General scheme of an image formed on the screen of a camera obscura.
Any object in front of the camera obscura reflects light, in all directions, that goes beyond the hole, constituting itself as a secondary source in an internal environment. The primary light source can be a lamp.
The face of the object that is facing the hole will have an image formed inside the chamber. This face reflects light in all directions, but only part of it passes through the hole. As in these conditions light propagates in a straight line, the image formed will be upside down.
In current cameras, as well as in our eyes, the images that are formed have the same characteristics as those obtained with the camera obscura: they are all upside down and have the right and left sides inverted, when observed from behind the screen.
If the darkroom hole is small, the images obtained will be very sharp. The problem is that the small amount of light that enters the camera will produce a dimly lit image. If we make a larger hole, the amount of light will increase, but the image formed will lose its sharpness.
Therefore, we can say that the function of the face of the camera that contains the hole is to control the entry of light that reaches the opposite face, on which the image will be formed. Each point on the face of the object facing the hole reflects light in all directions, but only rays emitted in the direction of the hole will be able to pass through it and reach the tracing paper, forming a complete image of the object.
The size of the image formed on the wall (screen) is closely related to the distance between the wall with the hole and the wall on which the image is formed. In addition, the size of the object and the distance between it and the camera influence the size of the image formed.
We can mathematically express these relationships depending on the height of the object ho, the height of the image hi, the distance from the object to the camera Do and the distance from the image to the hole Di, by the following relationship: