How many times have you seen something you liked, pointed your camera at it, pressed the shutter, and moved on? It's easy to do and I've done it--still do many times. The problem with that approach, like most activities, is that the brain needs to be engaged, not just the emotions...photography is thinking. So, what are you focused on, that is, thinking, when you're taking that next shot?
How about if "focus" is exactly what you're thinking about? What if sharpness and image crispness is of primary concern--what should you be thinking about to accomplish that? Well, here's what contributes to a sharp image... 1) a decent lens, 2) a still camera, 3) an aperature that includes sufficient depth of field to include what you want sharp, 4) a still subject, 5) low ISO (noise). Easy huh? Guess I need to think about this for a minute...
1) got a decent lens, check. 2a) have a tripod, check. 2b) don't have a tripod...better use a shutter speed at least as fast as 1/focal length. (200mm shot, use 1/200 sec or faster). 3) Let see...fast shutter, therefore wide open aperture, right? No. Two things working against you. One, you may need more depth of field, and two, most lenses are at their sharpest at f8 (lots of tech reports you can find to explain this). Ok, I'll choose f8 or maybe f5.6... check. 4) The exposure TRIO defines proper exposure--that includes the f-stop, shutter speed, and ISO. So I guess my ISO "choice" rounds out the specs. Of course, higher ISO means more noise, which negatively influences your perception of sharpness. That's ok...got the good camera and I have good noise-reduction capabilities in my post production software...check.
Hey, that's too much to think about... my camera has auto focus and that good enough to get great, sharp images, right? Depends. Depends on whether your subject is moving and whether you have your camera set to focus once when you press the shutter half way, or focus continuously until you press the shutter all the way down to capture the image. Hmmm...more thinking.
So, look at this image of the bird. Taken with a decent camera (actually a great camera--Nikon), and taken during the day (great news for shutter speed, aperture, ISO). But the bird is moving which means that the camera is moving too. And, the bird is not moving parallel to the camera (keeping focus distance constant), therefore the camera will need to maintain CONTINUOUS focus until the image is captured, or you'll have to be pretty quick on the shutter to get the focus and complete the image capture.
Is that too much to think about? Not really. Not any more than driving a car and attending to all the detail of the vehicle operation and the surrounds... What's common is that the more you PRACTICE THINKING the faster you're able to do it, and in some cases the more automatic that thinking becomes.