Exploring Photography: Controlling what is in focus using aperture, focal length and subject distance
posted Wednesday, September 19, 2018 at 10:00 AM EST
If you’re new to photography, one of the most challenging aspects of understanding a camera is figuring out how different settings affect the overall look of an image. One of the main reasons why a person buys a dedicated camera system is to get a “professional” look to their shots. In many cases, what this means is getting a soft background with a very sharp subject.
In the video below, photographer and educator Mark Wallace demonstrates the three things that affect how much of a photograph is in focus. These three elements are the aperture value of your lens, the focal length of the lens and the distance away you are from your subject. All else equal, the faster your aperture, the less depth of field you have, which means the more the objects in front of and behind your subject will be blurred. A “faster” aperture means a lower number f/stop. For example, f/1.8 is faster than f/2.8 and f/1.8 is a larger aperture, meaning a lens at f/1.8 allows more light to travel through the lens and reach the image sensor of your camera. The faster your aperture, the less sharp the out-of-focus areas of your image will be.
Further, all else equal, the longer your lens, the shallower your depth of field. Similarly, the closer you are to your subject, the shallower your depth of field and the blurrier your background will be. If you want to get a softer background, one thing you can do beyond using a wider aperture is to get closer to your subject. It is not always the case you want less of your image in focus, sometimes you want more depth of field. For example, when shooting a landscape image, you often want as much of the frame to be sharp as possible. This typically means stopping your lens down, which means going from a fast aperture to a slower one, such as moving from f/5.6 to f/16. Depending on your particular camera and lens combination, lenses sometimes get very soft at small apertures, such as f/22, which is known as diffraction. For more information on each element which affects what is in focus in your photographs, watch Wallace’s video below.