Defocus Blur

Now our final feature: defocus blur. Note, photographers call this depth of field, so be sure to only use the term defocus blur among your raytracing friends.

The reason we have defocus blur in real cameras is because they need a big hole (rather than just a pinhole) through which to gather light. A large hole would defocus everything, but if we stick a lens in front of the film/sensor, there will be a certain distance at which everything is in focus. Objects placed at that distance will appear in focus and will linearly appear blurrier the further they are from that distance. You can think of a lens this way: all light rays coming from a specific point at the focus distance — and that hit the lens — will be bent back to a single point on the image sensor.

We call the distance between the camera center and the plane where everything is in perfect focus the focus distance. Be aware that the focus distance is not usually the same as the focal length — the focal length is the distance between the camera center and the image plane. For our model, however, these two will have the same value, as we will put our pixel grid right on the focus plane, which is focus distance away from the camera center.

In a physical camera, the focus distance is controlled by the distance between the lens and the film/sensor. That is why you see the lens move relative to the camera when you change what is in focus (that may happen in your phone camera too, but the sensor moves). The “aperture” is a hole to control how big the lens is effectively. For a real camera, if you need more light you make the aperture bigger, and will get more blur for objects away from the focus distance. For our virtual camera, we can have a perfect sensor and never need more light, so we only use an aperture when we want defocus blur.