Depth of field in an image is how much of an image is in focus relative to the foreground and background. For most lenses, depth of field operates on the same plane as the sensor in the camera; the only exception to this is a tilt shift lens (TS). Because the plane runs parallel to the sensor, we can manipulate how deep the focus is in an image.

We have three opportunities control depth of field (DOF) for the intended result. These are: Aperture, Distance and Focal Length. These all work simultaneously to create depth in an image, however each can operate independently.

Aperture

For the purposes of this article, we’ll disregard how aperture controls exposure to focus of DOF manipulation. Let’s consider a prime lens; it may operate at an aperture of f1.8-f22. At f1.8, the aperture is at its widest; this creates a shallow depth of field. At f22, the aperture is at its narrowest, creating a deep depth of field. Manipulating this will allow you to directly control the depth of field optically, however it’s impossible to forgo depth in an image, so even at f22 there will be an element of blur (assuming an image with foreground and background data pixels), shooting at f22 vs f 1.8 with simply control the degree to which the out of focus areas are out of focus.

Distance

The next way to manipulate DOF is the distance of your subject (hypothetically a person) from your camera’s sensor. It’s important to know it’s from the sensor, not the front of the lens; at extremely short DOF distances measuring from the wrong spot will be the difference between an in and an out of focus image. All cameras have a symbol on top of the camera (a circle with a line strike-through) to denote the line of the sensor within the camera; this is your starting position.

From that point, if your were to position your subject close to your take a picture then move them further away for a second image, you’ll notice (other than a different framing to your subject), the background will have less blur, and your subject may even be sharper overall. When shooting a subject that is operable be sure to place it at a distance that allows for it to be in focus, but not so far away that it blends into the background (though adjusting your aperture may assist that). If your subject is inoperable (a flower, a building, etc.) then you will need to physically move your camera to the optimal distance from your subject.

However because we need creative control over foreground and background blur, it’s important to note these are both visible because of distance. A rule of thumb I’ve used in practice is: foreground blur is half the distance to your subject and back, background blur double the distance to your subject and beyond.

A way to test this is to place your hand in front of your eyes and focus on the grooves in your palm to then notice how your periphery blurs, then focusing your eyes on something beyond your hand to notice your hand now out of focus. Your camera operates identically to this.

Focal Length

Focal length (denoted on the lens) will help dictate background blur. Similarly to distance, the focal length will be a representation of the visible scene width (wide lenses to telephoto lenses). As it’s measured as millimeter’s, the same concept as DOF distance (above) applies in reverse. Having a wide lens, 10-30 mm range, will create less background/ foreground blur than a mid range (50-100 mm) or telephoto lens (200 mm+).

In this scenario the greater the distance in focal length millimeters the more blur would occur in an image assuming a consistent distance and aperture between focal lengths.

Next time your in the field, and you have a subject you can try these tricks out on I’d recommend trying to manipulate your depth of field using this three strategies to see how your camera reacts to build your practical understanding of how your camera operates.

Once you have an understanding of each method, I’d recommend combining strategies to see how you can compound these effects if you are looking to create more or less blur in your images. Sometimes you’ll seek a shallow depth at your daughter’s third birthday party to isolate her blowing out the candles… Sometimes you’ll need to get everything in focus because you’re on holiday and the sunset is just right.

Which method have you mastered?