There are many methods to achieve computational refocusing. We do it the simplest way.
I'm going to try to explain this intuitively and imprecisely, because the mathematical definition has been well explained elsewhere
.. First of all, light field imaging
treats light as rays, each with a direction and intensity. So think of the light around you as a bunch of long thin rays traveling through space. They shoot around in all directions, all the time, reflecting off things, refracting through glass, air, water, etc.
What we want to do is select certain
light rays from certain
angles to form our final image. To do this, first we need to actually capture rays which are coming in from different angles.
It goes without saying that the most obvious way to capture rays from different angles is to measure them from different points in space. What is not so obvious is how to recombine these intermediate images into the final, refocused image.
Let's try to think of this in terms of cameras
. Imagine you have an object sitting on a table. You have a camera centered on it. You take a picture.
When you move the camera left and right, the object moves right and left in the image, respectively.
Think about this for a minute -- by looking at the object like this, you have captured different rays of light moving in different directions from the surface of the object. And in the extremes of the left and right image, you can actually see around the object a little bit. If you took enough pictures, you could "see through" the object by choosing only the rays of interest.
The clever trick here is that by placing our cameras at exactly the same distance apart every time, we "know" the angle of incoming rays relative to each other. We don't know them in an absolute sense (like in radians or degrees), but that's not really very important -- if we overlay the images and shift them all relative to each other, we "select" different rays by blurring out the ones we're not interested in.
In other words, the important thing is that by making the distance between the cameras the same every time, we automatically preserve the angular relationships between incoming rays.