Ironically the distance at which the eyeball is most effective coincides with the distance that a rifle can be fired point blank at most targets, given the proper zero. If you have a target at a distance inside 200-300 yards, the eye may be used to estimate range and make corrections accordingly to good effect. The rifle’s trajectory at these distances undergoes radically less change inside these distances than it does farther out, where your eye won’t work quite as accurately.
Things that affect the apparent range to target are terrain and environmental factors. According to the reference materials I have on hand, targets uphill will tend to appear farther away than they actually are, and targets downhill will tend to appear closer. A valley between you and your target will make it appear closer. Looking around at stuff would suggest that a target in a narrow clearing with obstructions to either side of your field of view would create a tunnel effect that makes the target appear to be farther. A straight visual line, such as a path or road, leading to the target is supposed to make it look closer. I would have checked all these out as part of my research for this article, but I don’t have a rangefinder to verify my estimations.