Guest post by Sam Fischer
Autofocus can sometimes be a complex subject, especially when we consider just how many modes and options there are available. Looking at my autofocus settings I might feel a bit unsure to move beyond the basic autofocus settings. Some people may not even realize there are different types to use in different situations. So let’s take a look at autofocus in Canon cameras together.
What are autofocus points?
Autofocus points are the areas of the field of view where the camera can direct focus for maximum sharpness. Generally, the more expensive the camera, the greater the number of autofocus points. Basic DSLRs may have around 9 to 12 autofocus points to use. Advanced DSLR and mirrorless cameras often top 400 or more autofocus points. It’s always great to have as many as you can afford because the more autofocus points I have the more options I have for image composition.
Contrast versus phase detection autofocus
There are two main types of autofocus systems. The first one to consider is contrast detection based systems. These systems work by searching for the highest possible tonal difference between image pixels. The lens hunts for the greatest difference by panning in and out.
Once the greatest difference is found the image will have optimum sharpness and clarity. Contrast detection is overall more accurate than phase detection. But the disadvantage is that it is sometimes slower, especially if the lighting is not so good. Contrast detection is better for situations where being even slightly out of focus makes a big difference, like portraits and macro photography.
Phase detection works quite differently. This system work by using paired autofocus sensors to analyze light received from the subject. If the light is different between the pairs, the camera knows the focus is off and adjusts accordingly. Phase detection is faster than contrast detection and usually more accurate enough. This system works best where we need extremely fast autofocus speeds, such as sports and other action styles.
Many newer cameras nowadays, especially mirrorless cameras, have hybrid autofocus systems. The way they operate depends on the brand and model. But most use a sensor that has phase detection pixels or groups of pixels combined with a contrast detection base system. Fast and accurate, hybrid autofocus systems combine the best elements of both worlds.
One shot AF
This is the standard autofocus mode for Canon cameras. Once I hold the shutter button halfway the camera finds focus and then keeps it steady. Even if the subject moves out of the plane of focus the focus zone remains unchanged. Once I press down on the shutter fully the camera takes an image. So long as my subject is unmoving, moving only slightly, or I am using a larger depth of field, One Shot AF is plenty for a great image.
AI Servo AF
AI servo AF is often called Continuous AF by other manufacturers. Once the camera locks onto a subject it then tracks it constantly. This mode is perfect or wildlife and action photographers since the subject will often be moving in unpredictable ways. Some higher-end Canon cameras even have Lock-On AF, which can keep track of fast-moving subjects that have left the frame. Once you bring them back into the frame the plane of focus remains sharp and steady.
AI Focus AF
AI focus AF is the “auto” mode for autofocus. Using this, the camera will choose the best mode for a given situation. It will use One shot AF unless it detects movement and then switches to AI servo AF. While this may seem like the best mode to use all of the time there’s a disadvantage in the response time. The camera has a slight delay in noticing subject movement and switching to AI servo AF. So if you have the lens zoomed onto a subject and it suddenly moves fast then the camera may not respond fast enough to keep track.
Canon autofocus is a detailed subject but necessary for every Canon user to grow as photographers. Manual focus is the gold standard but autofocus does have its place in many situations. So don’t shy away from using the autofocus setting for crisp, beautiful images!