When you’re introduced to photography, one of the techniques everyone seems to want to learn is how to create images with a blurry background. The technical term for this is “shallow depth of field”. Normally, this technique is used to create an image where the subject is sharp, distinct, and highlighted by the blurring out of the background. While this is easily achievable when using DSLRs or even prosumer cameras, those who would like to stick with mobile devices and smart phones like iPhones may have a lot of difficulty creating such image. The challenge lies mainly on the technical specifications of all camera phones. But when I say it’s difficult, I don't mean it's impossible.
There are four ways in which you create a shallow depth of field. These are 1. Using a wide aperture, 2. Shooting at a longer focal length, 3. Minimizing camera-subject distance and 4. Maximizing subject-background distance. Let us look at these four factors in relation to iPhones and similar camera phone devices.
Using a Wide Aperture
Aperture size is simply the size of the lens opening. Lenses with larger openings are more effective in creating lens blur – the larger the aperture, the better. Camera phones and mobile devices will normally have an adequate aperture size that will create enough blur in the background but there's more to it. iPhones have an aperture size of f/2.2 and this lens opening is enough to create background blur but for its small sensor size. It is well known that cameras with big sensors are best at creating soft backgrounds. For this reason, your camera phone will usually keep everything in focus (or technically, it has a huge depth of field) unless you change the subject-to-camera and/or the subject-to-background distances.
Your camera phone will usually keep everything in focus. Image by Digo Souza
Shooting at a Longer Focal Length
Focal length is another consideration. The ability of the lens to magnify an image helps in creating a shallow depth of field. When the lens zooms-in closer to the subject, this creates more blur on the background. Therefore, shooting at longer focal length is better than photographing at a wide angle. Unfortunately, mobile devices and camera phones have wide angle lenses which cannot be changed. This is a major limitation in creating the effect we want to achieve. Usually, even with large apertures, devices have difficulty creating the blurry background effect because of a shorter focal length which is usually always the case.
A camera phone is great for wide-angle shots but lacks the telephoto reach. Image by Les Haines
This is why we need to compensate these limitations on a mobile phone by using distances to our advantage.
Minimizing the Camera to Subject Distance
The first step in creating a blurry background with mobile devices is to keep your camera as close as possible to your subject. The shorter the distance between the camera and subject, the blurrier background you get. Most cameras won’t allow you to go too close as it will just blur your subject. Go as close as you can while keeping your subject as sharp as possible. By moving closer to the subject, the blurriness of the background increases.
Minimizing camera to subject distance. Shot with a camera phone. Image by Andy B
Maximizing Subject-Background Distance
The second step in creating background blur is to extend the distance of the subject from the background. The farther away the subject is from the background, the more blur you get. Therefore, if you need to re-position your subject or move around so that you pick a background that is relatively far from the subject, do that.
Maximizing subject-background distance. Captured with a camera phone. Image by Maxime Raphael
A good rule to follow is to keep the camera close to your subject and the background as far as possible.