There is only one distance that is in the sharpest focus. Everything in front of or behind that distance is blurred. The farther we move away from the focus distance, the more blurred things get. The questions become: "How blurry is it? Is it within our acceptable limit? How far away from the focus distance do things become blurry in an unacceptable way?"
What we call Depth of field (DoF) is the range of distances in front and behind the focus point that are found acceptably blurred so that things still look as if they were focused.
The amount of depth of field depends on two things: total increase and opening. The total magnification includes the following factors: focal length, distance of the subject / focus, magnification ratio (which is determined by the size of the sensor and the size of the screen) and the viewing distance. The visual acuity of the viewer also contributes to what is. acceptably strong Enough to appear in focus instead of blurry.
The distribution of the depth of field in front of and behind the focusing distance depends on several factors, mainly the focal length and the focusing distance.
The proportion of any given lens changes as the focus distance changes. Most lenses approach 1: 1 at the minimum focusing distance. As the focus distance increases, the rear depth of field increases faster than the front depth of field. There is a focus distance at which the ratio will be 1: 2, or one third in front and two thirds behind the focus point.
At short focusing distances, the ratio approaches 1: 1. A true macro lens that can project a virtual image onto the sensor or film that is the same size as the object for which it is projecting the image reaches a ratio of 1 : 1. Even lenses that can not achieve macro focus will show a very close ratio of 1: 1 to their minimum focus distance.
At longer focusing distances, the rear of the depth of field reaches infinity and, therefore, the ratio between front and rear DoF approaches 1: ∞. The shortest focusing distance at which the rear DoF reaches infinity is called hyperfocal distance. The near depth of field will approach half the focal length. That is, the nearest edge of the DoF will be halfway between the camera and the focusing distance.
We must also remember that hyperfocal distance, like the concept of depth of field on which it is based, is actually an illusion, although it is rather persistent. Only a single distance will be in the sharpest focus. What we call depth of field are the areas on both sides of the sharpest focus that are blurred so insignificantly that we still see them clearly. Note that the hyperfocal distance will vary depending on a change in any of the factors affecting DoF: focal length, aperture, enlargement / size of the screen, viewing distance, etc. To know the reason, see:
Why did manufacturers stop including DOF scales on lenses?
Is there a "golden rule" I can use to estimate the depth of field while shooting?
How do you determine the acceptable Circle of Confusion for a particular photo?
Do you find the hyperfocal distance for HD resolution (1920×1080)?
Why do I get different values for the depth of field of the calculators compared to the DoF preview in the camera?
As well as this response to the simple and rapid method of estimating DoF for main lenses