Focus, Focus, Focus

By Ken VanBree

Most modern cameras today have auto-focus lenses that for the most part do an excellent job of providing well-focused images under standard lighting conditions. However, there are three main ways in which the auto-focus mechanism on modern cameras can be fooled: low light, low detail and objects at widely varying distances from the camera. To get the best results from your digital camera, it helps to know the limitations of auto-focusing in general and of quirks related to your particular camera.

When taking construction images, getting the image properly focused is second only to composition in determining the quality of the result. PhotoShop can do wonders to recover lost detail out of shadows and to correct the color balance of your image, but if the picture is out of focus, there is nothing you can do to improve the sharpness after the fact. If the image is just a bit blurry because the focus wasn't tack sharp, you will still be able to use the image for most purposes, but if it is severely out of focus, it is equivalent to not having an image at all.

The first situation that can fool most auto-focus mechanisms is low light. Most digital cameras can get acceptable results with shutter speeds up to two seconds, allowing the digital sensor to pull detail out of rooms that are almost too dark to walk around in without a flashlight. But although the camera may be capable of taking an excellent photograph in low light, the auto-focus mechanism may not work in those same conditions. One way around this problem is to bring in additional light, like a flash or a portable work light. That might allow the auto-focus mechanism on your camera to work properly, but can cause other problems with non-uniform lighting of the scene. Another method is to set the focus manually if your camera allows you to do so.

The second situation that can fool your auto-focus mechanism are subjects with little detail, such as new sheetrock walls. In this case you are shooting a uniform field of gray with only the screw-heads sprinkled throughout the scene. If your camera's auto-focus mechanism uses one or more small areas of the image to determine focus, it may have trouble focusing on the uniform gray area. You can help the camera by putting something with more detail, such as a tape measure, in the scene. But if that doesn't solve the problem, you may need to resort to manual focusing.

Even if there is plenty of light and plenty of detail, an auto-focus system can give poor results if there are objects at widely different distances within the scene, such as shooting framing before sheetrock. If the camera focuses on the nearest object in the scene, the farthest objects may be out of focus, or, if the camera focuses on the farthest object in the scene, the nearest objects may be fuzzy beyond recognition. To solve this problem you may again need to switch to manual focus and set the focus point to somewhere between the nearest and farthest, what photographers call the "hyper focal distance." The hyper focal distance will achieve maximum sharpness for all objects located between one-half the focal distance and infinity. For example, the hyper focal distance of a prosumer digital camera with a 24mm wide-angle lens set at f/8 is 11.8 feet. At these settings, all objects between 5.6 feet (11.8 divided by 2) and infinity should be acceptably sharp. This means that if you are standing in the middle of a room that is 12 feet square with 12-foot ceilings, you can set your camera at f/8, manually focus your lens at 11.2 feet and shoot in any direction, and your photos will be well focused.

The table below shows the hyper focal distance for various f-stop and lens combinations, where images should be acceptably sharp from one-half of the distance listed in the table to infinity. Notice that the use of wide-angle lenses at small apertures (i.e., large f-stop numbers like f/8 or f/11) produces the shortest hyper focal distances.

July 2005 Builder Architect Edition Issue

| Permalink