A reticle in the first focal plane (in front of the inverting system) changes in the same way as the actual image. The reticle and the image combine to form a single unit, the size of both increasing or decreasing in the same way. As a result, the subtension remains constant at all magnifications, making it easy to estimate the range.
![](https://i.ytimg.com/vi/lNKaaD_1pCQ/maxresdefault.jpg)