Depth of Field
Depth of field is a relatively advanced photography technique, but one which can really add a whole new dimension to your photos.
Depth of field refers to the amount of the scene which is in focus in your photo. A large depth of field will result in a photo which is mostly in focus, while a shallow depth of field will result in a band of focus with blurred regions in front and behind.
Here's a photo of my guitar as an illustration:
While the light painting may be very cool, that's not the point I want to demonstrate with this photo. Take a close look at the fretboard and strings.
At the front of the photo the strings are blurred. As you look towards the sound hole they get clearer, but once you reach the sound hole and beyond they are blurred again.
This photo demonstrates a shallow depth of field. Notice also that the blurred region behind the region in focus is about twice as large as the region in front of it. This is no coincidence, and is used as a rough rule of thumb by photographers.
Factors which affect depth of field
There are three main factors which affect depth of field in a photo. These are:
- Aperture size
- Focal length (zoom)
- Distance from the subject
The depth of field is shallow if:
- The aperture size is large (small f-number).
- The focal length is large (i.e. zoomed in significantly).
- The distance from the subject is small.
The depth of field is large if:
- The aperture size is small (large f-number).
- The focal length is small (i.e. zoomed out: wide angle).
- The distance from the subject is large.
To illustrate this, I need to show you a few home made diagrams. If you've ever studied physics, these may look familiar as ray diagrams.
Depth of field diagrams
First of all, this is how a basic converging lens works, taking the light from a distant object and converging into onto a point.
Now we can put in points representing the beginning and end of the region in focus, as so:
So, the depth of field is the region between the extremes. What happens if we make the focal length of the lens shorter (zoom out)?
As you can see, reducing the focal length of the lens reduces the depth of field (and interestingly, also brings the region in focus closer to the camera)
Now let's increase the focal length...
So when you zoom in the focussed region gets longer and moves away.
I won't do the same thing and change the other variables, but this should give you an idea of how it works.
Examples of depth of field
The photo of the leaf in the banner is a good example of this effect. Here's a bigger version:
Note how the leaf itself is in focus but the background (a rather overgrown garden) is completely blurred.
Here's another of a stork on a lamp post. The trees behind it are not in focus but the effect is much less severe than the photo above.
Phew. Like I said, it's a relatively complicated concept, but it's great fun when it works!