Depth of field (DOF) is a measure of the range of distances from a lens that appears to be in focus. This range varies depending on the lens aperture, lens focal length, and object distance. DOF is significant in scientific cameras and photography because it affects the sharpness of an image . However, in medical applications, this is an even more important factor. For example, a tool that allows a physician to see inside a narrow body cavity during a medical procedure requires precise control over the image. Otherwise, the subject may not be properly illuminated or in focus, limiting the healthcare professional's ability to make accurate diagnoses or treat patients. As such, the depth of field is usually a limiting factor within instrumentation and needs to be considered at the onset of a custom lens development project.
Analyzing DOF
There are three primary factors that affect the DOF:
Aperture size
Lens Focal length
Object distance
Within a lens, the aperture is a physical component that restricts how much light passes through the lens. In commercial and photographic lenses, it is usually adjustable and is marked with an "f-stop number." The lower the f-number, the larger the aperture and the shallower the DOF. Longer focal lengths produce narrower angles of view, which also produces shallower DOF. And lastly, when the lens is focused at closer object distances, the DOF will be narrower than when the lens is focused at farther object distances.
It's important to understand DOF because it can be a significant limitation on the usability of a lens system. For medical applications, depth of field is normally important to understand because it can be a significant limitation on what part of an object or patient can be imaged in focus at one time. The depth of field can be determined by calculating the "circle of confusion," which is the largest circle that can be drawn on the image plane that will still appear as a point to the observer. The circle of confusion is directly related to the aperture size, focal length, and object distance.
Aperture Size
Within the lens, the aperture is an opaque mask or mechanical component with an opening in the center to allow light to pass through. It is typically adjustable and is marked with an "f-stop" number. The lower the f-number, the larger the aperture and the shallower the DOF. For example, an f/2.8 aperture will produce a photo with a shallow DOF and a sharper image, while an f/22 aperture will produce a photo with a deep DOF.
Focal Length
The focal length or effective focal length is a measure of how strongly a lens focuses light. It is the physical distance from the optical center of the lens, or principal plane to the image plane where light is focused from infinity. Longer focal lengths produce narrower fields of view and shallower DOF, while shorter focal lengths produce wider fields of view and deeper DOF.
Object Distance
The object distance is the distance from the front of the lens to the object or subject being photographed. When the lens is focused at closer object distances, a shallower DOF will be produced than when the lens is focused at farther object distances. For example, if you are photographing a subject that is 1 foot away from the camera, the DOF will be shallow. However, if you are photographing a subject that is 10 feet away from the camera, the DOF will be deep.
Depth of field is a classic optical challenge that needs to be taken into account when designing lenses. By understanding how aperture size, focal length, and object distance affect DOF, optics manufacturers need to convey this important factor to clients so that the lens is just right for their desired application.
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