There are two problems associated with camera viewfinders.
You can improve your composition if you're aware of these problems.
Viewfinders are not accurate.
What you see through the viewfinder is not what you get in the photograph.
You see less of a scene in the viewfinder—and more of the scene when you look at the photograph.
You may have looked at one of your photographs, and disliked something distracting near one of the photograph's edges.
Perhaps there was an elbow sticking into the frame.
You may have wondered, "Why didn't I see that?"
The distracting element, such as a stray elbow, didn't appear in the viewfinder.
You have to do an experiment to compare your viewfinder view with what actually appears in your photographs.
Photograph something with well-define edges, such as a painting.
Place the edges of the painting exactly on the edges of your viewfinder.
Then, look at the photograph of the painting, and note where the edges of the painting are no longer on the edge of the frame.
This problem involves an important photographic tool called depth-of-field.
If you're a beginner, you may want to return here when you've learned more about depth-of-field.
When you look through the viewfinder, what's in the background will probably be out-of-focus.
That's because the lens opening is wide open, say f/4.
At f/4, there's lots of light entering the camera.
You can see the scene well in the viewfinder.
But, there's very little depth-of-field.
The background is fuzzy.
We don't pay attention to fuzzy backgrounds.
So, if the background is ugly, you may not notice.
Yet, when you press the shutter release, what was fuzzy, in the viewfinder, may become sharper, in the photograph.
When you look at the photograph, you may wonder, "Why didn't I see that telephone pole sticking out of her head?"
The depth-of-field changed when you tripped the shutter release.
If it's a sunny day, your camera may have selected f/22 as the best lens opening.
When you press the shutter release, the lens opening went from f/4, background fuzzy, to f/22, sharper background
Use depth-of-field preview.
Press this button, if your camera has this feature.
You'll see the actual depth-of-field that will be used in your photograph.
The image in your viewfinder may be dark.
Give your pupil time to adjust.
If you're outside, block the light from the sky that is striking your eye with your hand.