Depth of Field can the the photographers best friend or worst enemy – depends on whether you meant to get cousin Fred all blurry in the family photo or not!
In optics, particularly as it relates to film and photography, depth of field (DOF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. Although a lens can precisely focus at only one distance at a time, the decrease in sharpness is gradual on each side of the focused distance, so that within the DOF, the unsharpness is imperceptible under normal viewing conditions. (Source Wikipedia)
So what does this mean in relation to your photography? It means that the wider your aperture (the lower the number, eg ƒ/2.8) the shallower your Depth of field – conversely, the smaller your aperture (the larger the number, eg ƒ/22) the wider your DOF. There is a direct relationship between distance for the lens and image size to the DOF – the closer the subject, the shallower the DOF. For example, in Landscape Photogrpahy the subject if often a fair distance form the lens – therefore at larger apertures (like ƒ/2.8) you get a much larger DOF than in Macro Photography, where due to the closeness of the subject the DOF is extremely shallow, so much so that you may only get a part of the insect in focus.
The focal length plays a big part in DOF as well. To demonstrate, try this. Focus in close on a leaf at ƒ/2.8 from 3 metres away and take a shot – you will notice the area in front of and behind the leaf is blurry, whilst the leaf itself is in sharp focus. Now focus on an object in the far distance – say a tree on a hill. If you are using a 200mm lens, your DOF from 3 metres will be 20mm – at 200 metres it will be 121.39m ! Quite a big difference. Have a look at this Depth of Field Calculator that I found quite useful when writing this article.
The other thing that is useful to be aware of is the Distribution of Depth of Field. At a smaller focal lenght (10 mm) the distribution of DOF is roughly 30/70 in front of and behind the subject – at longer focal lengths, say 200 mm it is closer to 50/50.
This is particularly something to be aware of when photographing groups of people – you often hear people say, “The Bride & Groom where beautifully sharp, but cousin Fred in the back row is all blurry!” (poor Fred!). This is becasue to get the group in, the photographer used a smaller focal length (say 24mm) but had the aperture too wide (say ƒ/2.8 at 3m from the Bride & Groom in the front row – the DOF is only .76 m ). All you needed to do is close the aperture to, say, ƒ/9 and the DOF would have been about 20m (15.67 m @ ƒ/8 and infinity at ƒ/11).
This link to a Depth of Field Tutorial by Cambridge in Colour is very useful.
Depth of Field when used properly can make a photo – intentionally blurring a boring background can enhance an image immeasurable, whilst a tack sharp Depth of Field in a Landscape photograph can make your image an award winner. Again, (like exposure) it is a very subjective issue. I displayed (what I thought) was a beautiful image of a bid in a tree, with the background beautifully Out of Focus (OOF) in a Nature & Wildlife Forum. From an artistic viewpoint it was a good image – but to a bird watcher, he wants to see the detail of the foliage and the background – because he is interested in the Subject, not so much the artistic value.