Understanding depth of field is one of the keys to ensuring that your photos are always in focus in the parts you want to be sharp.
Depth of field (DoF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. It is determined by aperture, focal length, the camera’s distance to the subject, and the sensor size.
Knowing the parameters that govern depth of field will give you the creative tools to manipulate it as per your needs. You can be more creative in controlling DoF and ensuring that the camera rightly captures your vision.
In this discussion, we’ll learn in detail about DoF, the various parameters that govern DoF, and how to use them creatively.
The Aperture
The aperture is the opening in the lens through which light passes. The size of the aperture directly affects the depth of field. A large aperture (low f-stop number) gives a shallow depth of field, while a small aperture (high f-stop number) gives a deep depth of field.
The requirement of aperture and the subsequent DoF will vary from genre to genre and from frame to frame. Let’s say that you’re photographing a landscape scene.
The frame is expected to be sharp from corner to corner in a landscape scene. In other words, you need to use a small aperture to ensure that. Of course, there are other parameters involved in that, like hyperfocal distance, DoF calculator, and lens diffraction, that will come into play. Still, to keep this discussion simple, we consider that DoF is governed by just one parameter, and that’s aperture.
In a portrait scene, the background (and the foreground, too) is usually blurred out. This is done for two reasons mainly. One, you want to obliterate anything in the background that may be distracting. Two, you want to blur the knowledge to separate the subject from the background and, in the process, produce a beautiful bokeh (out-of-focus effect). In this situation, you need a large aperture.
We have learned that a large aperture produces a shallow DoF and vice-versa. When you focus on the subject’s eye (closest to the camera), everything in front and the back starts to get blurry from that point of focus onwards. As you move further away from the point of focus, the degree of blurriness gets bigger and bigger.
Focal Length
In photography, the focal length is the distance in millimeters from the front element of a lens to the point where an in-focus image will be formed on the sensor (or film). A shorter focal length will result in a wider field of view, while a longer focal length will result in a narrower field of view.
The most important thing to remember about focal length is that it is a measure of an angle, not a physical distance. This means that the size of your sensor (or film) will affect the field of view but not the focal length.
For example, a 50mm lens on an APS-C or crop camera will give an effective focal length of 1.5 x 50 = 75mm. However, in this case, what happens is that the field of view is shortened because the sensor crops out the edges of the image coming through the lens, and that mimics that the image is zoomed in. The actual focal length does not change. It remains a 50mm lens, just the ‘effective focal length’ changes.
Focal length also has a bearing on DoF. You will notice DoF is shallow when you’re using a telephoto lens. The longer the lens is shallower is the DoF. On the other hand, when you’re using a wide-angle lens, much of the frame is in focus. This is subject to, however, the distance between the subject and the camera.
If you’re standing at the same place and nothing changes except the focal length, DoF will become shallower with a telephoto lens and larger with a wide-angle lens.
Distance Between Camera and Subject
The distance between the camera and your subject is one of the most important factors in photography. It can determine the photo’s mood, the perspective, and even the amount of light that reaches the subject.
The ideal distance between you and your subject will vary depending on your photo type. For example, if you’re taking a portrait, you’ll want to be close enough to capture the subject’s features but not so close that you distort them. If you’re taking a landscape photo, you’ll want to be far enough away to capture a larger slice of the picture in front of you.
Distance between the camera and the subject also affects the DoF of an image. When you’re too close to the subject, DoF is shallow. The subject is sharp, but everything in the background gets blurry. You will notice this effect more easily when shooting macro photos.
With a macro lens like the 60mm f/2.8 prime or a 100mm prime, you will notice that the closer you’re to the subject, the blurrier the background becomes. This is why macro shooters use a small aperture to extend the DoF or use the stacking method when possible.
Sensor size
Sensor size also affects DoF, which photographers often overlook. But this is affected by the other parameters that we discussed above. Let’s elaborate on this.
Let’s say that two photographers are standing at a similar distance from a subject. One uses a D500 (crop APS-C camera with a crop factor of 1.5x), and the other uses a D850 (full-frame camera). They both use a 50mm f/1.8 prime dialed at the same exposure parameters, including the same aperture of f/1.8. In other words, everything else is the same apart from the camera.
The photographer using the D850 will experience shallower DoF than the photographer using the D500. Also, the D850 will capture a wider field of view than the D500, whose field will be slightly narrowed down because of the crop factor.
