Lesson 23: Auto & Manual Focus

Autofocus

By now, you are likely familiar with focusing your camera. It’s simple, right? Half-press the shutter release, the camera focuses, and then finish pressing the button to fire the shutter. And honestly that really is the basics of it.

There’s a lot more to focusing though, and while it’s not essential to simply taking a photo the knowledge can be incredibly helpful in speeding up your workflow and getting more functionality from your gear. Between the various focusing modes and types of autofocus there are a number of factors you can leverage to improve your photos if you understand the differences and limitations of each. So, to begin:

Phase Detect vs. Contrast

There have been different ways that autofocus has worked over the years, and the primary methods have been contrast detection and phase detection. Contrast detection is fairly self-explanatory: the camera detects areas of high contrast and shifts the focus until that area has the sharpest boundary between areas as possible.

With Phase Detect, it functions more similar to a rangefinder. The light from the edges of the lens are picked up by separate sensors multiple focusing sensors are incorporated into the imaging sensor) and when they converge they are considered “in focus”

So what does this mean?

In short, cameras with Contrast Detect autofocus are going to focus on areas of high contrast. If you try and focus on areas of lower contrast then the sensor will struggle to find focus, whether it’s due to similar lighting, low-light, or just a general lack of variance in the light’s contrast. With Phase Detect, it can be faster and more reliable as it’s lining up the areas of contrast, not judging purely on how much contrast there is. It will still struggle in low-light or when there’s not enough variation in light, but is generally faster and more reliable.

Autofocus Modes

There are also differences in how the camera applies the focus. The biggest difference, one that almost all cameras apply, is continuous focus vs. single focus (AI servo vs. One Shot for Canon users) These are the main concerns when shooting, as each has distinct advantages and are both available across camera systems.

• Single Focus (One-Shot): This is the standard mode for digital cameras, and easily the more commonly used mode. Essentially, when your camera focuses in this mode it will find the focus and then hold that focus until either the photo is taken or the shutter is released completely and focusing is began again. This mode is incredibly useful for still subjects, focus-and-recompose (which will be covered more shortly), and most general use.

• Continuous Focus (AI Servo): This mode isn’t great for still subjects, as it allows the camera to continue to change focus until the shutter is fully depressed. What it does excel at is focusing on moving subjects. As the camera continually adjusts its focus up to the point where the shutter is triggered, it will track moving subjects through the frame. Whether your subject is pets, children, athletes, or other fast moving subjects this mode can allow your camera to keep the subject in sharp focus.

• Eye, Face, and Tracking: Many cameras within the past decade have implemented focusing on a subject’s face or even their eyes, and they have improved detection for both people and animal faces. While for the most part this doesn’t change anything about the focus mode used it will allow for a more precise focus without the need to adjust the focus point manually. This is helpful for subjects that are in motion, but also is a fantastic resource for subjects that may move suddenly and quickly. There are absolutely methods to get precise focus on a still subject, but if your subject is moving or likely to move if you take too long to focus then using the face/eye tracking can help acquire focus quickly and consistently. 

• Manual Focus (with an autofocus lens): This is a bit generally unused simply because it usually doesn’t make sense to focus manually if you have autofocus available. There are times, however, where slight manual adjustments can make the difference between “good enough” and “perfect.”

  Usually, you will not be able to focus manually if autofocus is enabled. It requires a dive into the menu to make it work. And in most situations it’s completely unnecessary, which is why it is defaulted on many cameras to not be allowed without digging through menus. There are times though where you may want to use autofocus but it doesn’t quite focus where you want. If there’s a high-contrast area right beside where you want the camera to focus, if the subject isn’t perfectly still while the camera autofocuses, or if the focus isn’t perfect while using focus-and-recompose, then the ability to manually adjust can help.

• Back Button Focus: This feature is usually either loved or hated, but either way it’s a focusing mode that’s worth knowing about. Essentially, it decouples the focusing from the shutter button and maps it to a different button on the camera. For faster-paced photography such as sports or pets it’s generally less effective, but can still be used when pre-focusing on an area to capture any action as it comes into a pre-decided spot. It’s also fairly common among landscape photographers as it allows the photographer to set their focus before fine-tuning the composition. It will allow the focus to be set without needing to worry about the camera re-focusing when going to take the shot. It functions basically the same as using the autofocus-lock (AF-L) but without needing to hold the button to keep the focus constant.

  Most cameras require setting up back button focus through the menu system, but there are some that will allow for back button focus simply by switching the camera’s focusing mode to manual. In those cases, the default focusing button is typically the AF-L. In these cameras it’s much faster to switch between back button and standard autofocus, but in the end the effect is the same. It’s worth trying out, and if it’s something you prefer to use then it’s not likely you will be changing back and forth often anyway.