Why doesn’t a camera lens mix light from different parts of a scene into a noisy image?
Asked 2/15/2020
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I’m trying to understand how a lens forms a sharp image. In pinhole-camera explanations, a small aperture lets through only a few rays, so it seems easier to see how each scene point maps to one point on the sensor. But with a lens and a wider aperture, many more rays pass through different parts of the lens.
What confuses me is this: if those rays come through different parts of the lens, why don’t they represent different parts of the scene and create a noisy or smeared image? Why do they still converge to the correct point on the sensor instead of mixing information from different parts of the real-world object?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
8
The schema (and your interpretation) is misleading. If the blue rays are parallel then they come from a source which is infinitely far away.
When you shoot a real object, most of it is off-axis, and if the object is not very far away light rays from it hit the lens with various angles of incidence:
Originally by user75947. Source · Licensed CC BY-SA 4.0
user75947
6y ago
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They don’t come from different parts of the scene in the way you’re imagining. For any one scene point, many rays leave that same point and reach different parts of the lens. A properly focused lens bends those rays so they meet again at one point on the sensor.
So a wider aperture does not combine unrelated scene points into noise; it lets in more rays from the same scene point, making the image brighter. In that sense, each part of the lens contributes to the whole image.
If the rays in a diagram are drawn parallel, that usually represents light from a point very far away (effectively at infinity), such as a star. Those parallel rays still originate from the same scene point, and the lens focuses them to one image point.
Only points that are out of focus fail to converge exactly on the sensor; then they form blur circles instead of points. Also, very small apertures increase diffraction, which can soften the image slightly.
So the key idea is: one object point → many rays through different parts of the lens → one image point when in focus.
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