The most likely cause is shooting at f/1.8. Wide apertures give very shallow depth of field and many lenses are also a bit softer wide open. Try the same scenes at f/5.6 or f/8 and compare.

A few points from the replies:

• The tree image may actually be focused, but low contrast/haze can make it look dull.
• At the distance of the person shot, the whole subject should usually be in focus if focus is correct, even at f/1.8.
• Lenses often have a “sweet spot” a few stops down from maximum aperture.

To troubleshoot:

1. Test manual focus vs autofocus on a still subject.
2. Repeat shots at f/1.8, f/2.8, f/5.6, and f/8.
3. Use a tripod or otherwise eliminate camera movement.
4. Check whether the lens is dirty or has been knocked/dropped.

If stopping down and careful technique don’t improve things, the lens could be damaged or misaligned and may need service. But first, rule out normal wide-open softness and shallow depth of field.