The principle of the camera obscura (= pinhole camera) is like this:
The disadvantage is clearly, that the image on the image plane is very dark. The needed exposure times to take an image can well be minutes!
Idea: Lets use a larger hole :
Now, however, the image not only gets brighter (as intended) but also gets blurry, because the light not only passes through the center of the hole. So not only the correct position of the image is exposed to the light, but also the direct neighbours.
As a result, the image of an object point is not just a point, but instead a little disk, the so called “Circle of Confusion” (CoC).