So how does the camera obscura actually work and why is it relevant today? (Good question; I'm glad you asked!) Picture the darkest room in your house and imagine that you black out any source of light coming into that room. Then image that you poke a pin sized opening into one of the walls of that room; take a piece of electrical tape and cover that pin sized hole. What happens when you remove the tape from the hole? Light is going to travel through the small opening. The first thing to note about light is that it travels in straight lines (which you should recall from PH136). After the light travels through the hole, what happens next inside the dark room?
To answer this question, you have to think about what is on the other side of the wall. What is outside this darkroom; standing opposite the pinhole? Whatever that object or scene is opposite the hole will be projected into the room. The "catch" is that the projected image will be upside down! "Back in the day", this projection could be traced onto a piece of paper. We used the term "pinhole" although consider the actual size of that "pin"; some pins might be smaller or larger, correct?
The camera obscura will work with a variety of different sized pinholes. The smaller the pinhole, the more sharp the
picture would appear. However, if the pinhole was small, the darker
the projection would be. Does this sound familiar? Think back to what
you've learned regarding depth of field and your aperture settings. If
your aperture is wide open; say f/1.8, you'll have a lot of light coming
into the lens and will have a shallow depth of field. If your aperture
is closed down; say f/22, you'll have less light coming into the lens
and will have a deep depth of field. We can see in the diagrams and drawings of the camera obscura that the paper used to draw on is tilted at an angle. The artist, looking into the box, is then able to draw the upside down project - right side up! In your camera, there is a mirror that serves as a similar function to this tilted piece of paper. When light enters the "pinhole" of your lens, the aperture, the light projects the subject matter that you're photographing onto the mirror upside down. The mirror flips the subject matter being projected by the light right side up and onto your photo sensitive film... *cough cough* excuse me, onto your CMOS sensor:01. "Camera Obscura." Wikipedia. Wikimedia Foundation, 21 Mar. 2014. Web. 21 Mar. 2014.
02. "Jan Vermeer and the Camera Obscura." YouTube. RedCityProjects, 08 Mar. 2012. Web. 21 Mar. 2014.
03. "How Do Digital Cameras Work? | James May Q&A | Head Squeeze." YouTube. Head Squeeze, 10 Jan. 2014. Web. 21 Mar. 2014.