Influence of Focal Length & Aperture on Depth Of Field

Depth of field (DOF) is also known as the area of the picture that is in sharp focus. Let’s say we are looking at the picture of a landscape. There are snow clad mountains, forests and a river in the background. Everything is in sharp focus meaning there is a big depth of field in the picture. This is achieved by the confluence of three factors, the aperture of the lens, the focal length and also the distance between the subject and the camera. Another factor also influences the DOF and that is the circle of confusion. The circle of confusion is the circle which is out of focus and is easily identified by its lack of sharpness. It is important to note that the shift from soft to sharp does no happen in a visual jerk. Rather it is a transition that happens gradually across the frame.

Aperture and DOF

Aperture is the opening of the lens. It is not a mechanical part of the camera but a number that is assigned to explain the size of the opening of the lens through which light passes then hits the lens glass and converges on the focusing plane. It is expressed as an f/stop where stop is replaced by the number assigned. Wide angle prime lenses normally have a maximum aperture starting with f/1.2. More the aperture smaller is the f/stop number and less is going to be the area of sharp focus on the image and vice versa. Telephoto lenses on the other hand have a maximum aperture of f/2.4 or f/2.8 meaning they have deeper DOF to start off. When shooting landscape photographers prefer to use a higher F number which means they reduce the aperture and let more of the image be in sharp focus (bigger DOF).

Focal length and DOF

Focal length is the distance between the center of the lens or the focusing mechanism and the focusing plane. Focusing plane could be a 35mm film (in a film camera) or a sensor (in a digital camera). Focal length suggests how strongly the camera is focusing on a subject. It also affects the total area of the frame that is in focus and thus the increase or decrease in DOF with increased F number and decreased F number respectively.

Focal length also affects the field of view. Say you’re using an 18-135mm lens with your full frame DSLR. When you zoom in at the longest focal length (135mm) the subject appears larger because the magnification of the image is higher. At the same time the field of view is reduced. On the other hand when you zoom out (18mm), the subject appears smaller and you have a larger field of view.

The distance between the subject and the camera also affects the DOF. Every lens and camera system has a minimum focusing distance from the subject. When this is breached the subject can be out of focus.

Both shallow and deeper DOF are used in photography creatively. Shallow DOF is used creatively for the purpose of creating bokeh; the beautiful blurry backgrounds that isolates the subject and bring it in focus.

Read more on Depth of Field here