If a photograph is two-dimensional, understanding depth of field gives you a measure of control over the third dimension. Mastering it will give you more ability to render photographs with clarity and directness.
What is Depth of Field?
Depth of field is a measure of the distance in front of and behind the point of focus that remains acceptably sharp in a photograph. In photography, everything affects everything. If you change a setting, it’s going to have a consequence in some other facet of the process. The key to understanding DOF is understanding which facets affect it and how.
The boundaries of DOF are not sudden transitions; they simply represent the threshold at which classically accepted levels of sharpness are crossed. The transition is smooth. Thin DOF is generally desired by portrait photographers, as the separation between the subject and the background draws the attention to the subject. A wide DOF is generally desired by landscape, product, and macro photographers, where the entirety of a scene or subject should be well-defined. These are not hard-and-fast rules, however; many times, a photographer may want the opposite of the convention for any number of reasons. Think of them more as prototypical situations that illustrate the use of DOF.
The most often cited factor that controls DOF is aperture. Larger apertures, such as f/1.4 or f/2.0, result in a thinner DOF. This is one reason many portrait photographers love “fast glass.” On the other hand, smaller apertures, such as f/11 and f/16, result in a wider DOF. In other words, if you want more of a scene to be in focus, stop down your lens; if you want less to be in focus, open it up.
DOF is also affected by your subject distance. A closer focal distance results in a smaller depth of field, while a longer focal distance results in a larger depth of field. In others words, to isolate your subject more, get closer. Subjects are most isolated when the distance between the photographer and the subject is minimized and the distance between the subject and the background is maximized. Notice in the 2 photos below how the blur become more pronounced much more quickly when I'm five times closer to the deck rail, despite the equal apertures.
Focal length is a bit trickier. It’s commonly accepted that longer focal lengths produce a thinner DOF, but this isn’t exactly true. It’s a consequence of how we use telephoto lenses: we use an increased focal length to magnify a subject from afar instead of using a smaller focal length and moving physically closer. If the magnification of the subject (how much space they take up in the frame) is held constant, the DOF is almost unchanged. For example, if I use a 100mm lens at f/8 on a full frame camera and my focal distance is 10 feet, my DOF is 1.42 ft. If I cut my focal length in half to 50mm, but also get twice as close, so my focal distance is 5 ft., my DOF is 1.44 ft. Only in the wide angle focal lengths will there be an appreciable difference. In almost all cases, however, you can still associate longer focal lengths with thinner DOFs because of how we use telephotos.
Cameras with smaller sensors produce larger DOFs given equal f-numbers. Yet, sensor size has absolutely no bearing on DOF. Rather, think of it this way: a larger sensor requires a closer focal distance to maintain the same subject magnification. In other words, you have to get closer with a larger sensor to maintain the same perspective. This reduces focal distance and as discussed above, this reduces depth of field.
A lot of times, you may have stopped your lens down to a certain point and do not want to stop it down any further, whether that be due to diffraction or the amount of available light. At this point, since you can’t increase your DOF any farther through your aperture, you might be interested in how you can use it most efficiently. This is when you should calculate the hyperfocal distance. The hyperfocal distance is given by a formula relating focal length, f-number and circle of confusion size that dictates the focal distance you should set your lens to to ensure that everything between that distance and infinity is in focus. For example, if I am using a full frame camera with a 16mm lens at f/8, setting my focal distance manually to 3.55 ft. will ensure that everything from 3.55 ft. to infinity is in focus. It’s a great way to guarantee that nothing is wasted in landscape shots. Luckily, you don’t need to carry a calculator and this formula with you. I highly recommend DOF Master or the PhotoPills app for this and related calculations.
Focus and Recompose
One thing to be aware of when working with small DOFs is the focus and recompose method. This is the common technique of using one of a camera's focus point to find focus, holding the focus button down, then recomposing the shot. It’s often used on cameras with fewer focus points or points that are clustered in the center of the frame, where, for example, one might make sure focus is achieved in a subject’s eyes, then recompose the shot. The problem with this is that it rotates and possibly translates the DOF plane and in a situation where you are using a large aperture and a short focal distance (taking a portrait), this can be the difference between piercing eyes and a miss.
Front and Back
The ratio of near-to-far depth of field is never even. In other words, there will always be more in focus behind the subject than in front. At hyperfocal distance, the ratio is 1: ∞ (all focus behind), while it approaches 1:1 (an even split) as the subject distance approaches zero. In practice, it is approximately 1:1 at portrait distances, and approximate 1:2 (1/3 in front, 2/3 behind) for normal distances (this asymmetry is especially noticeable when using wide angle lenses).
Lastly, have you ever wondered what all those numbers on the barrel of your lens are? These are DOF markings and are tremendously helpful when you have an estimate of your distance to a subject and want to know how much error you can have on either side. The numbers that are arranged symmetrically are f-stops. To read your in-focus area, simply read the distances shown in your distance window that align with your chosen f-stop. In the example shown below, working at f/8 will place everything between 10 and approximately 13 ft. in focus. Street shooters often work this way, setting an approximate distance and shooting from the hip when action enters the DOF range. This is often referred to as trap focus.
Depth of field is one of the most important fundamental aspects of technique and composition. Using it properly can be the difference between a shot that draws the viewer in and a shot that is busy and disjointed. Remember: wider apertures, getting closer to your subject and using a longer focal length (in the normal fashion) will decrease your DOF, and isolate your subject more. Your technique has to be spot-on when the DOF becomes thin. Be careful of focusing and recomposing. Using smaller apertures, getting farther away from your subject, and using a shorter focal length will increase your DOF and allow more of your scene to be in focus. If you're shooting landscapes, always consider the hyperfocal distance and be aware of the asymmetry between front and back DOF. Remember that crop sensors give a larger DOF when used at the same perspective. Lastly, don't overlook the usefulness of the DOF markings on your lens; they can be a real savior in many situations. Mastering depth of field will give you an incredible amount of control over the look of your images.