Saturday, November 17, 2007

G9 and Hyperfocal Focusing

The small point-and-shoot “digicams” so popular today have a significant depth of field. That is, for a given scene, the distance from the camera that is acceptably in focus is much more than might be expected with, say, a 35mm film camera. In fact, sometimes it seems that everything is in focus. The G9 is no exception to this rule.

At first thought, having everything in focus seems preferable although there can be good reasons for intentionally producing out-of-focus areas. In particular, in landscape photography the camera is often set to produce pictures having deep depth of field. Typically, this is done by setting a small aperture (large f-stop number) such as f22.

The G9 does not have apertures of f22; in fact, the smallest aperture is f8. The G9 has been criticized by some for this apparent lack of smaller apertures but f8 is more than adequate for depth of field. If smaller apertures are needed for exposure then the G9’s internal 3x neutral density filter should be turned on.

In some of my early tests, I noticed that the G9 automatically selected focus distances of about 25 feet whereas the main subject was easily 300 feet away. When I manually changed to infinity focus, there was no obvious change in the picture. How could this be?

The key phrase to understanding this phenomena is hyperfocal distance. Briefly, when the focus is set to the hyperfocal distance then everything from half that distance to infinity will be in acceptable focus. Remember that old camera that did not require (or allow for) focusing? It was built with the lens set at the hyperfocal distance. The focusing instructions for my old Kodak Brownie camera were simply to be certain that the subject was at least six feet away (as I recall).

Some love to debate the exact nature and mathematics of depth-of-field, focusing, focal lengths, enlargements, circle of confusion, etc. An excellent source of information, including computer programs, is For now, let’s just consider that the depth of field depends on the focal length of the lens, the distance that the lens is actually focused at and the aperture.

For the G9 widest angle zoom setting, the focal length is 7.4mm. At 7.4mm with f4 aperture, the hyperfocal distance is about 8 feet. This means that with the camera set for 8 feet everything from about 4 feet to infinity is in focus! No wonder I was confused! For my scene, automatic focusing was selecting about 25 feet – which was actually OK even though the main subject was much farther away.

At the other extreme, the G9 telephoto zoom is 44.4mm. At 44.4mm and f5.6, the hyperfocal distance is nearly 200 feet; everything from 100 feet to infinity is in focus. Therefore, as is known from experience, focusing is much more critical when using telephoto lenses.

So when using the automatic focusing modes of the G9, be aware that automatic focus settings rely on the principle of depth-of-field and make use of hyperfocal distance.

1 comment:

eliburak said...

Interesting observations. I was aware of this but did not realize that the camera would know this and would focus up close. On diglloyds site I recently read an article on how small digicams are diffraction limited well before even their smallest aperture such as f/8. Therefore we're lucky that we get so much DOF at wide open apertures because we shouldn't be using the smaller apertures anyways, excluding aesthetic reasons.

Interesting post.