Range finder

Sure, the electronic range finders are flawless, but if you want to try to locate a bird that went down somewhere yonder try this:

The human eye is good at finding the middle.  If you are tryingto estimate to something along ways off, pick something in the middle and estimate to that.  You'll find that you are more accurate estimating smaller disatances You might even cut that distance in half again and estimate on that quarter and then multiply out.