Types of Autofocus


All of today's cameras use some form of autofocus, where the camera automatically detects the distance from the camera to the subject and sets the focus on the fly. With each generation of new cameras, engineers continue to advance the intelligence and capability of digital autofocus, transforming it from a clumsy yet interesting concept into a functional piece of photographic technology. Having said that, although the technology has improved greatly, it is by no means automatic or foolproof. It's important to understand a bit about how the focusing system in your camera works, so that you can get the best results and avoid focusing mistakes.

The primary types of autofocus are known as active and passive systems. Active autofocus sends out an infrared signal that is reflected by the subject, allowing the system to calculate the distance to the subject and set the focus. Passive autofocus systems determine the subject's distance from the camera based on the light and contrast in the subject matter.

Passive autofocus systems are less desirable in that they are limited by the available light; they have trouble focusing if there is little or no contrast in the subject matter or if the scene is too dark. Active autofocus works well in most instances, although its infrared signal can prohibit you from focusing through glass windows or other transparent objects.

Autofocus cameras define active areas within the viewfinder that act as focusing zones. When one of these areas is active, it reacts to areas of contrast and edge detail, reflecting back information to both passive and active systems for the reading. The number, placement, and sophistication of these focusing zones vary based on manufacturer and price point of your camera, but it's safe to say that all autofocus implementations are a marked advancement from where we were just a few years ago.

 Montalcino Ring
Nikon D2H
1/100 sec, f/10
EV -0.33 ISO 640
Focal length 75mm

 Olivetto Trees
Nikon D2H
1/80 sec, f/4.5
Focal length 200mm

 Duomo Statue
Nikon D2H
1/500 sec, f/4.5
EV -0.33
Focal length 75mm

PHOTOtip: Blurring on Purpose

Remember that it's not always necessary to keep everything tack sharp in order to create a great image. There are times when I intentionally create images that are soft, and sometimes blurred beyond recognition.

For example, many museums in Europe don't allow flash photography or tripods (even though they do allow you to use your camera on its own). I think it's their way of allowing tourists to get their snapshots without giving up anything good to the professionals. Thus, if I'm standing in front of a great sculpture and I don't have enough light to capture a sharp image, I'll introduce some intentional camera shake to make the image even more abstract. If I'm going to get camera shake, I'm at least going to get shake that's interesting. In the example shown here, I was in the museum of the church of Santa Croce in Florence, shooting sculptures from the original façade.

Intentional blur is an obvious way to imply motion within the image. A tripod-mounted camera with a slow shutter speed can keep the static portion of the scene tack-sharp, while adding blur to the moving subject. This approach allows you to blur just the subject, while avoiding the inevitable camera shake that comes from hand-holding a slow shutter setting.

In other instances, I experiment with abstraction by setting the camera to a very slow setting and shooting out the window of my car while the car is moving. Now, I don't necessarily recommend that you do this if you're the driver (I always have someone else at the wheel as I'm shooting these kinds of images). With some experimentation, you can achieve some interesting, fluid abstractions that would be very difficult to emulate, even with Photoshop's filter set. In this second example, I was shooting at 1/13 of a second at f/25, allowing for a well-exposed image with a great tonal range, in spite of the extreme abstraction.

Experiment with intentional blur in low-light conditions.

Motion-induced abstraction.


Controlling Autofocus

The skill you need with the auto focus technique is being able to quickly identify the desired focus point in your image and select the nearest focusing zone in the viewfinder. If the subject is not aligned with the focus zone of the viewfinder, the camera will focus on the background or the foreground, and the subject will be out of focus.

If your camera is older or more of a point-and-shoot model, you might not be able to select specific focusing zones, although your owner's manual should outline where the focusing zone is and how to optimize focus. In many instances, you can place the subject in the center of your viewfinder, hold the release button halfway to focus, and then recompose your image before pressing the release button the rest of the way down to take the picture. This approach to focusing is similar to a feature called Focus Lock, which is available on many advanced cameras. Instead of pressing the shutter halfway, Focus Lock allows you to center the subject and press the Focus Lock button to set the focus, then recompose the scene as desired.

Digital SLRs and advanced all-in-one digital cameras allow you to select which focusing zone is active as you're shooting. Some Canon models use eye-tracking technology that allows you to simply look at a zone in the viewfinder in order to activate it (the camera tracks your eye movements to determine the focus zone you want to use). Nikon has a button that toggles through all the available zones. Most systems work well with a little practice, provided that you understand what it's doing.

 Duomo Scaffold III (detail)
Nikon D2H
1/250 sec, f/7.6
EV -1.33
Focal length 78mm

 Duomo Scaffold III
Nikon D2H
1/250 sec, f/4.5
EV +0.33
Focal length 75mm

When to Switch to Manual

Although autofocus can be a life saver in certain situations when you need to react quickly to get the shot, you can also see that it has its limitations. As a result, be mindful that you can switch your SLR camera from autofocus to manual mode to set the focus exactly where you want it by adjusting the focus ring on the lens barrel. (If your camera doesn't offer a manual focus feature, it's a good reason to consider upgrading to a more advanced model.)

JOURNALentry: A Night in Siena

Porta Romana, Siena

Santo Spirito

While in Siena, I stayed in a small hotel just inside the old walls of the city, close to the massive southern gate known as the Porta Romana. I estimated its thick stone walls, rising several stories high, to be at least 1520 feet thick. I struggled with how I was going convey this monumentality in a single shot, and didn't even point my camera at it for the first few days of my stay.

I was returning from a day's shooting in San Gimignano when I saw the edifice illuminated at night for the first time, and realized that this was the perfect way to shoot it. The darkness enveloped the surrounding distractions, and the tourists and other pedestrians were at a minimum, clearing the stage for my close-up of the Porta Romana.

I set up my tripod and cable release and began shooting test exposures. I confess that in my film days, I would have pulled out my handheld meter and taken spot readings across the scene. With the immediate feedback I get from digital, I now start with an educated guess and tweak the results as needed. This is a situation in which you have to check the histogram in the camera's LCD so that you can be sure you're getting the proper exposure.

The first exposures were shot from the side, just to get the settings right, although I knew all along that I needed to set up in the middle of the road to get the composition and symmetry right. I ended up using a wide lens set to 12mm, with an exposure of 2.2 seconds at f/4. As I set up and triggered the shutter, a single Vespa motorbike appeared and drove past me, creating the perfect light trail to punctuate the composition.

I shot several compositions that night, including the shot of the church of Santo Spirito. In this shot, I created en extreme angle of view by setting the tripod on the lower steps in front of the church, and setting an extra long exposure to exaggerate the color and to pick up detail in the sky. The image shown is straight from the camera, with no color or exposure adjustments to create the dramatic effects. Once again, I used the 12mm focal length, with a 13-second exposure at f/4.


Low Light

Low light is probably the most common reason for switching to manual focus. Passive systems require enough light to bounce off your subject and reflect back into the system, and if they don't confirm a focus point, they usually don't allow you to shoot the image. In these circumstances, the only way to shoot an image may be to switch to manual mode and do your best to focus on your own. Although active systems create their own beam of light for focusing, the beam can misfire and hit the wrong area of the image. Although the active autofocus camera will let you shoot, the results from the misfired autofocus beam might not be visible in the dim viewfinder. Thus, with both active and passive autofocus systems, it's a good idea to take your time in low-light situations and switch to manual as necessary to get the shot right.

No Edges

Autofocus systems look for contrasting edges to determine where the subject is in relation to the camera. If you're shooting a soft or muted subject, such as a foggy meadow or diffuse sky, the camera might not be able to find enough edge contrast to set the focus. In these cases, you should switch to manual and set the focus yourself.

Some autofocus systems detect only horizontal or vertical edges and might have a problem if the appropriate edges are not visible to the focus sensor because of a lack of contrast. Problems with edge orientation can be avoided by turning the camera on its side while focusing. In either event, you can focus on another subject at the same distance and use the Focus Lock feature or switch to manual focus.

 Montepulciano Altar III
Nikon D2H,
1/60 sec, f/4.5
Focal length 75mm

Glass and Transparency

As mentioned previously, glass presents a problem for both passive and active focusing systems because both systems tend to get fooled by the reflections off the glass. There's not much middle ground on this one: Switch to manual whenever you're shooting through glass and set the focus as needed.




The Art of Photoshop for Digital Photographers
The Art of Photoshop for Digital Photographers
ISBN: 0672327139
EAN: 2147483647
Year: 2006
Pages: 141

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