* Shutter speed.
* Aperture size or F/Stop.
* ISO speed or film speed.

Shutter speed: When a shutter opens light passes through your lens onto the cameras sensor until the shutter closes again (I will use Sensor from now on but it equally refers to film). Controlling the speed of the shutter dictates how much light can pass through, the faster the shutter speed (measured in fractions of a second) the shorter the duration of the opening, resulting in less light being passed through to the sensor and your exposure time is reduced. Most modern cameras have a quickest shutter speed of 1/4000 (this equates to 1 4000ths of a second that the shutter is open) or even above (this will obviously change over time so may be wrong). For a more detailed explanation on shutter speeds please read Basics - Understanding Shutter Speeds.

Aperture Size: The amount of light hitting your sensor can also be controlled by having a larger aperture. A bigger hole allows more light through (in a shorter time) than a small hole; one full stop will increase/decrease the size of the hole by half and therefore alter your shutter speed by 1 stop (1/200 becomes 1/100 or 1/400 depending on whether you increased or decreased the size). Altering the aperture also has other effects as well, but is a way to increase or decrease the amount of light. (Please take a look at the article Basics ? Understanding Aperture for details of how this works and the other effects aperture has)

Film/ISO Speed: When film was much in use different film had different sensitivities to light, ISO100 (ISO standing for International Standards Organisation) was the common everyday film; If light was an issue then more light sensitive film was needed ISO200 doubled the sensitivity to light, 400 doubled that up to 6400 in pro shops. These terms have been carried over to the digital world, and similar speeds are available (in fact there are some extremely sensitive camera sensors equivalent to ISO12800+). This doubling of sensitivity means that for each increase in ISO your exposure time is reduced (so quicker shutter speeds are used). Unfortunately with digital sensors this has created its own issues; high ISO?s tend to produce ?digital noise? which are not all that pleasant in large printed images (Film grain had a unique quality that sometimes improved images digital noise does not). For more on ISO please read - Understanding ISO

Over Exposed Image

Over Exposed Image

Sample of Original shot over exposed İAshley Beolens

These settings can all be used in conjunction with one another in order to get an exposure that really captures a scene in a manner you wish. So if the light doesn?t allow you to get a movement free image (and that is what you desire) with just altering shutter speed, think about opening your aperture more (wider, lower f/number) or increasing the ISO sensitivity. Likewise if the scene is too bright reduce your aperture (higher F/number) less light will be coming in so you will have a ?better? exposed image (although it is worth reading the above link on aperture first to understand the possible effects with depth of field).

Exposure Value

Combining aperture settings and shutter speeds using single stop increments, 1 full stop increasing the size of your aperture (decreasing the f/number) means one full stop increasing the speed of your shutter speed (and the other way around as well) (see Understanding Aperture for details on full and 1/3 stop increments) will keep the light coming onto the sensor at the same level. This is known as Exposure Value (EV) and can be tricky to explain clearly and understand so here is an example of one of the many EV charts you could use.

If the light dictates you need a shutter speed of 1/1000 of a sec af f/1.4 then to achieve a shutter speed of 1/125 of a second would require the aperture to be set at f/4. The following chart shows the full comparison to show how each incremental stop changes things:

1/1000 at f/1.4
1/500 at f/2
1/250 at f/2.8
1/60 at f/5.6
1/30 at f/8
1/15 at f/11
1/8 at f/16
1/4 at f/22

This is obviously just one example, if the shutter speed was 1/2000 at f/1.4 it would be 1/1000 at f/2 etc. it is the principle that needs to be remembered not this one chart.

ISO and Shutter speed

In a similar vein ISO changes will affect shutter speed in the same way, each step up in ISO value will increase the potential shutter speed, while allowing the light to have the same effect (unlike changing aperture value though depth of field will not be affected but noise levels will). Again see the chart below.

ISO 100 - Shutter speed 1/125
ISO 200 - Shutter speed 1/250
ISO 400 - Shutter speed 1/500
ISO 800 - Shutter speed 1/1000

Again, as with the previous example the numbers are arbitrary it is the principle that counts, I could have chosen any shutter speed as the starting point and doubled up as the ISO changes.

Filters and other accessories

Beyond the settings of your camera it is possible to use a number of other items in unison with your camera to alter exposure; from flashes to help light the image to Neutral Density (ND) filters that allow much longer exposures (graduated versions can help expose one plane of your photo while stopping another being blown). But this is not an article about those.


You can use exposure to create some stunning images, look at most photographers collections and you are likely to see some really interesting portraits with dark backgrounds, with deep shadows creating the definition on the face or high key images with the subject isolated on a clear white backdrop, or even those creamy water pictures that seem to be all the rage at the moment (2012) this is all achieved by knowing how to control exposure.

One important thing to remember is that there is no correct exposure. There is exposure that is technically well done, but that does not mean it is the ionly option. Take for example and image of a twilight scene, technically correctly exposed it will be crisp and light enough, but as the time of day dictates in our heads that it should be in between day an night an under exposed scene will actually look more natural.

It is all about learning to control light, once you have mastered that your photography will come on in leaps and bounds. Please follow the links above to get a better understanding of exposure and how to really control it.

Sorry this wasn't as short as I thought it would be, and I keep adding updates! But I look forward to hearing if you have found this useful, please feel free to contact me if it has helped or you have other advice that I may have missed (or if I have made any glaring errors). The feedback link is at the top of the page.

Ashley Beolens - Sorry the image used isn't interesting, but I thought a simple image would be best as an example