If you’ve read anything about photography before now, you may have come across the term ‘shallow depth of field’, or just ‘depth of field’ - or even the acronym 'DOF'. Which can all be quite confusing, especially if you don't know what is meant by ‘field’ in this context.
What this refers to is your entire ‘field of view’ - or more specifically, the space between your camera and the far distance. Think of this entire distance as being like a loaf of sliced bread, with one end right in front of the camera and the other end as far away as you can see - and between those two extremes you have ‘slices’ of focus.
When you take a photo, you can choose how many of these slices are in focus. So, using a portrait with a blurry background as an example - you can see here that the 'slice' where the subject is standing is in focus, and the slice where the background is has gone out of focus.​​​​​​​​​​​​​​​​​​​​​​
​​​​​​​You can choose to have just one slice in focus, some of them, or all of them - depending on the artistic effect you want. When you only have one or two in focus, this is known as ‘shallow depth of field’ and will result in a blurry background behind a sharp foreground subject.
To choose the number of slices that you want in focus, you need to change your aperture. A big hole (wide aperture – e.g. f3.5) will give you just one slice in focus, whereas a small hole (narrow aperture – e.g. f22) will give you all the slices in focus, from where you are, to as far as you can see - like this example below.
In this situation the rock-pool was very close to the camera, so had this been shot on f3.5, or even f8 - and with the focus on the rock pool - the beach, the sea and the island on the horizon would have been out of focus.
This works on a sliding scale from one extreme to the other. So in between these two is a medium aperture - for example f8 to f10 - which gives you some, but not all, of the slices in focus.
An easy way to remember it is that f3.5 is three and a half slices of bread in focus (‘shallow’ depth of field) and f22 is 22 slices of bread in focus (‘deep’ depth of field). That’s just a rough approximation of course, but it gives you an idea!
The transition from sharp to blurry is gradual, so where you have a full row of items you will see a gradual change in the amount of blur, as in this wine bottle shot. ​​​​
The other thing to mention here is that if the focus point was on the furthest bottle, then this would be sharp and the ones in the foreground would be blurry - so this is why you need to be able to choose where your camera focuses.
Going back to the loaf of bread analogy, it’s also important to note that the slices of focus near the camera are thinner than the ones in the distance, so the effect of shallow depth of field is more noticeable on subjects that are close to the camera. This is why it’s much harder to get a blurry background on a subject that is further away.
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