Keeping Your Back-focus in Check

11.20.2006

Have you ever zoomed in, fine-tuned your focus, and then zoomed out only to watch your wide shot turn to mush? If so, you're seeing what is commonly referred to as back-focus error. This unwanted special effect occurs when the macro group, a set of elements at the rear of the lens, is improperly positioned. Back-focus problems are often the result of an accidental bump that moves the macro lever at the rear of the lens or failing to ensure that the macro lever is returned to its detent position after shooting a close-up.

Interestingly, it's possible to shoot for days or even weeks with your back-focus improperly adjusted and never suffer the consequences. That's because minor back-focus errors are mitigated when the iris is stopped down and even then are generally only apparent on wide shots. And then there's Murphy's Law, which says that back-focus problems only occur when you're live on the air or capturing a shot that cannot be repeated.

Correcting back-focus error can be as easy as snapping the macro ring back into the detent position. That's the first thing to check if your wide shots are looking soft. But on professional lenses, that detent position is adjustable. If the screw that fixes the detent (or the entire macro group) in place works its way loose, you'll need to restore the back-focus setting. And while this is something usually done in the shop with the aid of a special chart, it can be done in the field. We'll explain how in the next issue of Sharp Shooters Tips.