Beginners are often confused when it comes to Depth of Field (DoF). Since it is one of the most important ceoncepts in photography you should consider for every photograph you take which DoF is best for the subject. Therefore we will have a closer look at the parameters that control the depth of field. And why they sometimes cancel each other.
What is depth of field
In any photograph only subjects at a certain distance from the camera can be in focus. Objects that are nearer to the camera or farther away are more or less out of focus. However, they can nevertheless appear sharp to the eye. The parts of the picture that appear sharp are within the depth of field. All parts that appear blurred to the naked eye are outside the depth of field. The exact physical definition of depth of field is somewhat more complex but is not of importance for the general understanding.
Three parameters govern depth of field
These three parameters are:
o The lens aperture
o The distance from the subject
o The focal length of the lens
1 Lens aperture
The lens aperture is of course the most important factor for the depth of field. You probably know thsi already, but let’s say it again: The depth of field is small at large lens apertures (small f-stops, e.g. f2.8) and increases when the aperture gets smaller (large f-stops, e.g. f16).
2 The distance from the subject
Here the rule is quite simple: The closer you are to the subject the smaller the DoF or the farther away you are the larger the DoF. Provided, of course, that the f-stop is the same!
3 The focal length of the lens
Again the rule is simple: The longer the focal length of the lens the shallower is the depth of field, or vice versa, the shorter the focal lenght the larger the DoF. Of course, this is again only true if the f-stop is the same.
Note, that the distance from the subject and the focal length of the lens determine the magnification, that is how small or large the subject is displayed on the sensor. In other words, the magnification also determines the depth of field. And once more there is a simple rule: The larger the magnification, the smaller is the DoF and vice versa (again this is only true as long as the f-stop is the same). However, there is a little problem here.
Both factors, distance from the subject and focal length of the lens tend to cancel each other out. When using a telephoto lens, i.e. a long focal lenght, the distance from the subject must be large if you want to show it completely like the flower pot in the featured image above. However, when using a wide angle lens, i.e. a short focal length, the distance from the subject must be small if you want to show the subject at the same size.
Therefore it is not quite correct that wide angle lenses allways have a large depth of field and telephoto lenses allways have a shallow depth of field. It very much depends on the magnification!
When the subject matter in a photograph are all far away you will have complete depth of field at any aperture, as in the picture below:
Therefore you should simpy choose a medium aperture like f5.6 or f8 because the overall image quality will be best at these settings.
On the other hand, if the subject is very close to the background it is simply impossible to blur the background sufficiently even at the largest aperture of the lens:
Even at f1.4 the wall behind the bike will allways be sharp.
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