Comment by chandlerh
5 years ago
Great article. One thing that's always stumped me as an amateur photographer is how an object is "in focus" when the rays from that object coverage to the smallest point, thus the smallest amount of pixels are getting hit. It would seem to me you would want to collect the max amount of information and thus use more of the sensor. How does converging to less of the sensor give you a sharper photo?
Ideally, the mapping from object to image is injective. What you're proposing will lead to "hash collisions" aka blurriness, since each point in the object will bleed colors into neighboring points in the image.
The entire object doesn't get collapsed to a single point. Rather, a single point of the object radiates light in all directions. A lens then captures a fraction of that radiation and collapses it back down to a single point. Then we iterate over each point in the scene with a "for each".
The amount of total light is the same, but the smaller the points of focus, the easier it is to distinguish image elements from each other because each point overlaps less with its neighbors.
Here’s a metaphor. Let’s say you have 10 glasses, with 10 different amounts of water in them. When you hit them with a spoon, each glass makes a different musical note: 10 different notes.
If you pour the water from each glass individually into 10 new glasses, you preserve those 10 notes.
If you combine every two glasses into one, now you have 5 glasses, and only 5 notes. It’s the same total amount of water but you lost information.
If you take up a larger part of the sensor, then the distance between any two details must be larger so the areas that their light covers over the sensor don't overlap. If they overlap, you have no clue where the information is coming from.
Look up circle of confusion, Rayleigh resolution criterion and sparrows resolution criterion for the technical details of resolution.
It's more that the rays from a point on an object converge to the smallest point when in focus? The rays from the object as a whole are spread out over an object-shaped are on the image plane, unless the object is (effectively) at infinity, when they will converge to a point...