Why is it called the plane of focus instead of a sphere?
Asked 2/27/2016
5 views
2 answers
0
I often hear photographers refer to the area that appears sharp as the “plane of focus.” But if focus is set by subject distance from the lens, wouldn’t the in-focus locus in front of the camera really be part of a sphere (or spherical shell) rather than a plane? Why is “plane of focus” the standard term?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
4
My question is why does it have to be a plane?
When people talk about the focal plane in a photographic context, they usually mean the image plane, a.k.a. sensor plane, the area where the image is formed. Since film and sensors are (usually) flat, a field of focus that is also flat is desirable. The field of focus is often not flat, which is one reason that many lenses are sharp in the center but softer at the edges, but lenses often include elements that flatten the image as much as possible.
I'm guessing, though, that you're talking about the focal plane in front of the lens. You're right that the space in which the subject is in focus is shaped like a sphere. Wider lenses see a larger portion of that sphere, but they usually also have greater depth of field. Long lenses see only a small part of that sphere, which means that the field of focus is close to flat.
Originally by user4262. Source · Licensed CC BY-SA 4.0
user4262
10y ago
0
Generated from our catalog & community — verify before relying on it.
“Plane of focus” can mean two related things.
Most often in photography it refers to the image plane: the flat film or sensor surface where the image is formed. Because sensors and film are usually flat, lens designers try to make the sharp focus field match that flat surface as closely as possible.
In object space (the scene in front of the lens), your intuition is basically right: for a simple lens, points at the same focus distance form a curved surface, not a perfect plane. This is related to field curvature. A simple lens naturally focuses onto a curved field, while real camera lenses use additional elements to flatten that field so the image is sharp across a flat sensor.
So the term persists because photography is built around a flat recording surface, even though the actual sharp-focus region in front of the lens is not always a true geometric plane.
Recommended products
UniqueBot
AI10y ago
Your Answer
Related Questions
Why does focus sometimes appear as a horizontal or vertical band in macro photos?
What is the term for the area a lamp or flashlight illuminates?
Why not focus closer than the hyperfocal distance to get more depth of field?
What does a lens’s minimum focus distance actually mean if depth of field extends closer?
In photography, is the sensor/film plane correctly called the focal plane?