IntroSensorLens • Focus • ExposureRecordingExamplesComparePhones

Isaiah 40:26, "Lift up your eyes on high" (East transept),
Mission San Luis Rey de Francia, Oceanside CA, 2012.
Canon 5D Mark II,
Canon EF 17-40mm f/4 L @ 40mm
f/16, 1.6 sec., ISO 400

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Photographic Image Quality:
Focus

No image—regardless of the camera and lens quality—will be successful if it is not accurately focused. This does not mean that the entire image has to be sharp, but the most important parts of the image must always be sharp. In fact, out-of-focus areas are frequently desirable for aesthetic reasons, and overall sharpness might not even be physically possible. The camera simply can’t make these decisions accurately every time, no matter how sophisticated its autofocus system might be.

Fixed focus

The smallest compact “point-and-shoot” cameras do not offer variable focus lenses, automatic or manual. The most obvious reason for this is lack of space for all of the required mechanism. The less obvious reason is that their extremely short focal length lenses, mounted on a miniature sensor, can be designed to be in focus from a specified close distance all the way to infinity. (This will be explained in more detail under “exposure -> aperture” on the next page.) The closest possible point that will be in focus depends on the zoom setting—wider will be closer than longer. The only user responsibility is to consult the camera manual and keep the camera far enough away from the subject for the selected focal length.

Autofocus

In all fairness, modern autofocus cameras and lenses make a photographer’s life much easier—allowing faster, more accurate shooting while easing the burden on those of us with aging eyes. To understand their limitations, we need to be aware of two facts of physics: 1) only one plane, perpendicular to the lens axis, can be in perfect focus at any one time, and 2) it is not always possible for the autofocus software to find anything to focus on.

▸ Autofocus points: many cameras offer a number of points within the visible frame that can be used to set the focus. Like mega-pixels, the number of such points can be a marketing tool: my older high-end DSLR camera features 9 autofocus points, while the latest (much more expensive) version of the same camera has 61! If you are doing high-speed sports photography or video, this is important; otherwise it doesn’t make much sense. For hand-held still photography, I set the autofocus to use only the center point, press the shutter halfway to lock focus where I want it, then re-frame and shoot. Many cameras also offer multiple autofocus modes for single-shot or continuous shooting—again important for some tasks but not all.

▸ When autofocus fails: although not all autofocus system use the same technology, in general they are “looking” for edges—areas that have strong contrast between light and dark elements of the image. Thus they will struggle with images that contain very little difference between their light and dark values. On the other hand, the system might find good contrast in an area that has nothing to do with the real subject of the picture. In these cases, the photographer must step in to help out.

Manual focus

Most autofocus cameras also allow the user to manually focus the lens, in one of two ways—by “tweaking” the focus after the auto has finished, or by turning the auto off altogether and just using the focus ring traditionally. Switching between the two methods is done on the camera with some systems and on the lens with others. In either event, most cameras also provide some sort of manual focus assistance, which could be by enlarging a portion of the image in the viewfinder or LCD screen, or by displaying a specific color to indicate which edges are in focus (or both). Manual focus is most useful with with static subjects (especially with the camera on a tripod) where precise focus is critical to the success of the image—and of course in cases mentioned above where the autofocus has problems.

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