What changes when you focus a fixed-focal-length lens, and how does that relate to depth of field?
Asked 8/20/2019
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I’m trying to understand depth of field with a 50mm prime lens. When I turn the focus ring, what is physically changing inside the lens? Is the focal length changing, or is something else moving? How does changing focus distance affect depth of field?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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Lenses these days are complex contraptions but the principal lens model they try to approximate is a single ideal convex lens of the given focal length. When you are changing focus, you are changing the distance of that lens to your imaging plane.
The focusing model then has rays spreading out from each sensor point to all points on the ideal lens that are reachable through the aperture (which is idealised into the same plane as the ideal lens) and from this sort-of circle shape a cone of viewing rays converges to the corresponding point in the focused plane. "Viewing rays" can be cut short by something before the focusing plane or, if they can continue onward to some background point. So basically you have these cones from the lens/aperture converging to points in the focusing plane and spreading out afterwards again.
The closer your focus plane, the more rapid the cones converge and spread out again. And obviously a larger aperture also makes them wider.
The variable changing as you change focus is the position of the idealised lens before your plate/film/sensor plane. Early cameras had a bellows to accommodate moving the lens. Large frame cameras still have them because there are reasons to not just move the lens but also shift it (for avoiding "tumbling lines" without having to place the horizon in the middle of the image) and tilt it (to get an oblique focus plane rather than one parallel to the image). But the normal focusing change is just varying the distance of the lens to the imaging plane.
Originally by user86489. Source · Licensed CC BY-SA 4.0
user86489
6y ago
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Generated from our catalog & community — verify before relying on it.
On a fixed-focal-length lens, turning the focus ring does not normally change the focal length. What changes is the optical spacing: lens elements move so the image formed by the lens falls sharply on the sensor for a chosen subject distance.
For a simple lens, light from very distant subjects focuses at about the focal length behind the lens. For closer subjects, the sharp image forms farther back, so focusing adjusts the lens-to-sensor optical distance (or equivalent internal element positions) to bring that image onto the sensor.
Depth of field is related to aperture, focal length, and subject distance/focus distance. Changing focus does not mean you are changing focal length; you are changing the distance at which the lens is perfectly focused. That in turn changes the near and far limits of what appears acceptably sharp.
In short:
- focus ring: changes lens element position / focus distance
- focal length: generally stays the same on a prime lens
- depth of field: depends on aperture, focal length, and subject distance, so it can change as you focus closer or farther
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