Focus Lock How To

While you can maximize the depth of field with a shorter focal length lens, a narrower aperture and standing back from the foreground subject, it is all for naught if you do not attain focus on the foreground subject itself. Depth of field at narrower apertures does “come forward” a bit from the point of the actual focusing distance, but making sure the foreground is in focus at any aperture is your best bet.

Finding that right point of focus can be a problem when the front-most point of the scene sits at the side or edge of the frame, Care must be taken to either focus on that point using AF and locking focus, or to manually focus the lens on that point. The AF targets in your finder may cover a considerable amount of the frame, and setting single AF mode usually defaults the camera to closest subject focus priority, but even that might not snag the needed focus point in the shot.

There are two ways to insure that you have caught the proper close focusing point. One is to simply switch to manual focus and focus on the foreground subject, then set your aperture to get deep depth of field.

If using autofocus, choose one focusing target, move the camera to grab and confirm that you have focused on the foremost subject, lock focus (using the AFL button--autofocus lock),  recompose as you wish, and shoot.

Many cameras have an AFL button or, in most cases, you can lock focus with light pressure on the shutter release.

Being able to first find focus and hold it, and then recompose as needed may require some practice, but as you gain experience it becomes second nature and allows for a good deal of compositional control.

This photo of a thermal pool at Yellowstone poses numerous focusing problems for any AF setup. The water and mist might well cause the AF system to search back and forth because it lacks a definable (contrast) target. The composition requires focus at the near edges of the pool, so depth of field has to be from 4 feet to infinity.

To attain focus throughout the frame, the AF target in the center of the viewfinder was used and the camera was pointed down so that focus was confirmed (by a beep or lighted target) at the closest point; focus was then locked. Using this technique, a very narrow aperture with a very wide lens delivered depth of field from the edge of the pool to the row of trees in the background.

Settings: 20mm lens: at ISO 200, first, focus was set and locked at the lower edge of the frame, then exposure settings were made by taking a spot reading from the mist area with a +1 EV exposure compensation. f/22 at 1/60 second.

It is critical in this photo that the tree and rocks on the left edge of the frame are sharp. That made this area the important starting point of focus within the frame. To accomplish this, focus was locked on the foreground rock in the very lower left of the frame. A spot exposure reading in aperture-priority exposure mode set at f/11 was taken of the blue area of the sky and locked. With a 28mm lens, depth of field ranged from 5 feet to infinity.

Settings: 28mm lens, at ISO 200, f/11 at 1/250 second. Focus lock on lower left corner; exposure lock using spot metering off the blue area of the sky.

Creative Focus: Part 1

At first glance it would seem that focusing is a simple matter. You look through the lens, press the shutter release button to activate the autofocus mechanism and make the photograph. Or, when using a manual focus lens, merely turn the focusing collar on the lens until the image seems sharp in the viewfinder, and then press the shutter release. This would work fine if we lived in a two-dimensional world. Yet subjects in most scenes sit different distances from one another. Some may be close together while others could be miles apart. Working with focusing techniques you can either make subjects at great distances appear sharp within the picture or have a background that is as close as a foot to your subject appear unsharp.

Focusing is one of the most important creative options. The technique for creative focusing play is called selective focus or depth of field, and it is based on the idea of “circles of confusion”, or what our eyes perceive as sharp and unsharp in an image.

Think of a magnifying lens slanted toward the sun and a piece of paper receiving the rays of light through the lens. As you move the lens back and forth the rays from the sun form a circle or a point on the paper. When a lens is mounted on a camera it directs light toward the film or sensor. Those rays that converge on the film or sensor at a point, or at a near point, are what we perceive as sharp in the image. Those that form a circle beyond a certain diameter are perceived as unsharp in the image.

Our eyes tolerate a certain diameter of circle, or “blob” as being sharp. We do this at a certain distance and with a certain degree of magnification of the image. Change the viewing distance and/or the magnification and what appeared as sharp might later seem unsharp. That’s why when we enlarge prints that looked good in a snapshot size print (4 x 6 inches) to 11 x 14 inches the print may look slightly unsharp. Close inspection of image before enlargement can help prevent this problem.

Next: Viewfinders and Live View

Photo and Text copyright George Schaub 2010. Depth of field controls set up what is sharp and unsharp in the photograph. A deep depth of field, as shown here, means the eye perceives everything from front to back as in focus, or sharp.