Steps to Avoiding Back-focus Trouble
Setting back-focus in the field is something that every videographer should be prepared to do--and it’s not that difficult. You’ll know you’re in back-focus trouble if your wide-angle images are soft while the telephoto shots are okay. Here’s how to invest five minutes and save the shoot.
After double-checking that the macro focusing ring is properly seated in its detent position, aim the camera at something with sharply defined shapes or intersecting lines and open the iris fully. Dial in as much filter as is necessary to get the exposure under control. It is critical that the iris remain wide open, so activate the camera’s internal shutter or, if necessary, move to a darker location before continuing.
Next, locate the back-focus adjustment locking screw on the rear of the lens. It’s located on the macro focusing ring, opposite the macro lever, and will be either a knurled thumbscrew or a Phillips-head setscrew. Loosening this screw will allow the entire ring to rotate.
Now zoom in to full telephoto and, using either the viewfinder or an external monitor, focus the lens to attain the sharpest image possible. Zoom partway out and observe when the focus starts to go soft. Hold this shot and rotate the back-focus adjustment ring back and forth as necessary to improve the focus. Repeat this procedure, always first zooming in and refocusing, then pulling out and tweaking the back-focus, until you can zoom to full wide-angle without any shift in focus.
The final step is tightening the setscrew to lock in the newly adjusted back-focus, which must be done with extreme care to avoid changing the setting.