DSLR cameras often have a little depth of field preview button beside the lens mount. This 3-minute video by ZY Productions revisits the basics of a DSLR camera and looks at how this handy button works, as well as some lesser-known things to consider.
First off, pressing the button closes the lens’ aperture to your desired setting. So if you have an f/4 lens but you’ve set the aperture to f/8, pressing the preview button will close the aperture ring down. When you look through the viewfinder, you’ll see the image darken and depth of field increase because you’re looking through the lens with a smaller aperture.
But viewfinders generally aren’t able to show depth of field very well. So, instead, try using the preview button in Live View mode. This gives the best results, showing you exactly what depth of field you’ll achieve in the final image.
Also, the camera will compensate for the decrease in light, brightening the preview image you’re looking at on the LCD.
But if you never use this button for this reason, you can often actually reassign it with a number of other functions. Handy!