It is often said that “the lens is the eye of the camera.” This is true, in that light enters through and is controlled by the lens, and ultimately this light registers an image. However, there are many things that the human eye and human psychology of perception do automatically, which, on a lens, must be accomplished manually. Framing, focusing, and exposure are activities we rarely consciously think about with respect to our own eyes, but a lens requires us to deliberately set each of these functions; and with each we are presented with a range of possibilities that ultimately represent the creative potential of any lens and its contribution to the aesthetic palette of a filmmaker. Always keep in mind that when setting lens functions, there is often no absolute “right” choice; rather, you must find the appropriate settings for what you want to communicate. On every single shot, you get to decide, from a wide range of possibilities, the framing and visual perspective within the frame, how near or far things appear, how bright or dark your scene and subjects are, and what is or isn’t in focus. Knowing how lenses work will help you choose the right lens and the best settings to create expressive images. It is helpful to remember that the lens is much more than just the eye of the camera: it becomes the eyes of your audience.
Simple image formation by a lens. A lens gathers incoming light and focuses it on the image plane (the sensor). At the optical center, the image is not only flipped, but also reversed.A zoom lens changes focal length by shifting the position of the internal optical center elements (a) by adjusting the zoom ring (b) (top). Some cameras have a servo zoom motor controlled by a rocker switch (c) that lets you glide from one focal length to another (bottom).Changing focal length versus moving the camera to achieve a specific shot size has a significant impact on the field of view and the perspective of the image. The left column shows three frames from a stationary camera: wide angle (a); normal (b); telephoto (c). The right column shows three frames all taken with a wide-angle lens and moving the camera. (See this as a high-resolution interactive example online.)Lens markings: the focusing ring on a lens has a series of distances engraved in feet or meters (a), which are aligned to a witness mark (b). Zoom lenses have their focal length settings in millimeters etched into the zoom ring (c) and align to the same witness mark. The aperture ring can be found closer to the lens mount and engraved with an f-stop or a T-stop scale (d).Follow focus involves the marking and timing of a subject’s movement toward or away from the camera. Here, a camera assistant will shift the focusing ring as the runner hits pre-measured marks on the ground.The f-stop scale: as the f-stop number grows larger, the aperture opening grows smaller, and vice versa.Controlling DOF through aperture. Both of these images were shot with the same lens and focus distance (the double dot fret position marker). The left image was shot at f/22 and gives us a deep DOF, notice that the bridge pins are in focus. Adding ND filters to reduce the light entering the lens allowed the aperture to open up to f/4 which resulted in a much shallower DOF (right), now the bridge pins and foreground strings are out of focus.Two DOF tables for the Super 35mm sensor format (as found on the Canon C300). The top table is for a 30mm lens and the bottom table is for a 100mm lens. All DOF variables are represented on the table.Circle of confusion: although technically out of focus, if the diameter of a blur point is smaller than the ability of the sensor to perceive its blurriness, it will appear to be in focus because it is within “acceptable sharpness” for the imaging format.
