In the previous DOF Tutorial I was telling you that Depth of field is influenced by three main factors: 1) Aperture, 2) Focal length, and 3) the distance to the subject. Now I will show you details about each of these factors.
The relationship between aperture and depth of field is quite simple:
A large aperture like f/2 gives a shallow depth of field.
A small aperture like f/16 will give you a large depth of field.
Let’s review the examples shown before:
For the purpose of achieving depth of field, you will need to be able to control the aperture, and therefore I recommend shooting in Aperture Priority mode. In this mode, you can choose the aperture and the camera does the rest for you and chooses the appropriate shutter speed. If you are comfortable with choosing both shutter speed and aperture yourself, please feel free to shoot in Manual mode.
2) Focal length:
The second parameter that has an influence on the depth of field of a photo is the focal length of the lens you are using. The focal length of a lens gives an indication of the field of view that the lens covers.
A lens with a 28mm focal length for example, is called a wide angle lens and covers a big field of view. It allows to capture a whole landscape in a photo for example. 28mm is a short focal length.
On the other hand, a 300mm lens is a super telephoto lens and only covers a very small field of view. It allows you to zoom in and isolate your subject, like a flying bird for example. 300mm is a long focal length.
If you stand at a given distance from your subject and take a photo with a 28mm wide angle lens, the depth of field will be relatively large and many things around it will be in focus.
Now, if you take a photo of the same subject, from the same distance, but “zoom in” using for example a 300mm telephoto lens, then the the depth of field will be a lot thinner and the background behind your subject is more likely to be heavily blurred.
With a 28mm wide angle lens at F/4 both the bottle and the surroundings are sharp.
With a 200mm zoom lens at F/4 the bottle is sharp but the background is blurred.
What this means in practice is, that in order to get a shallow depth you should zoom in closer to your subject. To get a larger depth of field, zoom out.
3) Distance to the subject:
The third factor that will have an effect on the depth of field, is the distance to your subject – also called focusing distance – that we discussed earlier.
The rule is quite simple:
The closer you are to your subject, the smaller the DOF will be.
The farther you are from your subject, the greater the DOF will be.
So in order to get a shallow depth of field, you should move closer to your subject. Moving away will help obtain a larger depth of field.
This is the reason why the depth of field is usually so thin in macro photography. Macro lenses have a specific mechanism that allow them to focus at only a few inches from the camera. This effectively creates very small depths of field.
On the contrary when shooting landscape photography, you’ll notice that if you focus on the infinite, the depth of field will be very very big, usually ranging from a few meters away to the infinite.
Summary: How to achieve the desired depth of field.
For shallow depth of field:
a) use a large aperture like f/2,
b) move closer to your subject,
c) zoom in and use a longer focal length.
For large depth of field:
a) use a small aperture like f/16,
b) move further away from your subject,
c) zoom out and use a shorter focal length.
Please do remember that you can, if necessary, use all three together. In other words, if you want to achieve a shallow depth of field, you can try using a large aperture. However, if that doesn’t give a shallow enough depth of field, you can in addition move closer to your subject and zoom in.
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