Aperture 

A camera has two chambers that are controlled by two gates, one gate is called the aperture and the other is called the shutter. 

The aperture is gate number one and is found in your lens. Each lens has different aperture allowances. Photons are particles of light that bounce of every surface allowing us to see. 

To understand how aperture works we need to look at how the eye operates. The coloured bit of the eye is called the Iris and this is our aperture mechanism. This opens and closes our pupil allowing the right amount of photons in to the eye, too many photons will damage the eye too few and we will be unable to clearly make things out as it will appear dark. The iris is constantly being instructed to open or close and it does this hundred of times a minute. 

A camera is designed to take a snap shot unlike our eyes that relate more to a camcorder that is constantly filming. This is why it is important to set up the aperture correctly.    

Referring back to the chambers of a camera I need to explain exactly what happens when you press the action button. Once it is pressed all you are doing is moving the shutter mirror out of the way of the sensor. The sensor then is recording the light until the shutter is sprung back into position closing chamber number two. 

Now for the tricky bit - stay with me 

Chamber one as we know is controlled by the aperture gate (in the lens) which determines the strength of the light which will be recorded in the short space of time the shutter is left open. 

Let me brake that down. 

Lets say for example the aperture is open full. This means a lot of light is able to pass through the lens. Many light particles (photons) are now waiting to get into chamber two which is controlled by the shutter. Behind this gate is the sensor that is going to record the colour of all this light. Too much light and your image will be over exposed. 

But.. The shutter can also be altered to suit the aperture setting. As you can probably guess as a result of having the aperture open to its fullest you may need to shorten the amount of time the shutter is open (too much light hitting the sensor will mean an overexposed image). 

The shutter opens and closes extremely quickly (or slowly)  to compensate your aperture setting. The noise a DSL makes when you press the action button is the shutter (second gate) opening and closing. 

Lets get to the bottom of what aperture is actually used for. Aperture controls the depth of field, sorry to throw another spanner in the mix. Have you ever seen an image of perhaps a person and they are in focus yet behind them the background is blurred. This effect is not accidental and can be controlled using the aperture. 

Notice the examples opposite, each image has been taken using the aperture setting that is displayed. Before you begin experimenting I have to mention this before you start - The smaller the aperture number the wider the gate is open and vice versa. 

They are calculated by F/Stops and if all this was not confusing enough here is another spanner for your toolbox. Now that you know the larger the F/number the smaller the opening from this you need to know just how much light is going to be entering chamber one. To do this we need to work backwards once again. f/2 will let in 4 times as much light as f/4 and f/4 will let in 4 times as much light as f/8 and so on. 

To allow you to make sense of the above just remember this - the smaller the aperture (large f/number) the further your depth of field will be - everything will be in focus (depending on the lens) 

Everything from a large f/number down to a smaller f/number will bring the depth of which the focus is crisp to becoming weaker by each setting until just your foreground subject is in focus. 

As you can see opposite in the diagrams. The patterned balls are roughly equal distance apart. the smaller the aperture number will reduce the  depth of field. If  you increased the F number the opposite would happen, you will increase your depth of field allowing the background in your composition to remain in focus - Finally something that makes sense. 

The Patterned ball at the back is starting to become more in focus as the aperture gets larger. different lenses will act differently and give you greater depths of field.

Notice the images get darker the smaller the F - number gets. This is because the hole that lets light (photons) in to the camera via the lens is getting smaller meaning less light is able to correctly expose the image on the sensor. 

To compensate the reduced light when using high apertures you could either increase the ISO setting which makes your sensor more sensitive to light or your could decrease the shutter speed or both. Don't forget to use a tripod if your are slowing down the shutter speed as you run the risk of having blurred images.   

In order to understand aperture further I suggest that you set up your camera on a tripod with a subject about 6-10 feet away from the camera. Set your camera to Aperture priority (A) this means that you can alter the aperture and the camera will automatically change your shutter speed to get a decent enough exposure. All you have to worry about is the aperture settings. 

<take a few practise shots just to make sure your exposer is ok - if is not go to manual mode (M) and altar the shutter speed yourself, practise makes perfect> 

Put your aperture on the lowest f/stop and then take the picture. make sure your memory card is empty so your can fit the images on it. Increase the f/stop taking a picture on each setting until your at the highest setting. 

Upload your images to a computer so that you can see the alteration on a larger screen. Notice the background was in focus and gradually it becomes blurred. 

A word of advice, the use of aperture in this way can only be judged by experience as you don't have much guidance on how the image will look until you have taken the picture. This is because the view finder only shows you your shot and the LCD screen on the rear is often too small to get a clear indication of your tweaking. 

Search for Books on aperture.