This article is a part of a series about choosing a digital SLR camera:

• Choosing a Digital SLR Camera – Part 1: The basics about a Digital SLR Camera
• Choosing a Digital SLR Camera – Part 2: Resolution
• Choosing a Digital SLR Camera – Part 3: Auto Focus
• Choosing a Digital SLR Camera – Part 4: Sensor Size
• Choosing a Digital SLR Camera – Summary

Auto Focus

Following megapixel specification, the next major feature that manufacturers list would be the auto-focus performance of the camera.

Most modern DSLRs come with an auto-focus sensor that allows us to quickly lock focus on a particular area of a scene. It is highly convenient and incredibly accurate in most cases. The sensor in DSLR systems are considerably better than those of portable digital cameras as these sensors because of their design. DSLR auto-focus sensors (AF sensor, hereonout) use a passive auto-focus system. Passive auto-focus system analyzes the actual subject itself and seeks out areas with high contrast (clear distinction of dark and light edges) edges to lock focus on.

Portable digital cameras often use an active system by emitting infrared beams to determine focus distance, which is a lot slower and less accurate.

With both systems, if there isn’t enough light for the system to detect an object to focus on, it will not be able to achieve focus as all. Therefore, most cameras employ a focus-assist light that momentarily lights up the scene to allow the sensors to pick an area to lock focus on.

Differences in Auto Focus Systems

So what are the differences between entry-level and professional DSLR models if they all use a passive contrast-detecting AF sensor? In most cases, the differences are centered around the number of AF points and the sensitivity of each AF points.

Most entry level DSLRs will have a minimum of around three AF points. This is usually the minimum standard since auto focus film SLRs were invented several decades ago. The AF points are usually positioned in a horizontal pattern running from left-to-right of the viewfinder.

This positioning of AF points is logical and simple. Oftentimes, our subjects are positioned in either of the three AF points, the center or just off-center to either side. When we take a photograph vertically, the right most AF point now becomes the upper AF point, which locks on to the subject’s face perfectly if we’re shooting a portrait.

As we move onto more advanced models, the AF points usually increase between seven to 11 AF points. With a typical 9-AF point sensor, we’ll notice that apart from the standard three horizontal AF points mentioned earlier, we now have AF points on top and bottom of the viewfinder. In addition, some will have AF points diagonally between the edges of the frame.

The additional AF points give the user more options to select the area to lock focus on without moving the camera to recompose. Notice how the upper diagonal focus points lock onto the eyes of a subject perfectly in a traditional head-and-shoulder framing. Having more AF points usually provides more accuracy when utilizing the camera’s focus tracking on moving subjects (more on this later).

Professional models take the AF performance up several notches. Not only do most professional cameras sport between 30-50 AF points, the AF sensor often has their own dedicated processing engine which allow much faster AF performance and accuracy.

All cameras position the most sensitive AF at the middle AF point. The other AF points are less sensitive than the center, however, with more expensive models, even the off-center AF points are considerably faster than the center AF points in entry-level cameras.

How AF Works

Passive AF works by detecting contrast in a scene with certain AF points arranged in a pattern to detect contrast. The sharper the contrast, the easier it is for the AF system to find a focus point.

The image with a sharp edge contrast will enable the AF sensor to lock focus more accurately than the low contrast image, so in practice, the encircled area in Figure 4 would allow the AF sensors to focus faster than the area in Figure 5.

AF sensors can only lock focus if the arrangement of the AF sensor bisects the area of contrast. Most auto focus sensors employ either a horizontal, vertical, diagonal, or cross-type sensor in any single auto focus point.

Entry level cameras usually have three AF points, as mentioned earlier, with a cross-type sensor in the middle capable of detecting contrast in both vertical and horizontal pattern. This allow the center auto focus point to lock focus in virtually any scene as long as there’s enough contrast.

The edge sensors, however, are more specific. In most cases, the horizontal sensor on the side employ a vertically-oriented AF sensor, and that requires the area of focus to have a horizontal high-contrast area. If the area the sensor is trying to focus only has a vertical contrast scene, it will not be able to detect an edge within the sensor’s coverage to lock focus on.

With cameras that have more than three AF points, there usually would be a set of AF points above the center AF point. These AF points usually have a horizontally-oriented AF sensor, and these sensors can detect vertical high-contrast areas in a scene. If the area of the sensor is trying to focus on only has a horizontal contrast scene, it will not be able to detect an edge within the sensor’s coverage to lock focus on as well.

As you can see, as the number of AF point increases, the greater the chance for one or more of the sensors to lock an area with enough contrast on.

It’s not all about the quantity however, AF sensors differ in sensitivity with more expensive models using much sensitive AF sensors than the entry-level models. More sensitive AF points locks focus much faster and more accurately especially with faster lenses. This is a critical feature when depth-of-field is narrow when using wide aperture lenses and when using auto-focus in low-light.

Lastly, with higher-end models, all these auto focus points may employ cross-type sensors on most or even all focus points and increases focus accuracy and speed tremendously. You can deduce how much more accurate professional level camera’s AF system can be with over 30 AF points and over half of them are sensitive, cross-type sensors filling the frame.

In general, the increased number of AF points also work together in tandem for more accuracy as the surrounding AF points will supplement the distance reading of the selected AF point.

No matter how sophisticated the AF system may be, it isn’t fool proof, however. In scenes with little to no contrast, such as a blank wall, a clear sky, or a pitch-black room, the won’t be any areas for the AF sensor to lock on without the aid of a focus-assist light.

Single Shot vs Continuous Focus Tracking

Almost all cameras offer single-shot focus lock and continous focus tracking. Single shot simply means that the camera will attempt to lock focus when you half-press the shutter button right when you’re taking a photo. Single shot is best utilized for static or slow-moving subjects, when shooting fast moving subjects single shot will not be accurate enough as the moment the image is captured, the subject will no longer be in the same spot where you locked focus.

Continuous focus and servo tracking keep track of subject movement without locking onto the subject until the last moment when the shutter is pressed. This feature is best when shooting fast moving subjects that are moving towards or away from the camera’s position. Continuous AF may not be accurate when shooting static subjects, however, as the AF sensor will track any movement in the scene and may shift the focus to the moving element instead of your static subject.

Naturally, when we only have three AF points, the amount of tracking available will not be as accurate as having 30+ AF points, however, if we only plan to shoot landscape, still life, and portraits often, there’s no need to have more than three to 11 AF points.

So How Much AF Points Do We Need?

While having a lot of AF points may seem impressive, in many cases, the sensitivity of the AF sensors are more important. However, if you plan to concentrate on sports and high-speed events, then the number of fast AF points will be an important factor.

The specification of a camera’s AF system should match the type of subject we shoot most often. If we never shoot professional sports, for example, majority of non-professional camera’s AF system are more than sufficient. If we’re shooting portraits and landscape, we won’t even need more than three and it’s actually easier and faster to work with less AF points in such cases. If we are shooting a fast-paced sporting event or car races, for example, then it’s a wise decision to invest on a camera that is equipped with a sophisticated AF system, and finally, if we shoot macro for a living, then we won’t even need auto focus at all!

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