At AbelCine, we have a great deal of experience in digital cinema, with a strong focus on optics. Lately, I've been fielding a number of questions relating to a lens' back focus, more accurately described as the lens' flange focal distance. In this column, I'll address some of the subtleties of back focus and why it's so important for filmmakers.
Every lens is designed to go on a particular camera lens mount and sit at a specific distance from the sensor or film plane. This distance is described as the flange focal distance (FFD) or sometimes as the flange focal depth. Each lens-mounting system is associated with a particular FFD. The term "back focus" describes the FFD in terms of how "in focus" the rear projection of the lens is, or rather, how correct the distance is from the rear of the lens to the camera's sensor.
Have you ever noticed that when you hold a magnifying glass above the ground with sunlight shining through, the light becomes more or less focused as you move the glass up and down? The same is true of cinema or still lenses, where the lenses are designed to be in focus at a set distance from the sensor. You might think it would be obvious when back focus is incorrect, but that's not always the case.
Here's a common zoom lens question we get: "I zoom in and everything appears in focus, but then I zoom out and everything is soft. What's up with that?" This is a classic back-focus issue. Unless the FFD of your lens is extremely far off from its specification, getting your image in focus with an incorrect FFD is actually very possible. In a zoom lens, however, inaccurate back-focus settings are much more apparent. A cinema or ENG zoom lens is designed to stay within focus throughout all the variable focal lengths of its zoom range, but even the slightest variation in FFD can cause it not to hold focus through the full range.
On a 2⁄3-inch zoom lens, also known as an ENG lens, this is a common occurrence. The 2⁄3-inch lenses are designed to have a specific FFD based on the mount and prism design. Both lens and camera manufacturers design their products to be in spec. However, with variations in camera bodies and the possibility of things shifting over time, lens makers added the ability to adjust back focus quickly, directly on the lens. There's usually a tiny screw in the rear of these lenses that can be loosened to move the rear element (or group) slightly forward or backward, which adjusts the back-focus setting.
Correcting back focus on an ENG lens is simple: 1) zoom to the longest focal length of your lens; 2) get critical focus (sharpest possible); 3) zoom out to your widest focal length; 4) adjust the back-focus ring (the one with the tiny screw holding it in place) until your image is in focus again; and 5) repeat until the image remains in focus throughout your zoom range. This procedure is best aided by a large monitor and a focus chart, but also can be performed in the field, preferably in high-contrast situations. We recommend checking for back-focus issues before each shoot.
Now let's talk about zoom lenses designed for large-sensor cameras. These generally can be divided into two categories: cinema zooms and still zooms. Cinema zooms are lenses designed specifically for film and video use and are most commonly found in a PL mount, which has an FFD of 52mm. They're designed to allow smooth, repeatable adjustments to focus, iris and zoom. The focus scales on these lenses are designed to be very accurate, and focal distances are spread out across the focus ring. Because of this design, we expect cinema zoom lenses to focus accurately at a specific distance and hold focus through the zoom range. When back focus is off, the lens won't hold focus through the zoom range and could cause the focus scale to be incorrect. Unlike 2⁄3-inch zoom lenses, cinema zooms don't normally have a back-focus adjustment ring. Instead, the FFD is adjusted by technicians who place very thin pieces of material called shims between the lens body and lens mount; this is called shimming a lens. This adjusts the FFD in tiny increments; additionally, to compensate for scale inaccuracy, focus scales on cinema zooms are generally adjustable. Accuracy is important to make sure that focus is marked correctly and held throughout the zoom range.