Terminology Alert!
Do not confuse depth of field with depth of focus. The terms are not interchangeable.
Depth of Field is the distance from the nearest point in the scene that appears in acceptable focus to the farthest point in the scene that appears in acceptable focus.
Depth of Focus is at the other end of the business – at the film plane. It is the distance the focal plane can be moved forward or back within the camera, and still record an acceptably sharp image from the lens. It is quite independent of Depth of Field.
Let the equipment manufacturer and repair technician worry about Depth of Focus.
Photographers worry about Depth of Field.
From the practical photographer’s vantage, the things that really affect Depth of Field are lens aperture (f/stop) and focusing distance.
I have long questioned the proposition that shorter focal length lenses produce greater Depth of Field than do longer lenses if we keep lens-to-subject (focusing) distance and f/stop constant. What shorter focal length lenses surely do produce is an image in which more details are included, but the details are at smaller size. Blow the details up, and Depth of Field (as well as general sharpness) drops off.
What different focal length lenses really produce, in my view, is different perspectives.
All that out of the way, here’s a list of my Quick and Dirty (Q&D) focusing rules:
1) Focus on the primary subject.
This is the first rule, always. The limits of Depth of Field – DOF -- are usually defined by acceptable focus. Remember the proviso, “acceptable focus” – and remember the internet warning, YMMV.
2) DOF varies with aperture and focusing distance.
This is a complication. The smaller the lens opening, i.e., the higher the f/stop number, the greater the DOF. But the closer the focusing distance, the smaller the DOF. Maximizing DOF becomes a real balancing act.
3) Use a smaller aperture to extend DOF.
Smaller apertures produce greater DOF at any given focusing distance. So if we want to maximize DOF, then we minimize lens aperture diameter. Period.
And as a practical matter we don’t worry a lot about loss of sharpness due to diffraction. Most discernable sharpness loss at smaller apertures is due to camera or subject movement, both of which are aggravated by the lower shutter speeds that “stopping down” the lens aperture dictates.
That said, I most like the f/5.6 – f/11 range on most of my own lenses.
4) Learn and use the 1/3 - 2/3 rule.
Basically, this rule says that at any given aperture and focusing distance, the total DOF lies 1/3 in front of and 2/3 behind the actual point of focus. Some old photo manuals gave the rule as 2/5 front and 3/5 back, but that’s quibbling.
Point is, to maximize DOF we are advised to focus 1/3 (or 2/5) of the way into the scene, and hope for the best. Sometimes this works OK. But sometimes it doesn’t in dealing with subjects that have considerable depth – especially at closer focusing distances, which narrow the DOF.
5) Do learn the hyperfocal distance trick.
But don’t obsess over it.
The rule says that hyperfocal distance at any given aperture is the nearest point in the scene at which the focus is acceptably sharp, when the lens is focused at infinity. Focusing at the hyperfocal distance then produces sharpness that extends from halfway between the camera and the point of focus, out to infinity.
This is the 1/3 - 2/3 rule in play, combined with the principle that the greater the focusing distance the greater the DOF.
6) Do learn the Q&D approach to using hyperfocal distance.
As a practical matter, just focus first at infinity, and then “back off” the focus a tad so the point of focus is a bit short of infinity. The smaller the aperture the farther you can back off the focus.
7) Do recognize the limitations of using hyperfocal distance.
HD focusing works in "scenic" photography -- making pictures of vistas that reach to the distant horizon. For more intimate scenes, use the 1/3 – 2/3 rule.
G.
