When the lens is focused, it is actually moved back or forth a little such that the lens equation is satisfied, i.e. the object distance g between object and lens and the image distance h between lens and sensor are adjusted. However, the focus distance d which you can read and set on your lens measures the full distance between object and image plane, i.e.
|d||=||g + h||(F1)|
To calculate g and h for a given distance d, we solve equation (F1) for h and substitute it into the lens equation (L5)
|1 / f||=||1 / g + 1 / (d – g)||(F2)|
With some algebra, we end up with a quadratic equation
|g² – g d + f d||=||0||(F3)|
This equation has (at most) two different real solutions, given by
|g||=||d / 2 ± sqrt (d² / 4 – f d)||(F4)|
where sqrt denotes the square root. With equation (F1), we also get
|h||=||d / 2 ± sqrt (d² / 4 – f d)||(F5)|
For simplicity, we define the root term as
|r||=||sqrt (d² / 4 – f d)||(F6)|
If we add r in equation (F4), we must subtract it in equation (F5) and vice versa. In practice, the vast majority of lenses are constructed such that the distance g between object and lens can be (much) larger than the distance h between lens and image sensor. Otherwise, you could not focus at infinity (there are actually a few such lenses as the dedicated macro lens Canon MP-E 65mm). Thus, we set
|g||=||d / 2 + r||(F7)|
|h||=||d / 2 – r||(F8)|
Minimum focus distance
Note that equations (F7) and (F8) can only be solved if the expression under the root in equation (F6) is non-negative, i.e.
|d² / 4 – f d||≥||0||(F9)|
Division by d (positive for all meaningful cases) gives
Inequation (F10) defines a lower bound for the focus distance d, i.e. we cannot get any closer than 4 times the focal length. How close we can actually get also depends on the mechanical construction of the lens. The closest possible distance dmin that still gives a sharp image is called the minimum focus distance (MFD) of the lens. It is closely related to the magnification and a major characteristic of any camera lens.