# Why does a smaller aperture achieve deeper depth of field?

I am just wondering what the physics or general rules are that permit a larger depth of field to be achieved with a smaller aperture, assuming I'm right about that.

Relevance
• RickB
Lv 7

A sharp focus is achieved when the light which originates from a single point in the environment, ends up at a single point on the film plane.

For example, say there is a small red dot in the environment (a spot on a bird's feather, let's say). That dot emits red light in all directions, and some of it makes its way through your cameras's aperture. By the time it hits your lens, it has spread out a bit, so that the red light coming in at the top of the lens is aimed at a slightly different angle than the red light that comes in at the bottom of the lens. (To visualize this, it may help to draw a diagram, showing the light from the red spot as it travels in straight lines from the feather to your lens.)

It's the job of your camera lens to bend the incoming light by just the right amount, so that the diverging light beams from the red dot will converge again, and hit a single spot on the film plane.

However, that operation depends on how much the incoming beams are diverging. You can imagine (especially if you've drawn a diagram) that light beams originating from some source much closer than the bird (say a green spot on the sweater of a girl standing close) will diverge at a greater angle by the time they reach your lens. And (here's an important point) the divergence angle will be greater for larger apertures than for small. Because of the larger angle of divergence from the green spot, when your lens bends THOSE incoming rays, they don't converge to a point at the lens plane. The result is that the light from the green spot appears smeared and out of focus.

But now, try stopping down the aperture. When the aperture is very small, the incoming light from nearly any point in the environment, whether it is near or far, will diverge at only a very tiny angle by the time it reaches the lens. Because of this, the lens is able to deal with the "red dot" beams the same as with the "green dot" beams. It bends each of them by the same amount, and each set of beams converges at a small, clean spot at the lens plane. The result is that both nearby objects and faraway objects are in good focus.

• Anonymous