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Back Focus

Solution Keeping Your Back-focus in Check

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.

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.
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Article details
Article ID: 4
Category: Field Crew Help
Date added: 2012-01-18 23:25:44
Views: 515
Rating (Votes): Article rated 3.4/5.0 (15)

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