Depth of field (DOF) is a photography term that refers to a zone in front of the camera where the image is in focus, while items before and after this zone appear blurry. The size and location of this zone changes based on several factors such as aperture, focal length of the lens and distance of the subject from the camera.
Below is an example of the effect of using a shallow depth of field. The image on the left has a shallow depth of field and the background building becomes blurred. This is a common technique in portrait photography to isolate the subject. This is useful in architectural photography for shooting details (ie, a door knob). You can achieve this affect by using a big aperture (low number) like 1.8.
Three things that affect depth of field
Aperture is a prime control of depth of field. A large aperture (small number) results in a shallow depth of field (good for details) while a small aperture (large number) results in a larger depth of field (good for almost all other type of architectural photography). Aperture however is not as effective in controlling depth of field in architectural photography as it is in other types of photography. We’ll explain why below.
As the distance from the camera to the subject increases so does the depth of field. This is why it is difficult to achieve background blur with large subjects like architecture.
3. Focal length
The smaller the focal length of the lens the larger the depth of field. So using a wide angle lens will increase depth of field while a zoom lens will reduce it.
Why depth of field is different in architectural photography
Because architectural photography often involves large distances and wide angle lens a large depth of field is almost always achieved no matter what aperture you select. You can test this concept by using an online depth of field calculator. Enter some typical details for architectural photography like a distance of 50m and a 24mm lens. Now adjust the aperture. You’ll see that the far plane is fixed at infinity and changing the aperture only affects the near plane. What this means in a practical sense is that the concept of DOF and adjusting the aperture in architectural photography is more about deciding how much of the foreground is in focus.
The aperture 11 rule of thumb for architectural photography
Aperture 11 is the rule of thumb I picked up somewhere and it has served me well. Its a good balance between depth of field, shutter speed and image quality. Once you go beyond an aperture of 11 the change to the depth of field is minimal and you have to contend with other issues because smaller apertures let in less light and increase exposure times. You also move away from the lens’s sweet spot when it comes to sharpness.
A couple other things to mention.
For anyone really, really interested in depth of field here’s a couple areas for further bedtime reading: acceptable sharpness and the circle of confusion. These are important in understanding how something is considered in focus and also some of the limitations of the depth of field calculators mentioned above. Enjoy.