Exploring focusing options

Autofocus made the difference in getting this shot of the U.S. Navy Blue Angels. Taken with an EOS-1Ds Mark II, EF 100-400mm f/5.6L lens at 400mm, ISO 100, 1/1000 second at f/5.6.

Today’s modern lenses offer photographers amazing optical quality, and whether you choose to focus using one of the automatic focusing modes or to focus your lens manually, you can obtain razor-sharp images with the lineup of Canon lenses. Focus can be defined as the point at which light rays from the lens converge to form a sharp image. This is achieved by adjusting the lens or the distance between the lens and the subject. I usually use some sort of autofocus (AF) as I know my eyes aren’t as accurate as Canon’s AF, but I do manually focus in some situations where AF tends to fail or become a hindrance. In 3-9, you see how AF can render even a fast-moving subject tack sharp; in 3-10, I used manual focusing as the closeness of the subject and the shallow DOF made using AF mode impractical.

The 24-70mm f/2.8L also serves as a great macro lens as evidenced by this photo of an Iris. Taken with an EOS-1Ds Mark II, ISO 800, 1/320 second at f/2.8.

AUTOFOCUS SYSTEMS

Autofocus (AF) systems rely on one or more sensors to determine correct focus. Some Autofocus systems rely on a single sensor, while others use an array of sensors. Most modern digital SLR cameras use through-the-lens optical AF sensors, which also function as light meters.

The speed and accuracy of through-the-lens optical autofocusing is now more precise than what you can achieve manually with an ordinary viewfinder. Autofocus accuracy within a third of the depth of field at the widest aperture of the lens is not uncommon in professional Autofocus dSLR cameras. There are, of course, some situations where manually focusing provides better results than the Autofocus system and I explore these as well.

Most multisensor AF cameras allow manual selection of the active AF sensor, and many offer automatic selection of the sensor using algorithms that discern the location of the subject. Most AF systems are now able to detect if the subject is moving towards or away from the camera, including speed and acceleration data, and stay focused on the subject (AI Servo mode) — a function used mainly in sports and other action photography. Take a look at your autofocus options with the Canon dSLRs.

  • Focusing points.All of the Canon dSLRs have focus points (ranging from 7 to 45 depending on the model) that you can manually select. By selecting the Autofocus point(s), you can decide what part of the image will be selected for sharpest focus, which can change depending on the shooting situation.On the EOS 40D, the center Autofocus point is a high-precision cross-type sensor that is twice as sensitive to both horizontal and vertical lines of the subject as the rest of the AF-assist points. This combination assures extremely accurate focusing in most situations. The other Canon dSLRs use different combinations of the same technology depending on the model.The Mark III features 26 AF-assist points that help pinpoint areas for increased focusing accuracy. Also, with its greatly increased low-light capabilities, it can lock on to objects and focus on them with much less light than other cameras, including the previous Mark II series.
  • Focus lock. All of the Canon dSLR cameras are equipped with a focus-lock feature. This allows you to lock the focus on a particular subject or subject area, and then recompose prior to releasing the shutter. Different cameras have different numbers of AF points. You accomplish this by selecting the desired AF point (as shown in figure 3-11), focusing on the subject, and then pressing the shutter release halfway to lock the focus. Once focus is locked, keep the shutter pressed halfway to maintain focus, recompose, and shoot; it is that easy.

A diagram of the 45 AF points as seen in a 1D Mark III viewfinder; this wide array of points allows you to lock in on virtually any aspect of a subject to ensure a sharp image.

MODES

By selecting different AF modes, you can further fine-tune the AF operation of your camera for specific shooting situations. You would rarely want the same AF style for a hockey game as you would for landscapes or product photography.

  • One Shot AF. Used primarily for still subjects. Pressing the shutter release halfway activates autofocus and achieves and locks focus. With evaluative metering, the exposure setting (aperture and shutter speed) is set when focus is achieved. The focus and exposure setting is locked as long as you continue to keep the shutter release pressed halfway. One Shot AF was used to get a tack-sharp image with the stationary subject shown in 3-12.
  • AI Servo AF. Used when dealing with moving subjects where the focal distance keeps changing. When the shutter release is pressed halfway, the AF mechanism is activated and the camera focuses continuously. The exposure is set at the moment the shutter is released. I used AI Servo AF to make sure I got the shot in figure 3-13.
  • Predictive AF. If your camera has Predictive AF, you have even more fine control over your camera’s AF operation. If the subject approaches or retreats from the camera at a constant rate, the camera tracks the subject and predicts the focusing distance immediately before the picture is taken. With a manually selected AF point, the selected point tracks the subject and refocuses as needed.
This unusual sign for a sardine factory sits right on the highway outside of Prospect Harbor, Maine. Taken with an EOS-1Ds Mark II, EF 70-200mm f/2.8L lens, ISO 100, 1/400 second at f/9.

AI Servo AF mode allows me to track fast-moving subjects and capture them in stop-action images, such as this U.S. saber fencer attacking a Canadian opponent at the 2007 Pan American Games in Rio de Janeiro, Brazil. ISO 640, 1/640 second, f/2.8, taken with an EOS-1Dn Mark II using a 70-200mm f/2.8L lens.

MANUAL FOCUSING

You may wonder why you should ever bother using the manual focus (MF) feature when your digital camera has such a sophisticated autofocus system. Isn’t AF much easier to use and doesn’t it always provide the right focal distance for your photos? The fact is, autofocus isn’t perfect. Because an autofocus system looks for patterns onto which it can lock on to focus (such as contrasting lines, patterns, etc.), difficult lighting situations, subjects that are very far away, macro photography, or conflicting focal points may result in your digital camera autofocusing incorrectly or being unable to focus at all. Often you have to compensate by using your manual focus ring. Also, you may want to intentionally blur a photograph for artistic effect. In this case, you need to slightly defocus manually. Here are some of examples where MF is apt to deliver better results:

  • Low contrast subjects. Blue skies, solid-color walls, and so on
  • Subjects in low light. Night ballgame or a concert
  • Extremely backlit or reflective subject. Car with a reflective body
  • Repetitive or conflicting patterns. Skyscraper windows, computer keyboards, and so on
  • Overlapping near and far objects. Animal in a cage
  • Macro photography. Close-ups of flowers, stamps, insects, and so on.

In situations such as these you can change from AF to MF by using the switch located on the lens barrel. I preferred to use MF for the image in figure 3-14.

I wanted more of a soft focus effect in this image of a bride looking out the window just before her wedding. Taken with an EOS-1Ds Mark II, EF 24-70mm f/2.8L lens, ISO 400, 1/60 second at f/8, 580EX Speedlite

Category: Photography System

Comments are closed.