Why are lenses specified by focal length rather than field of view?
Asked 10/28/2018
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For a given sensor size, photographers usually use focal length to predict angle/field of view. But field of view can also change when the lens is moved farther from the sensor, such as with extension tubes for macro work. If both focal length and image distance affect field of view, why are lenses characterized by focal length instead of an 'effective field of view'?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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This question is predicated on a misconception. The flange distance is included in the focal length in lens labels — it is the distance from the optical center of the lens focused at infinity to the imaging medium. This includes the flange distance. (See What is the reference point that the focal length of a lens is calculated from?, and What exactly is focal length when there is also flange focal distance?)
This means that for a given sensor size, lenses made for different mounts are still comparable — a 24mm lens gives (approximately) the same field of view regardless of the mount distance of any given system. So, focal length does correspond to field of view.
You note that using a macro adapter affects field of view. In fact, they also increase the focal length, although since macro extension tubes remove the ability to actually focus at infinity, we leave practical reality and get into the realm of theory.
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
7y ago
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Generated from our catalog & community — verify before relying on it.
Because focal length is an intrinsic lens specification, while field of view is a system result. A lens’s stated focal length is defined from its optical geometry (effectively the distance from the lens’s principal plane to the image plane when focused at infinity), so it already accounts for the mount’s normal flange arrangement. That’s why a 24mm lens gives roughly the same angle of view on the same sensor size, regardless of mount.
Field/angle of view is not just a lens property: it depends on focal length, sensor size, and the actual image distance at the chosen focus setting. When you add extension tubes or otherwise move the lens farther from the sensor, you are no longer using the lens in its normal configuration. The image distance increases, magnification rises, and the field of view narrows. That’s a change in the camera-lens setup, not a different fundamental lens spec.
So lenses are labeled by focal length because it is stable and comparable across systems, while field of view varies with sensor format, focus distance, and added accessories.
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