Depth of Field
The range of distances that appears in acceptable focus at the same time through a scope, a band that narrows sharply as magnification climbs.
Depth of field is the span of distances, from nearer to farther than your focus point, that all look acceptably sharp at once. At low power a scope holds a generous band in focus, so a target and the terrain around it can both look crisp without any adjustment. As you turn up the magnification, that band shrinks quickly and only a narrow slice of distance stays sharp.
This is why high power makes precise focusing more demanding and why a side focus or adjustable objective becomes useful at long range. Setting focus carefully also helps remove parallax, since the same control that sharpens the image at a given distance is the one that aligns the reticle and target planes in a scope.