Why can a camera only focus on one distance at a time, and what occurs inside to change that focal distance? Does the same principle apply to our eyes?
In: Physics
When an object is close the light enters the camera at a different angle and the lenses need to change the distance between them to make the light focus on the sensor. Your eyes have lenses with muscles that do this automatically, and if your eye is the wrong length, you might need glasses.
When an image is formed by a lens, it’s only sharp in a narrow range. That range defines the focal point of the optical system. Hold your finger an arm’s length in front of your face and shift you focus from your finger to a distant object. When you focus on one, the other becomes out of focus.
A typical camera lens is hard and inflexible. So there is only one distance at which it can focus light. Your eye has a crystalline lens that is soft and jelly-like. There are cables attached to that lens, which when they loosen, allow the lens to achieve a more spherical shape which allows it to change focus from distance to near. The lens becomes harder over time, so the loosening of the cables have less of an effect. The lens is not as easily able to attain this spherical shape anymore – this occurs gradually and typically people notice the effect around the age of 40. Usually around this time (or 45 or 50), people need to start wearing reading glasses to help change the focus up close.
Cameras change their focus by moving the lens closer to and farther away from the film (OK, charge-coupled devices these days). Your eyes change focus by changing the shape of the lens (flatter for distance viewing, rounder for close up). The narrower the aperture (light inlet) in the camera or the pupil of your eye, the deeper the field of view (depth of the distance in focus). In photography, using tiny apertures to get deeper fields of view also bring the problem of not getting as much light into the camera to make the picture. Long exposure times are needed to let in enough light to make a decent picture, so any movement will be just a blur in such photos.
Actually, a [Light Field Camera](https://en.wikipedia.org/wiki/Light-field_camera) can focus on multiple distances at once. But most cameras can’t do this, nor can your eyes.
It is possible for a camera to focus on more than one distance at a time by using movements that alter the plane of the lens and the plane of the film. Large format cameras mostly have that ability.
With smaller formats, just about all cameras are designed to only focus one distance at a time. This is done by keeping the film/sensor plane and the lens plane parallel to each other.
To change the focus distance, the distance between the lens and the film/sensor must change. As that distance increases, closer objects come into focus.
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