DOF is an abbreviation from Depth of Field. It refers to the field or the segment of the photo which is in focus.
DOF is an instrument used to draw the attention of the viewer to the important elements in the frame, which is a tool unique to photography. Other means of artistic expression, such as composition and colour, were borrowed from the fine arts, but DOF is not present in either painting or drawing because it is a function of the lens. It is present in cinema and television, which also use lenses.
What does the DOF depend on and how can we take advantage of it?
It depends on three things – the aperture, the focal depth of the lens and the distances between the lens and the object on the one hand and between the object and the background on the other.
The wider the aperture, the narrower the depth of field. Sometimes, with very wide apertures, the focus area is a plane of one-two centimetres and anything before or beyond that plane is blurred.
Being able to accentuate on different objects in the frame is an advantage of lenses with more light power over their dark kit counterparts. Lenses with high light gathering power allow us to shoot at places which are far from photogenic. When the background is blurred, it no longer distracts or offends the eye of the viewer the way it would, had it been more visible or detailed. In the picture with the monks the cars on the right side of the photograph are hardly noticeable because they are in the defocus field.
The millimetres of the lens also affect defocus. The longer the lens, the more it blurs the foreground and the background. It is worth noting that it is a matter of the actual millimetres of the lens and not of focus length equivalents.
Same sensor, same aperture.
When the lens is long enough, it produces good defocus even with less open aperture settings. The next photo was taken at 300mm and aperture 5.6.
The third factor the depth of field depends on is the distance between the object and the camera and between the object and the background. The closer the object is to the camera and the further away from the background, the more blurred the background will be.
The price we have to pay for a good defocus, however, is that the object becomes ever more overpoweringly present within the frame. If the first picture is a full-length portrait of the young couple and they take up a relatively small proportion of the photo, in the last frame they have been cropped nearly to the waistline and take up a much larger segment of the frame.
The reason it is so difficult to produce portraits with a blurred background using compact cameras and phones is that their lenses have very short actual focal lengths. When you have a small-sized sensor, you need a lens with shorter focus length in order to have the same viewing angle and scale you would have with a camera equipped with a larger sensor. For example, if the actual focal length of the lens in your phone with a 4x6mm sensor is 4mm, that corresponds to approximately 24mm for a camera with a 24x36mm sensor (6 times larger sensor sides, 6 times longer lens to achieve the same viewing angle).
In reality the main factor determining the DOF is the actual clear aperture of the lens. The smaller the aperture, the larger the segment of space which is in focus. It is not a coincidence that short-sighted people squint their eyes in an attempt to let in light through a smaller opening and avoid defocus in the fundus due to the displacement of the focal plane of the eye lens.
Light convergence when you have a small aperture is much greater and therefore a larger segment of the space is in focus.
Here is what actually happens when we shoot the same scene from the same viewing angle using a compact camera and a DSLR one with the same aperture.
As was said before, the aperture (the light power of the lens) is a function of the diameter of the actual diaphragm opening diameter and the focal length. With the light power being equal – aperture 2, the actual opening of the telephone will be 4:2 = 2mm, whereas the one of the larger camera will be 24:2=12mm or 6 times greater. That is why portraits taken with telephones look flatter and do not have a clear distinction between foreground and background..
One should really use the camera as though tomorrow you’d be stricken blind.