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How Focus Works

Although autofocus has been with us since the 1980s (this was when images were captured on film, not solid-state sensors), prior to that breakthrough focusing was always done manually. Even though viewfinders were bigger and brighter than they are today, special focusing screens, magnifiers, and other gadgets were often used to help the photographer achieve correct focus. Imagine what it must have been like to focus manually under demanding, fast-moving conditions such as sports photography.

I don’t have to imagine it. I did it for many years. I started my career as a sports photographer, and then traveled the country as a roving photojournalist for more years than I like to admit. Indeed, I was a hold-out for manual focus right through the film era, even as AF lenses became the norm and autofocus systems in cameras were (gradually) perfected. I purchased my first autofocus lens back in 2004, at the same time I switched from non-SLR digital cameras and my film cameras to digital SLR models.

Manual focusing was problematic because our eyes and brains have poor memory for correct focus, which is why your eye doctor must shift back and forth between sets of lenses and ask “Does that look sharper—or was it sharper before?” in determining your correct prescription. Similarly, manual focusing involves jogging the focus ring back and forth as you go from almost in focus, to sharp focus, to almost focused again. The little clockwise and counterclockwise arcs decrease in size until you’ve zeroed in on the point of correct focus. What you’re looking for is the image with the most contrast between the edges of elements in the image.

The camera also looks for these contrast differences among pixels to determine relative sharpness. There are two ways that sharp focus is determined, phase detection and contrast detection.

Phase Detection

Digital SLRs use an optical viewfinder and mirror system to preview an image (that is, when not in Live View mode). In that mode, the camera calculates focus using what is called a passive phase detection system. It’s passive in the sense that the ambient illumination in a scene (or that illumination augmented with a focus-assist beam) is used to determine correct focus. (An active phase detection system might use a laser, sonar, or other special signal.)

Parts of the image from two opposite sides of the lens are directed down to the floor of the camera’s mirror box, where an autofocus sensor array resides; the rest of the illumination from the lens bounces upwards towards the optical viewfinder system and the autoexposure sensors.

As light emerges from the rear element of the lens, most of it is reflected upwards towards the focusing screen, where the relative sharp focus (or lack of it) is displayed (and which can be used to evaluate manual focus). It then bounces off two more reflective surfaces in the pentaprism or pentamirror, as discussed in Chapter 1. Some of the illumination is directed to the autoexposure sensor at the top of the pentaprism housing.

A small portion of the illumination passes through the partially silvered center of the main mirror, and is directed downwards to the autofocus sensor array, which includes separate autofocus “detectors.” A single detector for a single focus “zone” is shown in Figure 3.3. (Keep in mind that your camera may have 3 to 51 of these detectors, one for each focus zone.)

Figure 3.3. Part of the light is bounced downward to the autofocus sensor array, and split into two images, which are compared and aligned to create a sharply focused image.

A condenser lens directs the image from that focus zone’s region through sets of two apertures where a pair of reconverging lenses focuses the images onto a matching set of light receiving sections in the focus sensor. As the lens is focused, the pair of images gradually converges, as you can see in Figure 3.4, until they merge when the subject (in this case, Chief Pontiac) is in sharp focus. When the image is out of focus—or out of phase—the two images, each representing a slightly different view from opposite sides of the lens, don’t line up. Sharp focus is achieved when the images are “in phase,” and aligned.

Figure 3.4. In phase detection, parts of an image are split in two and compared. When the image is in focus, the two halves of the image align, or are in phase, as with a rangefinder.

As with any rangefinder-like function, accuracy is better when the “base length” between the two images is larger, so the two split images have greater separation. (Think back to your high-school trigonometry; you could calculate a distance more accurately when the separation between the two points where the angles were measured was greater.) For that reason, phase detection autofocus is more accurate with larger (wider) lens openings than with smaller lens openings, and may not work at all when the f/stop is smaller than f/5.6. Obviously, the “opposite” edges of the lens opening are farther apart with a lens having an f/2.8 maximum aperture than with one that has a smaller, f/5.6 maximum f/stop, and the base line is much longer. The camera is able to perform these comparisons and then move the lens elements directly to the point of correct focus very quickly, in milliseconds.

Figure 3.4 shows what happens with a horizontally oriented focus sensor. In an actual dSLR, some sensors may be horizontal, some may be vertically oriented, and some (or all) may be of the “cross” type. You can understand why focus sensor orientation is important if you visualize a particular subject not as a Native American chief, but as a series of vertical lines, as shown in Figure 3.5. With a horizontal sensor, it’s easy to match up the vertical lines so they are in phase, as seen in the figure.

Figure 3.5. Horizontal focus sensors are able to handle vertical lines easily.

However, if, in an absolutely worst-case scenario, a subject happened to consist entirely of horizontal lines, you can see that a horizontally oriented sensor would find it almost impossible to detect when the two images are in phase, as you can see in Figure 3.6. Of course, in the real world, even a subject with predominant horizontal lines would have some vertical information that could be used to achieve focus.

Figure 3.6. Horizontal focus sensors are confused by horizontal lines.

A partial solution is to include some vertically oriented focus sensors, which have the opposite characteristics: they are able to handle subjects with horizontal lines with aplomb, and stumble over vertical lines.

Cross-Type Focus Points

So far, we’ve only looked at focus sensors that calculate focus in a single direction. But what does such a sensor do when it encounters a subject that isn’t conveniently aligned at right-angles to the sensor array? In the past, the “solution” was to include a sprinkling of vertically oriented AF sensors in with the horizontally oriented sensors. The vertical sensors could detect differences in horizontal lines, while the horizontal sensors took care of the vertical lines. Both types were equally adept at handling diagonal lines, which crossed each type at a 45-degree angle. Today, however, most digital SLRs use at least one “multi-function” sensor (often in the center of the array) that has a cross-type arrangement.

The value of a cross-type focus sensor in phase detection is that such sensors can line up edges and interpret image contrast in both horizontal and vertical directions. The horizontal lines are still more difficult to interpret with the horizontal arm of the cross, but they stand out in sharp contrast in the vertical arms, and allow the camera to align the edges and snap the image into focus easily. In lower light levels, with subjects that were moving, or with subjects that have no pattern and less contrast to begin with, the cross-type sensor not only works faster but can focus subjects that a horizontal- or vertical-only sensor can’t handle at all.

Contrast Detection

Contrast detection is a slower mode and used by digital cameras in Live View modes, because, to allow live viewing of the sensor image, the camera’s mirror has to be flipped up out of the way so that the illumination from the lens can continue through the open shutter to the sensor. Your view through the viewfinder is obstructed, of course, and there is no partially silvered mirror to reflect some light down to the autofocus sensors. So, an alternate means of autofocus must be used, and that method is contrast detection. Some cameras do have a mode—usually called Quick mode or Hand-Held mode—that temporarily flips the mirror back down to allow phase detection autofocus, but unless you use that mode, focus must be achieved either manually (with the color LCD Live View as a focusing screen), or by contrast detection.

Contrast detection is a bit easier to understand and is illustrated by Figure 3.7. When the image is brought into focus (top), the transitions are sharp and clear. At the bottom of the image, the transitions between pixels are soft and blurred. Although this example is a bit exaggerated so you can see the results on the printed page, it’s easy to understand that when maximum contrast in a subject is achieved, it can be deemed to be in sharp focus.

Figure 3.7. Focus in contrast detection mode evaluates the increase in contrast in the edges of subjects, producing a sharp, contrasty image (top) from a soft, blurry, low contrast image (bottom).

Contrast detection is used in Live View mode, and may be the only focus mode possible with point-and-shoot cameras that don’t offer a through-the-lens optical viewfinder as found in a digital SLR. Contrast detection works best with static subjects, because it is inherently slower and not well-suited for tracking moving objects. Contrast detection works less well than phase detection in dim light, because its accuracy is determined not by the length of the baseline of a rangefinder focus system, but by its ability to detect variations in brightness and contrast. You’ll find that contrast detection works better with faster lenses, too, not as with phase detection (which gains accuracy because the diameter of the lens is simply wider), but because larger lens openings admit more light that can be used by the sensor to measure contrast.

We’ll look at contrast detection again when we explore Live View modes.

Other AF Options

Some cameras have additional focusing options. Here’s a quick summary.

  • Avoiding endless focus hunting. If you frequently photograph subjects that don’t have a lot of detail (which complicates focus) or use very long telephoto lenses (which may be difficult to autofocus because of their shallow depth-of-field and smaller maximum apertures), you may find the camera constantly seeking focus unsuccessfully. Some cameras have a function that tells the camera what to do when it is having difficulty focusing. You can order the camera to keep trying (if that’s what you really want), or instruct the camera to give up when the autofocus system becomes “lost,” giving you the opportunity to go ahead and focus manually.

  • Show/hide AF points in the viewfinder. If the camera is selecting the focus zones, you may find the constant display of the selected zone distracting. Some cameras allow you to specify whether the AF points, the grid, and other elements are illuminated in the viewfinder with red highlighting under low light levels. Some people find the glowing red elements distracting and like to disable the function. You can select Auto (highlighting is used in low light levels), Enable (highlighting is used at all times), or Disable (red highlighting is never used).

  • Activate/deactivate the autofocus assist lamp. The autofocus assist lamp may be useless in some cases (when your subject is more than about six feet from the camera) or distracting in others (say, at a concert or religious ceremony). Your camera probably has a feature that determines when the AF assist lamp or bursts from an electronic flash are used to emit a pulse of light that helps provide enough contrast for the camera to focus on a subject. You can select Enable to use an attached speed-light to produce a focus assist beam. Use Disable to turn this feature off if you find it distracting.

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