There are many variables involved when computing depth of field (DoF). By specifying focal length and sensor size you have reduced that number by two variables. But there are still several other variables to take into account which prevent a simple "rule of thumb" for computing DoF:
- Shooting distance - The DoF changes with the shooting distance, both in terms of how much of the DoF is in front of and how much is behind the point of focus and in terms of the total DoF. At near focusing distances the DoF approaches 50% in front of the point of focus and 50% behind the point of focus for a 1:1 ratio. At medium distances the DoF is roughly 33% in front/67% behind the point of focus for a 1:2 ratio. At the hyperfocal distance and beyond the DoF reaches from the point of focus to infinity and the ratio becomes 1:∞. Note that the hyperfocal distance will also change with display size and viewing distance.
- Display size/Viewing distance - The same exact image will have a different DoF when displayed as a small image than when displayed as a large image viewed from the same distance. Conversely, the same exact print will demonstrate a different depth of field when viewed at varying distances.
Understanding what DoF is and what it is not is important here.
In a way, depth-of-field is an illusion. What we call DoF is the area where things appear, to our eyes, to be in focus. There is only one plane of focus. Everything in front of or behind the point of focus is out of focus to one degree or another. This is based on the ability of the human eye to resolve certain minute differences at a particular distance. If the slightly out-of-focus blur is smaller than our eye's capability to resolve the detail then it appears to be in focus.
There are many things we can do to an image after taking the picture that will affect the Depth of Field (DoF). Any time you crop an image and display the crop at the same size as the original you are altering the Depth of Field because you increased the magnification. Any time you increase or decrease the display size at the same viewing distance you alter the DoF. Anytime you change the viewing distance of the same photo you alter the DoF! Most DoF calculators that don't include inputs for display size and viewing distance are based on the assumption of an 8X10 inch print viewed from a distance of 12 inches by a person with 20/20 vision.
When you magnify a portion of an image by making it larger or moving closer to it you allow your eye to see details that before were too close together to be seen by your eyes as separate pieces of the image. There is no magic barrier beyond which everything is equally blurry and inside of which everything is equally in focus!
Since things are gradually blurrier the further they are from the point of focus, as you gradually magnify the image the perceived depth of field gets narrower as the near and far points where your eyes can resolve fine details moves closer to the focal plane.