Depth of Field

Everyone loves the sharp subject and background blur look, and some people love the look so much, they buy a DSLR. Technically, it's called Depth of Field, which measures how much of a subject is in focus. However, not many people know how to get that awesome look. So here are some things that affect the depth of field for a photo:

1) Aperture
2) Focal length
3) Subject distance from background and camera



Aperture

The most straightforward setting to adjust the depth of field. A large aperture such as f/2 creates a shallow depth of field while a smaller aperture such as f/11 creates a deep depth of field. To read more about aperture, see my other post here.


Advantages
- Settings easy to change
- Faster shutter speed when large aperture is used

Disadvantages
- Large aperture lens are relatively more expensive
- Slower shutter speed when small aperture is used


Now for the technical stuff.

The diagram above compares a large aperture and a small aperture effect on the depth of field. The red lines signifies light rays, and where the light rays crosses each other is the camera sensor is, and the point of focus. Light rays does not have to cross exactly to be considered a focus beam. There is a tolerance level for the light rays, and as long as the gap between the light rays is small enough, it is considered in focus.

In the diagram with large aperture, the light rays quickly exceeds the tolerance level for sharp focus, so only a small area is in focus and it creates a narrow (or shallow) depth of field. The diagram right under shows one with small aperture, and the light rays does not exceed the tolerance level for sharp focus as quickly, so it creates a wide (or deep) depth of field.



Focal length

Adjusting the focal length is another way to change the depth of field. A longer focal length such as 85mm on a telephoto lens will create a shallow depth of field, while a shorter focal length such as 15mm on a wide angle lens creates a deeper depth of field. Assuming no there is no change in distance between camera, subject and background, the subject will be magnified at longer focal lengths.


Advantages
- Focal length easy to change
- Cheaper to get a telephoto/zoom lens than one with large aperture.

Disadvantages
- Good telephoto/zoom lens can be expensive
- Longer focal length increases the chance of motion blur
- Subject may be distorted at shorter focal lengths (< 24mm)
- Subject may look "flat" at longer focal lengths (< 200mm)
- Camera may have to be quite a distance away from the subject at longer focal lengths.


Comparing the effect of different focal lengths on the depth of field of the subject. Pictures in all focal lengths except 85mm, have been cropped to match the level of magnification of a 85mm focal length.

15mm, f/5.6

24mm, f/5.6

50mm, f/5.6

80mm, f/5.6

Subject distance from background and camera

If isn't not possible to increase the aperture and the focal length of the lens, but you still want to have a blur background, what do you do?

You can either move the subject further from the background or move your camera nearer to your subject. Do note that moving the background away from the subject makes the background look smaller, and moving your camera nearer to your subject magnifies it, filling a bigger part of the frame.

Advantages

- No cost to move anything

Disadvantages

- Some background cannot be moved

- Not as easily changed as aperture and focal length

Using this photo again as an example, the bottles are placed exactly 30cm away from each other. As you can see, the items in the background gets blurrier as it moves away from the subject.

50mm f/5.6. Effect of distance on background blur.

So which is the best method of all? I would recommend a combination of all would be the best. If you like to be really technical, you can input your settings into this depth of field calculator to give you a rough idea of what to expect: http://www.dofmaster.com/dofjs.html

Once you understand the basics of depth of field, we can move on to the next level, that is creating a bokeh. And it will be in the next tutorial.