The autofocus system of a camera is designed to intelligently adjust the camera lens to obtain focus on the subject. The main goal of autofocus is to create sharpness at the focus point. This tutorial will help you improve the quality of your photos by explaining all you need to know about autofocus and how it works thereby enabling you to make the most of its positives and avoid its shortcomings.
How Does Autofocus Work?
The autofocus function is performed either by a dedicated AF sensor as in the case of DSLRs or on the imaging sensor itself, in the case of mirrorless cameras, or DSLRs set to live view mode. Basically, cameras use two types of autofocus, these are phase detection and contrast detection.
Generally, phase detection is believed to be the superior method for all focusing scenarios especially for continuous autofocus and subject tracking. Phase detection has the ability to calculate when an object is in focus and also when an out-of-focus subject is back-focused or front-focused, thereby letting the camera know the direction to move the lens so as to bring it in focus.
Contrast detection just as the name implies works by looking for contrast alone. It works based on the principle that maximum contrast occurs when an image is in focus. This technique can be as accurate as the phase detection technique when used for non-moving subjects but in the case of moving subjects, it is generally slower and less accurate. This is because it can only tell when something is focused or not but not the direction it needs to move the lens.
It is important to state that most modern mirrorless cameras use a hybrid technique of both contrast and phase detection. Canon uses the Dual Pixel AF which is a phase detection technology on both its mirrorless cameras and DSLRs. DSLRs from the stable of Nikon still use contrast detection when in live view mode, but the new mirrorless Z series features on-chip phase detection.
Panasonic Micro Four Thirds mirrorless cameras make use of an advanced contrast detection technology that is called Depth from Defocus. The technology analyzes the blur pattern of compatible lenses to determine whether an object is front or back-focused.
The autofocus sensors of a camera are the driving force behind achieving accurate focus. These sensors are laid out in various arrays across the field of view of your image. Each sensor of a camera measures relative focus by assessing changes in contrast at its respective point in the image. Maximum contrast us assumed to correspond to maximum sharpness.
The working system of autofocusing is explained below.
- First, an autofocus processor (AFP) makes a small change in the focusing distance.
- Second, the AFP reads the AF sensor to assess whether and by how much focus has improved.
- Third, the AFP sets the lens to a new focusing distance using the information from the second step above.
- Fourth, the AFP may alternatively repeat steps 2-3 until satisfactory focus has been achieved.
It takes a fraction of a second to complete the entire process from 1-4. In the case of difficult subjects, the camera may fail to achieve a satisfactory focus and will give up on repeating the above steps, resulting in failed autofocus. This scenario is the dreaded “focus hunting” where the camera focuses back and forth repeatedly without achieving focus lock.
The occurrence of focus hunting does not mean that the focus is not possible for the subject being used, reasons, why autofocus may fail, are primarily determined by some factors which are examined below.
Factors Affecting Autofocus Performance
A very important factor that can affect how well your camera autofocuses is the photographic subject. The effect is often more than any variation between camera models, lenses or focus settings. The three most important factors that influence autofocus are the light level, subject contrast, and camera or subject motion.
You should note that each of the three factors is not independent. It is possible for you to still achieve autofocus even for a dimly lit subject if the subject also has extreme contrast or vice versa. This has a very important implication for your choice of autofocus point. Selecting a focus point which corresponds to a sharp edge or pronounced texture can achieve better autofocus, assuming all other factors remain constant.
Number and Type Of Autofocus Points
Robustness and flexibility of autofocus primarily are as a result of the number, position, and type of autofocus points made available by a given camera model. In the case of high-end SLR cameras, they can have 45 or more autofocus points, whereas other cameras can have as few as one central AF point. Also, for SLR cameras, the number, as well as the accuracy of autofocus points, can also change depending on the maximum aperture of the lens being used.
This is an important factor to consider when choosing a camera lens. Multiple AF points can work together to improve reliability or they can work in isolation for improved specificity depending on your chosen camera setting. Some cameras also come with an “auto depth of field” feature for group photos which ensures that a cluster of focus points are all within an acceptable level of focus.
Continuous And Ai Servo Vs. One Shot
The most widely supported camera focus mode is the one-shot focusing. This mode is best used for still subjects. It is, however, susceptible to focus errors when used for fast moving subjects as it cannot anticipate subject motion, in addition to potentially making it difficult to visualize these moving subjects in the viewfinder. This mode requires a focus lock before the photo can be taken.
Many camera models also support an autofocus mode which continually adjusts the focus distance for moving subjects. This mode is called “Ai Servo” for Canon cameras while Nikon cameras refer to theirs as “continuous” focusing. This mode works by anticipating and predicting where the subject will be slightly in the future. It is based on estimates of the subject velocity from previous focus distances. The camera then focuses on the predicted distance in advance so as to account for the shutter lag.
By doing this, the probability of correct focus for moving subjects. So for shooting action photos, the Ai servo or Continuous mode is best.
Autofocus Assist Beam
The autofocus assist beam is a very useful feature found on many cameras. It is actually a small flashlight which uses either visible or infrared light to help a camera’s autofocus sensors. This is when there isn’t enough ambient light for them to work optimally.
AF assist beams, however, come with the disadvantage of much slower autofocus. The use of AF assist bean is only recommended for still subjects. This is because when you are using flash for the AF assist, the AF assist beam may have trouble achieving focus lock if the subject moves appreciably between flash firings.
The main goal of autofocus is to activate maximum sharpness for an image. This tutorial has discussed all you need to know about autofocus and how it works. You can use the knowledge gained to better improve your image qualities. You do this by taking advantage of the positives of autofocus while avoiding the pitfalls.
More on autofocus below:
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