Your camera has some pretty sophisticated technology for achieving sharp images. There are times for manual focus, but many photographers rely daily on the accuracy of the “AF” system built into the camera. Most cameras default to attempting to find focus as soon as you half-press the shutter button.
In this mode, the camera will acquire focus once. As soon as it’s locked focus it doesn’t try finding it again.
This mode will continuously attempt keep the subject in focus as long as the shutter button is half-pressed.
Many cameras have a sort of “Auto” mode where the camera will attempt to guess which mode works best.
Just what it sounds like. For landscape, product, and night-time photography, manual focus can be a significantly simpler approach.
Focus areas determine what area of the frame is intended to be in focus. This is one of the most common reasons beginning photographers end up with improperly focused shots.
This mode bases focus on a single point. On touch screen cameras, touching the screen can be used to quickly select the focus point. “Eye AF” is an automatic but single area selection mode that intelligently finds the eye closest to the camera.
Different numbers of points can be selected, or a “zone” of the image can be given focus priority. The wider you go, the more you chance missing focus if the depth of field is shallow and change is noticeable between points of focus within the zone.
These are intelligent focus area modes that use the camera’s processing power to attempt to recognize and track the subject.
AF can be broadly categorized into Active and Passive. The former isn’t used much in modern cameras. To estimate focus, it relied on actively measuring a signal (oftentimes infrared) shot from from the camera, bounced off a subject, and returned to the camera again. Most of the cameras you know use passive AF which can also be divided into two types:
These rely on the incoming image being split into two parts which are compared against each other for focus. On older cameras, only vertical focus was analyzed and “cross-type” sensors which worked both vertically and horizontally were few and far between. DSLR cameras use a separate AF system located below the mirror of the camera. Newer mirrorless cameras often use on-sensor phase detect AF or contrast-based AF.
This method relies on the image going into and out of focus for the sensor to effectively analyze the point at which contrast (sharpness) is greatest. It can actually do this quicker than you might imagine, but it’s not as good for video since the point of focus must be passed and then returned to.
Low light, reduced contrast, and fast motion will often made AF systems struggle. Blur due to misplaced focus can sometime look similar to motion blur, so check both your shutter (1/focal length) and focus settings.
Borrow Lenses has a great guid to selecting between these modes on your specific camera brand.