The Human Eye
The human eye registers what it sees; just like a film camera. Both the camera and the eye have similarities and differences that should help a filmmaker understand the fundamentals of cinematography, as well as the basics of optometry.
The Principles of Camera and Eye
Similar to a camera, the eye is a dark box that has an opening through which light enters and creates an image. In the eye, the opening is called pupil. The pupil is analogous to the aperture in the camera. The size of this opening is regulated by the iris in the eye and by the diaphragm in the camera. Quite conveniently, the diaphragm of the camera is usually called an iris also. In cinematography, the words “iris” and “aperture” are used interchangeably to refer to the size of that opening.
When the camera is in automatic, the aperture behaves as the eye’s pupil. It closes in a well-lit environment and opens in a dark one. If you’re driving your car through a tunnel, your pupil will expand to let light in. If you leave the tunnel to intense sunlight, your pupil will immediately respond by contracting and blocking sunlight, otherwise the bright light would dazzle you, and you wouldn’t see much, if anything. The camera in auto mode does the very same thing.
Once the light passes through the pupil, it reaches the retina and the actual process of conjuring the image starts. The retina is full of optic nerves that, when hit by light, form the image and send it to the brain. In a film camera, the retina is equivalent to the film stock. In a video camera, the retina corresponds to the CCD chips. The tape or card where the image is recorded and stored would be analogous to the human brain.
A healthy human eye has the uncanny ability to shift focus along its entire field of view. This happens so naturally and effortlessly that people don’t notice it. But a certain level of attention will make this point obvious. In one second, a person can be reading a letter a few inches from his eyes. In the other, he can be observing a landscape hundreds of miles away. In both cases, the image is sharp and clear for the healthy eye. The camera mimics this when its automatic focus is on.
When automatic focus is active, the camera will average the distance of the foreground and background in frame, do the needed equation, and finally guesstimate what the operator is trying to focus on. When the automatic focus is on, the operator loses control over the camera by giving it the autonomy to act based on its programmed processor. This is not professional videography. During a pan, or if someone or something passes in front of the lens, the focus will shift and create a noticeable distraction. Professional filmmakers and videographers must be in control at all times. If pooling focus is needed, the first camera assistant will do it. Not the machine.
Also, note that both the camera and the eye can focus in the distant infinity and see it sharp. However, if an object is too close to the lens or the pupil, it could appear blurry depending on its proximity; thus both the camera and the eye have a minimum distance required for sharpness.
An easy experiment to better understand focus goes like this: Close one of your eyes while keeping the other one open. Hold your finger very close to your open eye, but don’t let it go blurry.
When you do this, you should quickly realize that you can select what’s in sharp focus: it will be either the pencil or the background, but not both. When the background is sharp, your finger goes blurry, and vice versa.
The same is true for a video camera. The difference is that with the camera you have to manually pool focus by adjusting the ring.
Have you ever noticed that you see slightly better when you squint? Well, farsighted people have, and it’s true. This happens because the pressure on the pupil changes its focal length and increases the depth of field, therefore allowing greater sharpness in one’s field of view.
In the camera, the same effect happens. When the aperture is closed, the overall sharpness of the image is greater than when the aperture is open.
(In his book Optics, Eugene Hecht suggests that the f-number of the human eye varies from about f/8.3 in a brightly lit place to about f/2.1 in the dark. An f-number is unit used to measure the size of the lens opening. )
White Balance (video cameras only)
The human eye is so perfect that most people cannot comprehend white balance and its purpose. Since the eye automatically adapts to each new environment, people don’t understand that different light sources emit lights of different colors. For instance, sunlight is bluish whereas tungsten light is amber. Though the eye adjusts to each light, the camera doesn’t.
Even the average consumer, a videographer using a small handycam, probably doesn’t know much about white balance. Non-professional camcorders are notorious for lacking manual settings. The manual white balance is a missing feature in most handycams.
In a nutshell, every time before shooting under a different light source, white balance must be set. To white balance, check out your camera’s manual for specific instructions. Usually the process is simple: make sure the camera is in auto mode, fill the frame with pure white, which can be a sheet of paper or someone’s T-shirt, and then press and hold the white balance button. Most cameras also have pre-settings for daylight, tungsten, and fluorescent, which can be quite useful when the operator is in hurry. For more elaborate instructions on white balance, check out our White Balance page.
Note that film cameras don’t have white balance. When using film, the director of photography has the option of using daylight-balanced stock or tungsten-balance stock.