For a variety of reasons, your backfocus can be messed up. Anything from using someone elses camera to swapping out a lens. I’ve had to adjust my backfocus in the field several times.
Put the camera on sticks. Zoom in and focus on something really far away. Tree branches, a fence, anything with nice fine/thin lines. I also like to use my neutral density filters to make the focus point as critical as possible. Once you’ve zoomed in and focused, zoom back out wide. If you’re picture is soft, you have to loosen (but don’t remove!) the set screw at the back of your lens and then twisting that ring, not the normal focus section of your lens, adjust your focus. Again, look at the wide shot in your viewfinder and find nice fine lines in the picture to help you set a good focus. Once you have the picture focused, tighten that set screw you loosened. Be gentle. It’s easy to move that ring out of focus a bit when you are re-tightening the screw. Zoom back in to the same point you focused on before. Focus and pull wide again. Your picture “should” be nice and crisp. If not, repeat the procedure again. Sometimes you have to do it twice to “be sure”. You’ll be surprised how easy it really is to set your own back-focus.
It’s a lot easier on the eyes to do this in the engineering shop with a chart and nice big color monitor. The problems I’ve had with some shop-set back-focus is most shops aren’t very big so they set the chart up only ten feet away or less. With the chart that close, you may still end up with a “soft” wide shot when you get back to the field.
Remember, the further away that chart or point of focus is, the better you’ll be when setting your back-focus on the lens.
November 19, 2001 by: Dinosaur
What Lensmith does works in a pinch, but for better accuracy I would recommend setting backfocus in a lower light setting. Outdoors your depth of field may be too great for an accurate backfocus adjustment. Use the same proceedure that Lenssmith outlined, but work in a setting where your iris is nearly wide open. If you don’t have a resolution chart you can use a full page newspaper ad with lots of detailed vertical and horizontal lines hung at about 20 feet from the camera.
Try to use a decent color monitor when making the adjustment so that you are seeing both chrominance (chroma) and lumeinence (brightness). By using the viewfinder for adjustment reference, you can be fooled by a misadjusted viewfinder diopter lens.