Depth of Field Physics
We were recently asked about the inner workings or physics behind depth of field, in other words, why does a smaller aperture create more depth of field whereas a large aperture results in a smaller depth of field.
For those who aren’t familiar with depth of field it’s simply the amount of “stuff” that is “in focus” when you take a picture. A large depth of field means the entire picture appears to be in focus. A small or shallow depth of field means only a small portion of your subject is in focus. To learn more about the topic, you can it all in our handy depth of field guide.
When a camera focuses, the focus is on one point. The light comes from the subject (represented by the X & Y) goes through the lens and the light is then put against film or a digital sensor. When the image is in focus, the light is converging (coming together) just right. When the light doesn’t come back together just right, the result is a blur.
When the opening or aperture is smaller the light does not diverge as much because it is now narrower by nature. So when the rays finally arrive at the “Focal Plane” of the film or sensor, they’re not able to diverge as far to be as noticeably out of focus. So your perception is, with the smaller aperture opening, that most of the image is in focus–even though there is still just one subject or point in focus just like with the more open aperture. The difference is the effects of being out of focus are much more exaggerated when the aperture is wide open.