Aperture and depth of field were the first things I learned when I took my camera off automatic. In a nutshell, aperture is the opening in the lens which allows light into the camera. It’s analogous to your eye; your pupil expands and contracts to let certain amounts of light in for your vision. Aperture is expressed by f-stops which are those numbers you see on your camera, your lens, or when reading the settings of how a photographer captured an image.
The numbers can range from around F 1.4 to F 22 (more or less) depending on the lens and the manufacturer. This is where I had my first difficulties with f- stops, those numbers. Numbers like 1.4 or 2.8 are considered large f-stops meaning the lens is open wide while numbers like 8 and 16 are considered small f-stops meaning the opening is smaller. My brain was bent on thinking the exact opposite way. How could the number 16 be smaller than the number 4 I thought. In photography it does and after practicing thinking photographically and not mathematically I got the hang of it.
Aperture also directly impacts your depth of field. Depth of Field (also known as DOF) is the front-to-back zone of a photograph in which the image is sharp. With a large f-stop – for example F 1.8 – you will have a shallow depth of field which means only a small amount of your image will be well in focus. Depending on what you want your image to look like this can result in that wonderful background blur known also as bokeh or it can result in having only the tip of the nose of your portrait subject in focus. There is wiggle room in this because how close or far away you are from your subject also effects your depth of field. You can download DOF cheat sheets for your computer or your phone to assist you in calculating what your DOF will be based upon the model of your camera, how far away you are, length of lens, and f-stop here.
So what does this look like in action? Here are three shots that I took changing only the f-stop on my camera. Using my Canon 5D Mark III and Sigma 50mm art lens, my focal point was the knob of the pipe and other than a bit of wiggling from shooting hand held, I tried to keep the composition of the shots the same. Looking at each image, as I stopped down my f-stop (as my aperture changed) my depth of field changed; more of the knob, pipe, and background came into focus. I only used three stops here but I think you can see the concept. Depending on what type of lens you have you may not be able to have as large an aperture as I used here or you might have a lens with a fixed f-stop range such as a f/3.5 – 5.6 but whatever you have you can get out there and experiment and create wonderful images.
Here are a few links about aperture, DOF, and f-stops: