Depth Of Field

I shot the following images as a simple way of showing depth of field and how to understand the relationship between aperture and shutter speed and how of we use aperture to create a shallow (small) DOF, and a wide (large) DOF. I have shown on each image below what aperture (shown as an F-number) I have used, whilst showing the background with a box around it in order to show how the background is blurred with a small aperture and it gradually gains focus the higher the aperture. The lens I was using only goes as low as f4, which gives a reasonable depth of field, but I would always use 1.8 - 2.8 for best results. A shallow DOF causes the background of an image to be completely out of focus, therefore bringing instant focus to the subject you are shooting; making for much better images, especially portraits. 

These are some simple shots to show as a starting point. I have added what shutter speed I used for each shot, I will go into more detail as this is what is important regards the relationship between shutter and aperture. Hopefully this will give any beginner a rough starting point.

Aperture F.5 Shutter speed 500th of a second

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Aperture F8 Shutter Speed 250th of a second

Aperture F.11 Shutter Speed 125th of a second

F.16 Shutter Speed 60th of a second

You should be able to notice the difference in the amount of blur in the background in each image, as shown above. Aperture and shutter speed work in increments. F numbers also, when we talk of f-numbers we use the term 'stop' so a 'stop' up or a 'stop' down. The same with shutter speed. Each time we change the f-number, we also change the shutter speed, otherwise we would end up with under or over exposed images. The higher the f-number, the lower the shutter speed; this is because with a higher f-number, lets say f22, the lens is almost closed which means that not much light is passing through, therefore to counteract this we must use a slower shutter speed which allows for a longer exposure time, which enables more light to reach the censor. Using a lower f-number, lets say f4 the lens is almost wide open which is letting more light through, therefore to counteract this we would use a faster shutter speed as this will let less light in - this is the basis for a getting a good exposure.