2.4.4: The Lens

 

Another powerful tool a cinematographer has to work with is, of course, the camera. And there is a lot that goes into how that particular apparatus works and the nuances between different formats and manufacturers. But I want to focus on the one component that is interchangeable and allows for endless variety: the lens. No matter what camera a cinematographer chooses, it’s the lens that determines the clarity, framing, depth of field, and exposure of the image. Just by changing the lens, without moving the camera at all, you can radically transform the look of a shot.

The principle behind a camera lens is pretty simple. A piece of curved glass (or several pieces depending on the lens), held in place on the front of the camera, focuses light through an adjustable aperture (a fancy word for “hole”) and onto light-sensitive material (film or a digital sensor). The aperture controls the amount of light entering the camera, and the glass “elements” control the sharpness of the image by moving closer or further away in tiny increments from the aperture. The overall distance between the sensor and the point at which the light passes through those glass elements is called the focal length^[1] and is measured in millimeters. So, in a 50mm lens, the distance between the sensor of the camera and the point where the light passes through the glass of the lens is 50 millimeters.

The focal length determines both the angle of view and the magnification of the image. The shorter the focal length, the wider the angle of view and the smaller the magnification. The longer the focal length, the narrower the angle of view and the greater the magnification. Any lens below 35mm is generally considered a “wide-angle lens” because of its relatively short focal length. Any lens above 70mm is considered a “telephoto lens” because it greatly magnifies the image.

Lenses can be divided into two basic types based on how they treat focal length: zoom and prime. Zoom lenses allow you to adjust the focal length by sliding the glass elements closer to or further away from the sensor, thus greatly magnifying the image or widening the angle of view without swapping out the lens itself. Prime lenses have a fixed focal length. What you see is what you get. Now I know what you’re thinking. Why not just slap a zoom lens on there and choose your own focal length? But actually, cinematographers almost always use prime lenses when filming. For one thing, zoom lenses tend to have many more glass elements than primes and that can affect the quality of the image. But more importantly, prime lenses force the cinematographer to be more deliberate and intentional about the angle of view and magnification of a particular shot.

Confused yet? Maybe this will help:

Still confused? Here’s an explanation in just 23 seconds:

The angle of view and magnification is important in terms of what’s visible in the frame, but just as important is what appears to be in sharp focus. Lenses also allow cinematographers to control the depth of the image by either isolating a subject as the only element we see clearly in a particular shot or allowing us to see everything in the background and foreground equally. This is called depth of field, the range of distance in front of the camera in which subjects are in sharp focus.

Take a look at this image:

Note how the figure of the man lighting his cigarette is isolated from the background, focusing our attention on the spark from the lighter. This is an example of a narrow depth of field. The range of distance in front of the camera in which subjects are in sharp focus is relatively small, creating less depth in the image.

Now check out this image:

Note that everything seems to be equally in focus, allowing us to pick out all of the details of the set design. This is an example of a wide depth of field or deep focus.

But since cinematography is all about moving pictures, this is not necessarily a binary choice. A cinematographer can change the depth of field within a shot to shift our attention from one subject to another. This is called a rack focus or pull focus:

Now that you know what it is, you’ll see it all the time in film and tv. In fact, there’s usually one person on set whose only job is to manage those shifts in the depth of field within a shot. They’re called, appropriately enough, a focus puller.