There’s a name for the area of a picture that’s in acceptable focus. It’s called depth of field (often abbreviated as DOF) and it’s one of the central considerations in the taking of a picture.
Depth of field is controlled by the aperture (also known as the ƒ-stop) of the lens. A wider aperture—larger opening—is denoted by a smaller number, such as ƒ/2 or ƒ/2.8. Lenses with large maximum apertures are called fast lenses because they allow in a larger amount of light and permit the use of faster shutter speeds. These wide apertures also produce shallower depth of field.
Smaller apertures, denoted by larger numbers, such as ƒ/16 and ƒ/22, allow in less light and require the use of slower shutter speeds or higher ISO settings to compensate and deliver correct exposures. Small apertures also produce deeper depth of field.
In practice, there are three choices when it comes to depth of field: Do I want a lot, do I want a little, or does it even matter?
You may want a lot of depth of field—a greater area of sharpness—if you’re photographing a landscape or travel photograph in which you want the overall scene to appear sharply in focus. This is partly why landscape photographers use tripods—so they can combine a small aperture and low ISO (for minimal noise and maximum image quality), which requires a slow shutter speed.
If you’re photographing a portrait or otherwise trying to put the center of attention on a single object while minimizing the impact of other scene elements such as the background, you’ll likely want less depth of field. Therefore, you’ll choose your lens’s maximum aperture or something close to it. This is why portrait lenses are often very fast, with maximum apertures of ƒ/1.4 in some cases—because minimizing depth of field puts the focus literally and figuratively on the subject and nothing else. In the examples shown here, you can see how the background becomes more prominent, and often distracting, in the images shot with smaller apertures when compared to the same scenes photographed with wider apertures that produce shallower depth of field.
What about that third option—the one where you don’t really care whether it’s a lot of depth of field or a little? In this case, you might consider using the sharpest aperture on your lens. This is typically the aperture 2 to 3 stops from wide open—often in the neighborhood of ƒ/8. It produces an “average” depth of field, neither shallow nor deep, and maximizes the ability for very sharp focus because the apertures at each end of the spectrum (ƒ/2, for instance, and ƒ/32) are less sharp than ƒ/8 will be.
Your camera and lens may help you achieve the exact DOF you want thanks to a couple of features that are often baked in. Many lenses, such as the Fujifilm XF 23mm f/1.4 lens shown here, print a depth of field guide right on the barrel. This shows the range that will be in focus at a given aperture (indicated here by the numerals 4, 8, 11 and 16) and translates directly to the distance measurements on the focus ring. Focus at 5 meters at ƒ/8 and the guide shows your depth of field will include everything from 8 feet to infinity. This is a very helpful way to calculate just how much depth of field you’ll have based on where you’ve focused and what aperture you’re using.
An even more accurate way to determine what will be in focus is to use your camera’s depth of field preview button. On some cameras, such as the Fujifilm X-Pro2, pressing the shutter release halfway will provide a depth of field preview in the moment before exposure. On some cameras, there’s a separate DOF preview button on the body, near the lens mount, that when pressed will manually stop down the aperture based on the exposure setting in order to show the actual depth of field. (Cameras with TTL optical viewfinders open a lens to its widest aperture by default for the brightest preview, then adjust to the taking aperture when the shutter is released.)
You can preview the depth of field and manually adjust the aperture and focus point while doing so in order to adjust the depth of field to include everything you want and nothing more. Just don’t be surprised when the image gets darker during DOF preview, as the lens is actually stopping down and allowing in less light.
Most photographers think of a lens’s focal length as having an impact on depth of field too. Technically, this isn’t true if the subject occupies the same area of the sensor (closer to a wide lens, farther from a telephoto) but in practice, it sure seems that way. And what that means is that wide-angle lenses produce a wider angle of view and appear to provide greater depth of field, while longer lenses appear to narrow depth of field.
If you were photographing a person up close with a wide lens or far away with a telephoto lens (such that they occupy the same area of the sensor) the depth of field would technically be the same, but that’s not how people use these lenses in reality. So, a wider lens and wider angle of view deliver the appearance of a greater area in focus, and long telephoto lenses (such as 85mm, 105mm and 135mm portrait lenses) show a narrower angle of view that appears to compress the scene and deliver shallower depth of field. It’s safe to say that in most instances, a wider lens will provide a greater area in focus, and a longer lens will deliver the appearance of shallower depth of field.
In any case, whether you want a lot of depth or just a little, the best way to take control is to learn to make manual camera settings and dial in the exact aperture, shutter speed and ISO combination you desire. Short of that, choosing Aperture Priority will allow you to dictate whether you want a lot of DOF (ƒ/22) or a little (ƒ/2.8) and the camera will adjust the shutter speed accordingly. More on using Aperture Priority exposure mode next week.