|
Depth of Field (part 1)
In photography, something is 'in focus' if all the light rays entering the lens and originating from a point converge at a point on the image sensor. If the rays do not converge at a point but instead form a circle (called a blur circle or circle of confusion), then the image of that point in the scene will be 'out of focus'. In reality, the rays may not perfectly converge but still be perceived as in focus as long as the blur circle is small enough.
The image above clearly shows some of the text in focus, and other text getting progressively more out of focus as it gets nearer or further from the camera.
Depth of field (DoF) is an optical property of lenses which means that only objects within a certain range of distances from the camera will be in focus (as clearly shown in the photograph above). For images such as landscapes, a large depth of field is usually desirable, however for portraits, photographers often prefer a very shallow depth of field to make the subject stand out against an out-of-focus background. Cameras that have landscape and portrait modes on them will often adjust the camera's setting to try and achieve this effect.
The depth of field can be controlled by changing various things such as:
The actual quality of the out-of-focus area (known as the ‘bokeh’ of the image) also varies from lens to lens and is an important factor in the quality of the final image.
Controlling Depth of Field
The simplest way to control the depth of field in your image is to change the aperture of the lens. A large aperture (low f-number) will give a shallower DoF than a small aperture. On DSLRs with a fast prime lens, the DoF can be so shallow that the accuracy of focussing becomes critical in getting a sharp picture (for example, in a portrait where the head is slightly turned, one eye could be in focus and the other might not.
Another way to change the depth of field is to use a longer focal length. As you increase the focal length of the lens, the DoF decreases. This is why focusing is so critical on long lenses - for example, a 300mm lens set to f/2.8 and focused on a subject four metres away will have a depth of field of less than two centimetres.
|
The Relationship Between Focal Length and Depth of Field
An interesting observation is that if you increase the focal length of the lens (zoom in on your subject) the DoF will decrease, however, if you then move away from your subject so they are the same size again in the viewfinder, the depth of field returns to almost exactly the same as it was before.
Example: You have a DSLR with an APS-C sized sensor (and Canon, Nikon, Sony or Pentax consumer or enthusiast level camera)
You set your lens to 30mm lens at f/2.8 and take a picture of your subject who is standing 3m away:
DoF = 1.18m (Focus range: 2.52m to 3.70m)
You zoom in on your subject by setting lens to 150mm, changing nothing else:
DoF = 0.04m (Focus range: 2.98m to 3.02m)
You now walk back so you are 15m from your subject (they are now the same size in the frame as the first picture).
DoF = 1.15m (Focus range: 14.45m to 15.60m)
This highlights that both the focal length, and the distance from camera to subject affects the amount of depth of field you will get in the image. Therefore if you increase the focal length in order to reduce DoF, but then move back to reframe your image as it was prior to zooming in, you lose the shallow depth of field you wanted by zooming in the first place. |
Below is a table showing distance to subject along the top and lens focal length down the side - computed for an aperture of f/4, and on a camera with an APS-C crop factor (the calculations were actually made for a Canon DSLR with a 1.6× crop factor, although the difference with a 1.5× Nikon, Sony or Pentax will be small. For a camera using the four-thirds system, the depth of field will be greater). It shows three values: the closest point in focus, the furthest point in focus, and the percentage of the depth of field that lies behind the subject - an indication of the near:far ratio where 50% means the same amount must be in front of the focus point as behind it. This will never be less than 50% as there can never be more in focus in front of the focus point than behind it.
We have selected a fairly large aperture as, if you are interesting in the precise amount of DoF available, there is a good chance you are looking to blur a part of the images, and will therefore be using a large aperture.
If you look at the numbers in this table, you will see some patterns emerging.
-
For a given distance between camera and subject, as focal length increases the split between near and far approaches 50% (i.e. distant objects in the scene go out of focus very quickly as you zoom in).
-
As camera-subject distance increases, the area in focus stretches out more quickly behind the subject than in front of it.
-
Wide angle lenses can have a huge depth of field (for example the common 18-55mm kit lens at 18mm can have a focus range of 2m to infinity (even with a pretty large f/4 aperture)
This last point raises an interesting question. An 18mm f/4 lens can focus from 1.99m to infinity as long as you set focus to a distance of four metres. If you focus on infinity, then an object two metres away will be quite blurred (the depth of field would be from 3.93m to infinity).
So to maximise your depth of field, you need to focus at some distance from the camera but well short of infinity and which is dependent on the focal length. We saw from the table that for 18mm lens at f/4, focusing at four metres will keep everything further than this distance in focus. At 30mm f/4, it’s actually somewhere between 10 and 20 metres. At 300mm, the table does not go far enough - in fact you would need to focus at 1093 metres from the camera for the far focus point to be at infinity.
This minimum distance at which you can focus and still retain everything back to infinity in focus is called the hyperfocal distance. The near point of acceptable focus just happens to be half the hyperfocal distance.
Now usually we like to keep maths and equations to the blue 'nerd' boxes, but I’m afraid one has escaped here:
H = (f² / (N.c) ) + f
where H is the hyperfocal distance, f is the focal length, N is the f-number (the aperture) and c is the circle of confusion limit (often taken as 0.03mm).
Therefore for a 50mm lens with an aperture of f/16, the hyperfocal distance is…
H = (50² / (16 * 0.03) ) + 50 = 5258mm
Therefore if we focus the lens to about 5.3 metres, everything from 2.65 metres to infinity will be in acceptable focus.
|
Photographs
This is a site about photography so I'm sure you are expecting to see plenty of pictures.
For now, why not take a peek at the flickr galleries belonging to the two authors of this site.
Colin's Flickr Page
Phil's Flickr Page
"One photo out of focus is a mistake, ten photos out of focus is an experiment, one hundred photos out of focus is a style"
- anonymous
|
