What does this little button on the side of your DSLR actually do?
This is usually found to the left or the right of the lens on the cameras body. When pressed it will physically close down the aperture to whatever you have it set at. The reason for this is to give you an idea of the depth of field that you will likely get with this setting…much like my experiment for short sighted people earlier using the hole in your hand!
Switch your aperture to F8 or smaller (It won’t work with a wide open aperture because there is nothing to close), turn the camera around and look in the lens whilst pressing and releasing the depth of field preview button…see it opening and closing?
Now when you look through the camera and do the same, you should see the image going from blurred to sharp, especially if you focus on something in the foreground. A handy tool sometimes…but rarely will you use it once you understand depth of field.