Understand How Autofocus Works: Part 1

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This post is part of a series called Understand How Autofocus Works.
Understand How Autofocus Works: Part 2

Autofocus is easily one of the most useful advances in modern photography. In the majority of current camera systems, not having autofocus is nearly inconceivable. Learning to control this technology is a crucial skill for any photographer.

What is Autofocus?

To begin answering this question, we must first ask, "what is focus?" In photography, there exists this concept of being "in focus," which refers to the image being rendered with high clarity, distinctiveness, and fine detail. Achieving precise focus on a subject is a goal that photographers generally aim for.

Having good focus in our camera systems is like having perfect vision, the object of interest is rendered with perfect detail. When we have less than perfect vision, or imperfect focus, the world seems universally blurry. Luckily, unlike our eyes, the focus on lenses can be adjusted so that focus rests at the distance where we need it, so perfect clarity is always possible, it just becomes a matter of being able to obtain it. This is how autofocus can help.

In the heart of it, autofocus is any technology which automatically (without the photographer) changes the focus distance of the lens. This feature can be more accurate than “eyeballing” focus and tweaking manually and can be used to drastically improve focusing ability for moving objects, which our eyes and reflexes, struggle to track.

Using Autofocus

Most people have encountered autofocus already. It exists on almost every modern imaging platform, from advanced medium format Hasselblads to knock-off smartphones, and is almost always the default mode of obtaining focus. Put simply, if you aren’t sure if you have autofocus, you do.

Doesn’t it seem strange that after going out and buying that fancy DSLR, the autofocus is less flexible that that on a cameraphone? With modern smartphones, the user can simply tap the subject with their finger, and a nice little box shows up, with the phone soon to adjust focus so that whatever we tap renders clearly. What a nice trick.

This is the viewfinder screen of a D3100 camera which has an 11-point autofocus system More advanced cameras currently employ up to 61 autofocus points
This is the viewfinder screen of a D3100 camera, which has an 11-point autofocus system. More advanced cameras currently employ up to 61 autofocus points.

Looking at a DSLR though, the viewfinder is limited to only focusing on a limited number of points on the viewfinder, such a hassle! Without going into too much detail, this is because DSLRs use a different method of autofocus than point-and-shoot cameras or smartphones, an advanced system which eschews the need to process what the lens sees using the digital sensor.

While it may seem like a weakness at first, this mode of autofocus is actually much faster and more accurate. This article will focus on the autofocus system in DSLRs instead of the one in smartphones (who needs instructions on how to tap somebody’s face on an iPhone?).

Now that we know that we rely on fixed points to focus, we are introduced two key problems. How do we choose which point is used and what happens if my subject doesn’t happen to perfectly rest on a predefined point?

Manual vs. Auto Focus Point Selection

First, we need to see what mode is selected on the mode dial. Most modes belong to the so-called "automatic scene modes," where the camera attempts to changes its settings to suit the chosen environment. Naturally, these modes will choose their own focus. (There are some exceptions to this, such as the “macro” mode, but in general pre-programmed modes enforce automatic autofocus.)

For example, pull out a DSLR, and set it to the most basic mode, Auto. When you press the shutter halfway (which enables the autofocus), there will be a beep followed by a few randomly chosen autofocus points in the viewfinder being highlighted. These are the focus points that the camera thinks contains the subject. If that isn’t true, then tough luck.

In order to self-manage autofocusing on a DSLR, we must use one of the “manual modes” (typically P, A/Av, S/Tv, or M). In these modes, the autofocus point can be selected manually by a knob or pad of some sort in the back. The exact method varies from platform to platform.

DSLRs typically have a dial similar to this The text-based manual modes will afford the highest level of control to the user including manually selecting autofocus points
DSLRs typically have a dial similar to this. The text-based “manual modes” will afford the highest level of control to the user, including manually selecting autofocus points.

Alternatively, Leave the Focus Point in the Center

Being able to choose the focus point gives much more flexibility, but some people prefer to just leave the point in the center. In order focus on subjects that aren't in the center, the trick is to focus first and compose second.

This is the simplest and often first-taught method of obtaining focus, which works in the following three steps.

Step 1

With your camera in Single Servo or One Shot focusing mode. Set the focus point to the center of the viewfinder. The middle autofocus point is often a cross-type autofocus point, which makes it more accurate than the rest of the autofocus points.

Step 2

Place the point on the subject of interest, and press the shutter half way, in a process called pre-focusing. Once this is done, the “AF lock” of your camera will be engaged, effectively freezing the focus distance at the current distance, regardless of how you move your camera.

Step 3

With the focus distance locked, you are now free to frame the scene in any way you wish. Usually having the subject dead center makes for boring photos. Try the rule of thirds. Once you're happy with the composition, press the shutter button the rest of the way down.

We wish to use the center autofocus point to focus on the subject at first but are free to later compose as we wish after the AF lock engages This is called pre-focusing
We wish to use the center autofocus point to focus on the subject at first, but are free to later compose as we wish after the AF lock engages. This is called pre-focusing.
This option is also available if Continuous mode with Nikon camera by using the AF Lock function. It can also be used with the technique called, "rear button" focusing where the half shutter press focusing is disabled.

Appropriate Times to Change Autofocus Points

It’s very rare that any focus point will be exactly where you want it, even with the newer 51-point systems. So if we have to recompose after pre-focusing anyways, what is the point of ever having more than one point?

The first reason is that there could be instances where recomposing is not a physically valid option. While the “focus first, compose after” method is great for a majority of situations, there are times where the framing must be exact, and cannot be eyeballed.

In these situations, where the composition has been painstakingly chosen and the camera cannot change positions, the order of operations has been changed to “compose first, focus after,” this is where having a flexible autofocus system with many points becomes very helpful.

The main purpose of having an advanced autofocus system, though, isn’t about saving time. It’s to give photographers the ability to properly photograph fast-moving objects. With obvious implication for wildlife and sports photographers, proper use of autofocus is crucial for anyone taking photographs with non-stationary subjects.

Say you want to take a photo of a running child. By the time the focus has been acquired, the child very likely moved on (forget about trying to recompose after pre-focusing in this circumstance).

Even with the very fast autofocus speeds in modern systems, there is no way you can take more than one shot at a time with this focusing paradigm. How can you take advantage of your camera’s fast frame rate to burst out a sequence of well-focused shots to choose from later?

Most DSRLs now support, in addition to the aforementioned one-shot autofocus, the option to engage a very powerful feature called focus tracking (named AF-C “continuous focus” in Nikon and Al Servo in Canon systems).

How this works is that after the subject has been first focused on, the camera will attempt to track the movement of the subject and automatically select the autofocus point which is closest to the subject, all in real time!

This will continue as long the shutter is half-pressed and held. When in use, the camera will adjust the lens to maintain focus on the subject by predicting where the subject will be using its current speed and acceleration information.

In this way, it’s possible to take a series of photographs in quick succession, without having to worry about focus, and maximize the likelihood of having a great shot.

Improving Your Autofocus

Tip #1: Aim For the Edges

The tip that had the most impact on my autofocus was learning to position the autofocus point. Because the autofocus sensors use contrast to determine whether an object is in focus, it only works well when the point is placed on something with contrast to begin with!

For example, when I aim my camera’s autofocus point towards the edge of an object, the focus is instantaneous and extremely accurate. But if I try to aim it towards the middle of an object, where the color and tone is constant, the sensor has no way of determining whether what it’s seeing is sharp or not.

Think about it, the autofocus sensor only has the information on that exact point to determine focus. It would be comparable to looking through a straw and trying to gauge whether you have perfect vision. It may be possible if you see the edge of and objects and it’s blurry, but it would be impossible if all you see is a white wall.

To reuse an old example we can consider what would happen if I tried to focus on these two various points directly The left image will focus easily because of the sharp color contrast of the USB drive to its background The right image however will fail to focus on the black-to-black background and result in focus seeking where the lens zooms in and out without ever locking the focus By default cameras will actually prohibit a shot to be taken unless focus is acquired even if the shutter is pressed all the way
To reuse an old example, we can consider what would happen if I tried to focus on these two various points directly. The left image will focus easily because of the sharp color contrast of the USB drive to its background. The right image however, will fail to focus on the black-to-black background and result in focus seeking, where the lens zooms in and out without ever locking the focus. (By default, cameras will actually prohibit a shot to be taken unless focus is acquired, even if the shutter is pressed all the way.)

Tip #2: Turn on AF Illumination in Dark Situations

Most DSLRs come with some form of AF illumination, and turning it on differs between models. This helps by shining a small light in dark situations so that the camera can focus well. If everything is black, the camera runs into the same problem as the situation in tip #1, it has no idea if what it’s looking at is in focus or not.

Be warned though, having the AF illuminator is likely not appropriate in any situation where a flash is prohibited. You may not have the best time explaining the differences when confronted, so I normally leave it off.

Tip #3: Buy Faster Lenses

As much as this may seem like a fix-all solution to scam money, there is actually a very direct way faster lenses contribute to better autofocus. Faster means that the lens has a larger maximum aperture (smaller f-stop number, such as f/1.8), which means that the lens has a larger opening.

When the camera tries to autofocus, it will always open the aperture to the largest it can be, to allow in more light, regardless of what you actual settings are when you take the picture. So having a larger potential maximum aperture, regardless of what you actually use, will make it easier for the camera to autofocus effectively.

In fact, while using low-end DSLRS with small maximum apertures lenses, such as f/5.6 kit lenses, autofocus will simply not even work on any points except the center, and even pro-grade bodies with advanced autofocus systems can barely handle any lens if the maximum aperture is smaller than that. This is one of the main reasons large-aperture lenses are so sought after.