If you’re shooting landscapes, you would typically select a small aperture to increase the depth–of-field of your shot. Instead of focussing on the horizon, you’ve probably heard of the hyperfocal distance and you put the focus closer by to make best use of the front focus and the back focus. You might even have a calculator for this on your smartphone.
All very good, but sometimes you would like to preview the dept-of-field as you’re not interested in blurring out the background completely or getting the largest possible depth-of-field, but want to have a certain, specific to this shot level of detail in the background. Do you use f/5.6 or f/8 and make a gamble as so many details influence the result? A lot of people seem to be unaware that most cameras or lenses incorporate a button or lever to preview the depth-of-field. If you do not know that I’m talking about, check your camera’s manual and see if it supports previewing the depth-of-field. Most people have given it a try and noticed that the viewfinder goes dark and left it at that. The viewfinder goes dark as the only thing this button or lever does is to close the aperture, typically you focus with open aperture to make the viewfinder as bright as possible. But if you pay closer attention, you will also notice that apart from going dark, the background now snaps into more focus. All of a sudden it is not a gamble anymore and you see exactly what the camera will record.