A spotlight is placed at $ 2f $ distance from a $ + f $ focal length lens. When the point of light moves along the main axis of the lens, moving away from it, the distance from the image to the lens varies from:
Answer: $2f$ to $f$
I can't see it. If the object is at $ 2f $ (at the anti-main point) and I move it away from the lens, the image will decrease and get closer to the other focus, but it never goes beyond that focus regardless of how far I move the object away from the lens?
In the thin lens approximation, the image distance $d_i$, the object distance $d_o$ and the focal length $f$ are related by the following equation:
$$\frac{1}{d_i}+\frac{1}{d_o}=\frac{1}{f}.$$
This implies, in particular, that $\frac{1}{d_i}<\frac{1}{f}$, or $d_i>f$, if $d_o>f$. So, answering the question, the image will get closer to the other focus (i.e., the focus at the other side of the lens), but it never goes beyond that focus regardless of how far the object is moved away from the lens.
Source: http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lenseq.html