Depth of field
Depth of sharply depicted space (depth of field) is the distance between the closest and farthest object in the scene, which are perceived in the photo as sharp. Everything that lies closer or further than the depth of field zone is more or less blurred.
It should be understood that the concept of IPIG is quite arbitrary. There is no real and unambiguous depth of field in nature, since the lens at a time can be focused only on one specific distance, and not on some abstract range of distances. It is more appropriate to speak more about a focusing plane of infinitely small thickness, closer and further which the image begins to blur.
So where does the depth of field come from? In theory, any point on which the focus of the lens is pointed should be projected onto the surface of the film or sensor precisely as an unconditionally sharp point. A point lying out of focus will be displayed as a so-called. scattering spots (circle of confusion). The farther the point is from the plane of absolute focus, the larger the diameter of the circle will be. Those. the size of the scattering spots gradually increases with distance from the focusing plane. In practice, the scattering circles, the size of which is smaller than the resolution of the lens-matrix system, look indistinguishable from points, which allows us to talk about the depth of field as a kind of conditionally homogeneous range that includes not only the plane of absolute focus, but also adjacent to her objects.
Thus, the depth of field depends entirely on the maximum size of the scattering circle, at which it is still perceived by us as a sharp point. Perception is very subjective – for the diameter of the permissible scattering circle, you can take 1/1500 frame diagonals according to the established tradition, or (which is more reasonable) start from the pixel size of the matrix, and even better rely on your own eyes: if the object in the photo seems sharp to you personally, then you have the right to consider him trapped in the IPIG. Fine arts (including art photography) is generally an area where your subjective opinion has an immeasurably greater weight compared to any mathematical calculations.
What factors affect the depth of field? There are not so many of them:
Aperture: the smaller the relative aperture of the lens, i.e., the larger the aperture value, the greater the depth of field, since when the lens is apertured, the diameter of the scattering circles decreases. With a full aperture, the depth of field is minimal. It is changing the aperture value that is usually the most convenient way to control the depth of a sharply depicted space, because does not affect the composition and perspective of the picture.
Shallow depth of field
Shallow depth of field. Aperture f / 4.
Large depth of field
Here the depth of field is much larger. Aperture f / 22.
Lens focal length: the larger the focal length, the smaller the DOF, and vice versa – the smaller the focal length, the larger the DOF. The reason is obvious – telephoto lenses enlarge the image, including scattering spots.
Focusing distance: as the focusing distance increases, the depth of field increases proportionally, and when approaching the object, the depth of field will narrow. The ratio between the focusing distance and the depth of field, all other things being equal, always remains unchanged.
Resolution: the more pixels the sensor of a digital camera contains (or rather, the smaller the size of an individual pixel), or the smaller the silver halide crystals in the film, the better the small circles of blur are distinguishable. Decrease the resolution and the depth of field will increase.
It may seem that the depth of field also depends on the frame size, but this impression is erroneous. In fact, in the pictures taken with full-frame cameras, the depth of field is less than on cameras with a crop factor (with the same aperture and equal equivalent focal lengths of the lenses), and compact cameras, on the contrary, depict sharp almost the entire scene. However, this is not strange. The fact is that in order to obtain the same image angle, cameras with a smaller frame size (large crop factor) must be equipped with a lens with a proportionally smaller focal length, which inevitably leads to an increase in depth of field. On the contrary, longer lenses are placed on large cameras, and the larger the true focal length of the lens, the smaller (ceteris paribus) the depth of field.
Artistic Use of Depth of Field
We think about the depth of field during shooting in two cases: when we want to separate an object from the background using a small depth of field, or, on the contrary, we need a large depth of field to make the whole scene sharp.
Shallow depth of field
If you need a minimum depth of field, open the aperture wider, i.e. to its minimum value (f / 1.4-5.6, depending on the design of the lens). If possible, use the maximum focal length available to you.