Have you ever taken a photo with lots of people, pets, objects, etc. in the frame? Have you ever snapped a photo, then found that the one thing you wanted to be clear turned out to be a big blur? Everything, except the important thing, was in focus?
Your camera's auto focus point selection mode is not helpful. It basically surveys the frame and guesses what you might want in focus. Sometimes it may be right, sometimes it will be very, very wrong. I always manually choose my focal point using AF Point Selection (Manual Selection), but, for illustration purposes, I switched to AF Point Selection (Auto Selection.)
Take a look at the following photos. Take one guess as to which character I wanted in clear focus?
Did you guess Krusty the Clown (in the back row)? Ding, ding, ding! Notice, not once was he in focus. I moved my camera around, aimed it in different directions. Each time, the camera decided that I must not want Krusty in focus.
So, what do we do about this? A simple settings change is all it takes. Grab your trusty camera manual if you need it (especially if you shoot with a Nikon. I speak Canon.) Look at the top of your camera. See the symbol that looks like a square with a dotted cross? Click the corresponding button. It should look like this:
All the focal points are lit up. This tells you that the camera is ready to use any of them as it sees fit.
Hit the 'Set' button. Now, it should look like this:
Notice that the center button is the only button lit up? That means that you should look through your viewfinder, find the person/object that you want in focus, and be sure that red square is right over them (preferably over on eye if it's a person.) Half-press your shutter to focus, then take your picture.
I changed my settings to AF Point Selection (Manual Selection), and here's what happened:
Ta-da! My camera didn't have to guess what I wanted to be in focus, because I made it happen.
Just remember...when you're shooting, and you've decided what you want to focus on, you must half-click the shutter to focus, but don't move the camera once you've done that. Just depress the shutter fully to take the shot. Otherwise, you could be left with a blurry shot. You can choose to use a different, single, focal point if you prefer an off-center composition. When starting out, it's often just easier to choose the middle point.
Why are certainly things in focus and others are not?
Here it gets a bit more complicated, but I'll try.
Take a look at the photos below to understand this diagram:
In the image above, I placed the figures in 3 rows (similar to the diagram.) Harry and Benny are in the first/blue row. I focused on Harry, but, because Benny was next to him he was also in focus. If there were other figures placed in the blue row, they'd be in focus, as well. None of the other figures are in focus, because they are behind the front row.
In the above photo, I focused on Ginny, in the middle/pink row. Notice that neither Harry and Benny in the front, nor Krusty or Dobby in the back, are in focus. Only items on the same focal plane will be sharp.
Last one...I focused on Krusty in the back row. He and Dobby are in focus, while none of the other figures are clear.
So, what does it all mean? Decide what you're trying to get in focus (think Krusty.) Make sure you have your center focal point active (NOT auto-selection!) Make sure that red square is right over that important subject. Half-press to focus, fully press to snap. Understand that only items that are next to your subject may be in focus...
What if you have 3 rows of subjects, and you want them all to be in focus? Stay tuned!