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Summary of how to control dof (depth of field).
What is dof ?
What factors affect dof?
Using dof creatively
Dof requirements vs equipment choice
Depth of field is possibly one of the most important, yet complicated aspects of digital photography.
In practical terms it isn't strictly necessary to be fully understood in order to control its creative effect over your photos.
So I begin this article with a summary of the variables that you can adjust to control dof – more detailed explanations of the “why and how” follow later – only to be read if you are interested in gaining a greater depth of understanding.
Image sensor size. Smaller sensors as found in compact cameras and camera phones – give deep depths of field – if you aspire to photos with a nicely blurred unfocussed background as often seen in professional portrait images – this simply may not be possible with your compact camera or camera phone.
Aperture. Higher f number (smaller aperture) gives deeper dof – until the diffraction limit is reached, after which further reduction in aperture size decreases image sharpness. If you desire shallow dof, using a lower f number and ensuring the background is distant from your subject will give the greatest sharpness versus blur effect between subject and background.
Moving closer to focus on your subject will give a shallower dof.
Using a wide angle lens compared to a telephoto lens will normally give a deeper dof – particularly when focussing on relatively distant subjects.
Using a telephoto lens and moving further away from a subject will normally give a shallower dof. But even in the cases where technically it doesn’t (this is explained later in this article) the greater magnification of the distant background objects (which will be blurred) will give the visual effect of a shallower dof.
Let us consider this question in three stages,
standard universal definition of dof
Think of light coming from millions of tiny points from over the entire surface of subjects within the field of view in front of a lens. As the light from each point travels towards the camera, it diverges in ever widening circles until it enters the lens.
The lens then reverses the process and the light converges back to a precise point (and then begins to diverge once again)
The exact point where these light rays converge depends upon two things
the distance of the subject from the lens
the focal length of the lens
If the image sensor is positioned exactly at the point of convergence (the focal point) then all the precise points of light effectively recreate the image of the subject – and the image will appear sharp.
As the sensor can only exist in one particular position at the moment the photo is taken, it follows that only subjects placed precisely on the focal plane will be totally sharp.
When a subject is not on the focal plane, as its light rays reach the image sensor, they're either still converging (if the subject is in front of the focal plane) or diverging once again having already passed through the focal point (if the subject is behind the focal plane).
In both these cases, instead of precise points of light, the light rays will be defocussed circles of light. (imagine slicing the top off a cone to be left with a circle rather than a point).
These fuzzy circles of unfocussed light are known as circles of confusion.
Their size can be mathematically predicted and it is this that forms the basis of depth of field calculations.
If circles of confusion are still leaving YOU confused! Try watching this neat little clip from the Kinetic Image.
Whether the circles of confusion are big enough to cause part of the image to appear unsharp depends on numerous factors
how good the eyesight of the observer is
the distance from which the observer views the image
the resolution of the image
the size of the image (ie the more magnified an image becomes the more obvious out of focus areas will be.)
So, images consist of the following fives zones
Parts of the image where the subjects fall exactly on the focal plane.
Parts of the image where the subject lies in front of the focal plane but is still focussed well enough to be considered sharp by the observer.
Parts of the image where subjects are behind the focal plane but still focussed well enough to be considered sharp by the observer.
Parts of the image where the subjects are in front of the focal plane and appear unsharp to the observer.
Parts of the image where subjects are behind the focal plane an can be seen to be unsharp by the observer.
The depth of field therefore, is the term used to describe all parts of the image where the subjects are in zones 1,2 and 3 – that is to say the region where subjects are focussed to a degree of sharpness that is acceptable to the observer.
When the zone of acceptable sharpness is large, the dof is described as deep.
When the sharp zone is small, the dof is described as shallow.
For any given aperture, focal length and sensor size combination, it is possible to calculate the focus distance that yields the maximum possible dof. This is called the hyperfocal distance. All subjects from half this distance to infinity will appear acceptably sharp in the final image.
By computing the expected diameter of the circles of confusion and then taking into account the degree by which these are magnified when viewing the image at a particular size and distance, it is possible to produce comparative dof data for different lenses, apertures and sensor sizes.
Even tho' the actual regions that can be perceived as acceptably sharp may vary from person to person – being able to compare data indicating the expected affect upon depth of field for different variables is extremely useful when planning a shot and choosing appropriate equipment type and settings.
The primary factors that will affect the size of the circles of confusion falling upon the sensor are
image magnification – related to focal length /angle of view/ sensor size
focus / subject distance
Thereafter, the relevance of the diameter of the circles of confusion simply relates to how much the image (and therefore the circles of confusion) will be magnified in order to attain the standard image size and viewing distance.
Historically on an 8” x 10” print viewed from around 13 inches with circles of confusion around 0.001” (0.03mm) were considered to be the acceptable limit.
The mathematical equations for calculating dof are relatively complex, so rather than go into them here, I shall instead summarize the effect upon dof based upon some typical scenarios.
At the end of the day, all the maths does is compute the effect upon the the size of the circles of confusion due to the change in angle at which the light rays approach the image sensor
If the angle is large – dof will be shallower
If the angle is small – dof will be deeper.
So in summary depth of field is affected in practical terms as follows
Diagram below shows how at constant subject magnification the depth of field for a longer focal length telephoto lens becomes progressively shallower compared to a wide angle lens as subject distance increases.
Note. When subject distance is close, any distant background objects are subject to greater magnification by the telephoto lens. So although dof may be the same for both wide angle and telephoto lens, the magnified blur of background objects may give the impression of a more shallow dof for the telephoto lens.
Taking control over depth of field is one of the most creative aspects of photography, with both deep and shallow dof being used for effect, depending on the intent of the photographer.
Deep depths of field lend themselves to landscape photography where often the photographer wishes all objects within the scene – from very close to the far far distant to be sharp. Therefore wide angle lens (low magnification) and small apertures (high f numbers) tend to be used. Also the inherently deep dof from smaller sensors facilitates this style of photograph from cameras using smaller image sensors.
Shallow depths of field are frequently used to defocus objects that would otherwise be a distraction from the primary subject in a scene - frequently desirable in portrait photography.
To obtain these shallow dof effects typically requires cameras with larger image sensors and the associated larger aperture diameters / longer focal lengths that these image sensors require.
Once you've mastered depth of field, you can take some great photos that you will be truly proud of. When you have a collection of your favorites, create some lasting memories over at picture.com!
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