Start from the point which is in exact focus, at a distance "F" from the camera. And say that a certain distance toward the camera from that point is the largest distance change that will be tolerably in focus. That distance is a certain percentage of "F".
That same percentage, but moving away from the camera, will allow a greater distance change.
The same thing will happen when you increase the level of lack of focus. To be out of focus at the same level whether moving toward or away from the camera, the distance moved away from the camera will be larger than the distance moved toward the camera. The percentage of distance moved, with respect to the distance "F", will be the same in both cases.
In other words, there will be a larger distance away from the camera than toward it that will be tolerably in focus. The 1/3, 2/3 rule and the logrithmic character of the rule are the result of the geometric relationships regarding optical focus (and a bit of differential calculus). But the essential principle is the description above.
-yoram
-------------- Original message from Jorj Takacs <jorj@xxxxxxxxxxxxxx>: --------------
> A simple question yet I cannot seem to find a simple answer.
>
> Why is there more DOF behind the point of focus than in front?
>
> Thanks in advance.
>
> Jorj
>
