A NAB is a great opportunity to see a lot of different products and to get an idea where manufacturers are heading in product development. And, as you’ve discovered, it can also produce more questions than answers, particularly as video and cinema-style cameras have crossed over.Back focus does relate to zoom lenses. The term is also used when talking about prime lenses. Checking and adjusting back focus on a zoom is different from a prime. First, let me talk about zoom lens back focus and, in particular, for 2/3-inch video cameras—I call them video zoom lenses.
Since many video zoom lenses are interchangeable (as opposed to being built into a camcorder) and made by different manufacturers, the tolerances of the lenses—how they mount on the camera and how the optics perform—are critical to focus. And, as zoom ranges have grown, these tolerances have needed to become tighter and tighter. But tighter tolerances come at a cost, literally. So, zoom lenses are designed to have some after-manufacture adjustments to keep optical performance at a high standard.
Back focus is one of these adjustments. An adjustment ring on the lens is used to make sure the lens focus at one end of the zoom range matches the lens focus at the other end of the zoom range. The adjustment is a fairly simple process. A back-focus chart (a Siemens Star Chart is the usual option) is set up at a distance from the camera, the focus is evaluated at the full telephoto range, and then the lens is set for wide-angle. At that point, back focus is adjusted.
Here’s the method recommended by Canon (my appreciation to Larry Thorpe at Canon). First, place the chart at least 75 feet away from the camera. The exact distance isn’t critical. The chart and camera need to be mounted so they’re both stable. It’s also important to make sure the chart is parallel to the image sensor of the camera. Then, make sure that the aperture on the lens is set at wide open. This ensures the depth of field is at its narrowest. By using a narrow depth of field, any back-focus error won’t be hidden in the depth of field.
Make sure there aren’t any filters on the camera and that any extenders, if present, aren’t engaged. Then, zoom in on the chart and adjust focus for the best sharpness at the center of the chart. The Siemens star pattern really helps make it easy to see when you’ve achieved the best focus. By paying attention to the center of the chart, you’ll see that there are sharp spokes leading into the middle; they then become a small blurred circle at the very center. As you adjust the focus ring on the camera, the blurred circle will get smaller and smaller, and then start growing bigger. Rock the lens focus ring back and forth until you dial in the smallest circle. This can be very subjective.
Now, without touching the focus ring, zoom to full wide angle. At this point, you need to loosen the back-focus adjustment ring near the back of the lens. It’s usually locked with a set screw. Once the ring is loose, adjust the ring while looking at the Siemens star to achieve the best possible focus. Once you have a sharp focus, slightly tighten the set screw. Then, repeat the process several times. Zoom in on the chart, check focus by rocking the focus ring, then zoom out and check it again. Lather. Rinse. Repeat. It’s very important to repeat the process, as there are a lot of optics moving around in that lens in order to keep the image in focus, keep the amount of light being transmitted through the lens consistent and keep the flare (light reflecting inside the lens) to a minimum.