The Low-down on Autofocus

Most cameras use an autofocus (AF) system to make sure you get sharp pictures.  Over the past few years, we’ve seen technology that provides faster, more accurate, and more sophisticated focusing than ever before thanks to major technological advances.

Autofocus Systems

There are two main types of autofocus systems.  af-assist-beamThe active system is most commonly found in compact cameras.  It works by giving off an infrared beam that bounces back from the subject to a sensor on the front of the camera.  This beam helps the camera to determine the camera-to-subject distance.  It achieves this by measuring the angle or strength of the beam or the time difference between it being emitted and it being received.  This is a fast system and works in almost all lighting conditions. . . even total darkness.  However, it does have some disadvantages.  The main one being that it has a limited range of only a few metres.

The passive system is commonly used in more sophisticated cameras.  This would include systems such as SLRs.  It works by detecting subject contrast to help with autofocus.  The passive system is also known as the phase-detection system and is a very reliable and precise system for focusing from close-ups to distant or moving objects.  One drawback is that it can struggle in low light situations or with subjects that lack any contrast (eg. a plain wall) but in these situations many cameras have an AF-assist beam, which fires a patterned beam onto the subject and allows the autofocus system to focus.

Focus Points and AF Sensors

The focus point is the part of the frame where the sensor for the autofocus is located.  Traditional cameras would often use a single AF point at the centre of the frame.  Canon_XSi_AFToday, this system can still be found on many cameras (usually those at the lower end of the price range).  However, what is more common these days is a multipoint autofocus system.  This type of system has a number of autofocus sensors arranged in a pattern within the frame.  One of the benefits of this is that the increased coverage makes it easier for the AF system to lock onto the subject.  With only a single-point AF system, focusing on an off-centre subject involves locking the focus and then recomposing the frame.  Most of these multipoint systems allow the photographer to choose between leaving all the AF sensors active or selecting individual points.

There are two main types of AF sensors.  Cross sensors are the most sensitive because they have vertical and horizontal sensors that allow for faster and more accurate focus.  Line sensors, on the other hand, measure in one plane only.  Even though they can be suitable for most situations, they don’t match the accuracy of a cross sensor, particularly in low-light situations or with moving subjects.

Autofocus Modes

Many cameras offer you a choice of various autofocus modes that have been optimized for a particular shooting situation.

  • Single-shot AF (also known as S or one-shot AF) is the standard autofocus mode.  It will lock onto the subject until you take the picture or release the shutter button.  If your subject moves, this mode will not compensate for the change in distance.
  • Servo AF will constantly monitor and track your subject.  This is a really good mode particularly if you are photographing moving objects.  More advanced cameras will have predictive autofocus where the camera constantly monitors the rate of change in the subject distance so that when you finally press the shutter button, the camera predicts where the subject will be.
  • AI focus is a combination of the two previous modes.


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Ms. R.

About Ms. R.

Danielle Rochford has been a certified teacher for over 15 years, practicing her craft with middle school students in British Columbia, Canada. She is also a self-taught landscape photographer and founder of D-Roc Photography and Design. She has spent the past five years teaching online in such areas as English, Math, and Photography. She hopes to continue sharing her expertise with others now on Udemy. Danielle encourages her students to be interactive and provides many opportunities for discussion and feedback. In her spare time, she loves to travel and enjoy the solitude of nature. You will often find her traveling through Jasper, AB on a photography adventure.

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