The standard definitions of focus are: 1) The position at which rays of light from a lens converge to form a clear and sharply defined image on a focal plane. 2) The action of adjusting the distance between the lens and subject to make light rays converge to form a clear and sharply defined image of the subject.
These definitions are fine. But they're merely definitions of a unit without explanation of how it exists. If the PJ can't distinguish "sharp" from "soft," the PJ will always have a problem. Let's fix the problem.
If you're on the edge of your seat, please get a life. However, it's critical for PJs and advanced amateurs – especially when we discuss advanced issues. I'll try to boil quantum physics down to simple terms (as if gravity is simple), so cut me some slack on the comparisons. The point is to understand the general theory.
Before we get too deep into the mechanical aspects of focus, let's understand where focus occurs. Technically, it's the point where rays of light converge. Each point of convergence is called a focal node. A set of focal nodes makes the focal plane. The focal plane is inside the camera body next to the film plane, which can occasionally be on an airplane. :-)
PJs need to know where these points occur for complicated issues. However, we'll generalize today. Few PJs actually care about the focal plane. It's a constant (until something goes wrong). The real concern for PJs is the "plane of focus."
Yes, they sound alike, but they're not the same. The focal plane is on the tripod and the plane of focus is somewhere between the lens and infinity. Techno-geeks made up the names or these would have edgy names like "the red zone." So, we're stuck with two drastically different, yet similar-sounding, concepts of focus. Live with it.
Circles of confusion
Sharpness is determined by the diameter of "circles of confusion." Those areas of an image with the smallest circles of confusion are said to be "sharp." Those areas of an image with the widest circles of confusion are said to be "out of focus." Near misses (and bad glass) are "soft."
The diameter of a circle of confusion is determined by many factors. Among these factors are aperture, the positions of lens elements within a lens, glass quality and subject distance (from the area of focus).
Even if the circles of confusion are tight and sharp when captured, they can be further degraded on an enlarger or in a scanner. Consequently, attention to detail must be maintained from image capture to the final output, or the image can lose enough sharpness at each step to make the final image weak.
Cone of light
Most folks have used a magnifying glass or other convex lens element to direct (refract) sunlight and burn symbols onto a leaf or piece of paper. Through this exercise, we learn the closer the lens element (magnifying glass) is to the substrate (a leaf or paper), the wider the circle of light is.
As a convex lens element moves away from the substrate, the circle of light has a smaller diameter. The circle continues to become smaller until it hits a rough focal point and the rays of light converge. After the focal point, the circles begin to grow in diameter again as the lens element moves further from the substrate.
We learn to adjust the distance of the lens element incrementally from the substrate to find the focal point. Once we find the focal point, the light rays converge and create intense heat in a specific location. Then, the heat burns the substrate. For most folks, this is considered to be the point of focus.
But it's not.
The leaf-burner has taken straight sunlight waves and used a single convex lens element to produce a cone of light. Somewhere near the tip of this cone, light converges enough to create adequate heat to burn the substrate. Without mechanical devices (measured in nanometers), true focus can't be consistently achieved this way. Focus is soft but effective.
Inside a camera, this point of convergence is along the focal plane. Light has been refracted through the lens elements of the camera and brought to a set of focal nodes along the focal plane. Even if the plane of focus (in front of the camera) is set at a position where nothing is visibly in focus, the focal plane is still accurate because it's calibrated and in a fixed position (unless the camera was recently dropped... off a cliff).
Plane of focus
We've established the focal plane is inside the camera at a fixed position. Next, we must understand the "plane of focus." We understand the plane of focus is an area between the front of the lens and infinity.
Therefore, this plane is obviously mobile. It is the area PJs generally call "focus" in a scene. It is the flat area within the scene with the smallest circles of confusion as determined by the PJ's placement of lens elements (definition two). Again, I'll avoid addressing the vast array of precision glass elements, electronic components, gears
Hourglass of light
As the magnifying glass example shows, light directed through a simple, convex lens creates a cone of light leading to a finite point of convergence (a focal node). Once the light passes this finite point it again diverges.
Therefore, focus is the point (or node) of convergence between two cones of light. Visually, it's the pinch-point of a simple hourglass.
As sand can be controlled through hourglasses, light can be controlled through high-quality lenses
An hourglass with a small opening and fine-quality sand (smaller particles) is most accurate. Likewise, a lens with a small aperture and fine quality glass (lens elements) creates the smallest circles of confusion and is most accurate.
Life is good so far. Now it starts getting crazy because light doesn't play by normal atomic rules. Light is both waves and particles. Furthermore, gravity doesn't affect it as much as other particles. Consequently, it reflects in all directions simultaneously
So we must stop thinking about the sand and concentrate on the actual hourglass' shape. Furthermore, gravity no longer applies because PJs basically lay the hourglass on its side for photographic purposes and it still works fine.
I warned you.
If focus is the pinch point, the diameters of circles of confusion can be measured in relation to their proximity to the focus point. In other words, the further away from focus an image element is, the larger the circles of confusion are and the more out of focus it appears.
Eventually, the circles of confusion become so large and overlap so much that they become negligible (this is why the dust on the lens and blades of grass in front of the lens don't show up in the final image).
As the entire image is only a complicated pattern of overlapping circles, it's important to know where the smallest circles of confusion are. This area is focus.
Out of focus
So far, we've determined what and where focus is. It's the area with the smallest circles of confusion in a plane of focus somewhere in front of the lens. On future entries, we'll discuss how to make the plane of focus deeper through depth of field. But for now, we're trying to determine the actual area of focus.
To do so, we must identify what is not in focus or out of focus. With a good lens, something is always in sharp focus. However, it may not be placed exactly where the PJ wants or expects. Likewise it can be somewhere outside the viewing range within the scene.
Although everything outside of sharp focus is called "out of focus," PJs need to know the difference between out-of-focus and not-in-focus to understand the cause of a focus problem and how to avoid it.
While I wrote this, I attempted to address both manual and autofocus cameras. It would probably confuse some people. So, I'll save the autofocus discussion for another day. Let's just keep it simple and say this is a plain manual SLR camera with a plain manual lens in bright sunlight (to avoid the second definition of "soft").
Not in focus
PJs manually move the lens elements to an alignment by turning the focus ring of the lens. The PJ either uses a split prism to align objects at focus or uses her/his eyes to proximate focus on a ground glass viewing screen within the viewfinder (technically within the pentaprism).
When the PJ has the subject in focus, s/he presses the shutter release button and a latent image is recorded on film or disk.
However, when the film is developed, the PJ may notice the subject's eyes aren't in sharp focus. Instead, the subject's ear is in focus. The part of the frame expected to be in focus is "not in focus." The circles of confusion are larger than expected and another portion of the scene recorded the smallest circles of confusion.
Again, this is easily determined because some other part of the image area is sharply focused. Frequently, the word "soft" is used to explain this condition.
Out of focus
This happens for both manual and autofocus cameras for different reasons. Again, we'll only consider the manual focus explanation.
"Out of focus" occurs when nothing visible in the scene is in focus. Typically, this occurs when the lens elements align the plane of focus closer to the lens than any subject within the scene. It can also occur when the plane of focus is set behind the subject within the scene.
In both cases, the plane of focus simply is outside the range of the subject area and/or background. If any subject was visible where the plane of focus occurred, it would be in focus. However, in this case there is no portion of the scene aligned within the plane of focus.
We'll discuss the causes and remedies for both of these problems soon.
Enough for now,