The important thing is, just how much out of focus! We know objects closer or further than the point of focus don't just go totally out of focus all of a sudden; they become more blurry gradually as the distance from the point of focus increases.
The distance behind, and in front of, the point of focus that objects maintain ACCEPTABLE sharpness is called Depth Of Field (DOF). There's that word, "acceptable" again.
Depth Of Field INCREASES with smaller lens apertures (e.g. f11, f16) and DECREASES with larger lens apertures (e.g. f2.8, f4). The photographs on the next pages illustrate how aperture and DOF affect sharpness.
If you have absorbed all the above you will by now have let out a loud groan upon the sad realisation that the laws of optics, light and exposure are all working well and truly against you.
The Sad Facts (in no particular order):
Slow shutter speeds allow smaller apertures; smaller apertures = greater depth of field = acceptable sharpness over greater range, BUT slow shutter speeds = greater chance of camera shake (or subject shake if subject is live).
Fast shutter speeds minimise chance of camera or subject shake, BUT fast shutter speeds = bigger apertures = less depth of field = more chance of unshapness, especially close-up.
Yeah, life's tough, but all is not lost. If you have sufficient light, you can use both a short (fast) shutter speed AND a small aperture, giving the best of both worlds - minimum camera/subject shake, maximum depth of field.
Not fed up yet? Great; please go to Page 5.