Focusing on the Problem
At the most basic level, a camera makes a record of light. When you photograph a scene, light reflected by the scene hits your camera’s lens. The lens bends the light and redirects it to the recording medium. In a film camera, that medium is a strip of chemically treated plastic. In a digital camera, it’s an electronic sensor.
Light from the scene bends as it passes through the lens. The light rays all converge on a single point. If you want the subject of your photograph to be in focus, you need that point to be on the surface of your film or on the digital sensor. You can adjust where the rays converge by moving the lens closer to or further from the medium. When you focus a lens in a camera, what’s really happening is the lens is moving closer to or further away from the rest of the camera.
The curvature of the lens determines how far away from the lens’s surface light will converge to a point. A flat lens won’t make light turn in as sharp an angle as a rounded lens will. That’s why the convergence point for a flat lens is further from the lens than it would be with a rounded lens. Depending upon what you want to photograph, you’ll need the right lens to make sure your subject is in focus.
The standard approach to photography totals up all the light striking an image. There’s no one choice with a traditional camera that can capture everything so that you can switch the focus after you’ve taken the photograph — what you see in the final image is what you get. But if you could record all the individual light rays so that your image has all the visual information of a scene, you could switch from one focal point to another with the right software.
That’s the foundation for the Lytro camera. It captures not just the total amount of light within a scene, but the actual light fields.
Fields of Light
When we look at a lit object, our eyes detect the light bouncing back from that object. That light travels in all directions — if the object is transparent, the light continues through it as well. If you were able to isolate a single point in space within this lit scene, you would discover rays of light crossing through, traveling in all directions.
A light field describes this phenomenon. You can think of a ray of light as something that has five dimensions. Three of those dimensions are the spatial dimensions we’re all familiar with — what roughly translates to height, length and depth, or the x, y and z axes. The other two dimensions refer to the flow of light along the ray.
The geometric distribution of light is what we call a plenoptic function. The word plenoptic comes from the Latin word plenus meaning full or complete. Optics refer to the behavior of light. To capture the complete light from a scene, we would need a plenoptic camera.
A literal plenoptic camera is just a thought exercise. Opaque physical objects block light — they cause occlusion. To capture a light field, the camera — and photographer — would have to inhabit every perspective around a subject simultaneously without blocking light. We haven’t figured out how to position a camera to capture every point of view around a subject or prevent occlusion. But the Lytro camera does simulate some plenoptic functions.
The device has a few things in common with your average digital camera. It has a lens and aperture through which light passes. It also has a sensor that detects light. But between the two is an array of microlenses. These lenses are much smaller than the Lytro’s main lens — a prototype plenoptic camera that was the predecessor to the Lytro had microlenses that were 280 times smaller than the main lens [source: Ng].
The lenses in the array direct light from the back of the main lens to the sensor. The sensor captures the light field between the Lytro’s lens aperture and the sensor. The resolution of the photographs depends both upon the power of the sensor and the number of microlenses in the array.
Once you’ve captured an image, it’s time to crunch some numbers.
For more detail: How the Lytro Camera Works