Hi everyone! You are reading a photography lesson that answers the question “How do cameras autofocus?” This is one of the lessons that my Guided365 students are using to improve their photography daily. The Guided365 is a project where you receive a daily photography lesson along with a photo prompt designed to make sure you understand the lesson. The topic for day 112 is below. Read more about the Guided365 here.
OBJECTIVE: HOW DO CAMERAS AUTOFOCUS
Today, I want you to learn a bit more about how cameras focus. Remember that we already discussed how the glass elements inside the lens move in relation to the sensor to provide the sharpest photo.
What Do Cameras Focus On?
Let’s take this process a step further. How does the camera know where the glass elements should be in order to provide the best focus? That’s where AutoFocus comes in.
Remember that AutoFocus isn’t the same as as Automatic Focus Point Selection. Automatic Focus Point Selection is when we let the camera choose where in the scene to focus on. Thinking back to the beginning of this class, Automatic Focus Point Select often chooses to focus on whichever part of the scene is closest to your camera.
AutoFocus, on the other hand, is where we tell the camera where we want to focus and the camera figures out exactly how much to move the glass elements in the lens to focus on that point. This is the method we have been using throughout the class. (The opposite of AutoFocus is manual focus. This is where your hand rotates the ring on your lens and your eye decides when you have proper focus. We’ll talk about manual focus soon!)
How do cameras autofocus?
Say that you want to take a photo of a tiny hamster. You compose your shot, set exposure, and then select the focus point over the hamster’s eye.
Your camera measures the distance from the eye to the lens and calculates how to adjust the lens so that anything at that distance will be in focus. The camera then tells the lens how much to move the glass to focus for that distance.
Note that I did not say this: The camera then tells the lens how much to move the glass to focus on the hamster’s eye. When a camera and lens set focus, they do it for a specific distance. If your subject or your camera moves, the distance from the subject to the camera changes, but the camera is still focusing for that original distance.
Later, we’ll talk about a focus setting that attempts to track a moving subject. But for now, I want to you repeat this to yourself 3 times:
A lens focuses for a distance. A lens does not focus on a subject.
Ok. Got that? It means that focus problems are often caused by movement of either your subject or your camera. There’s one other important thing to know about how cameras set focus.
How Do the Focus Points Inside My Camera Work?
The number and type of focus points your camera has are big pieces of your camera’s marketing. For instance, this is what Canon says about the 6D’s AF points:
- AF Points
- Center: cross-type at f/5.6; vertical line-sensitive at f/2.8.
- Upper and lower AF points: vertical line-sensitive AF at f/5.6.
- Other AF points: Horizontal line-sensitive AF at f/5.6.
What does this mean? It means, first off, that I have 11 focus points to choose from when setting focus. Those points are like the points shown in the graphic below, except that there are 3 points on each of the 2nd and 4th columns. The graphic below represents a camera with only 9 points.
The center focus point, which is square-shaped, is cross-type. That’s the best. The camera measures the distance it’s going to focus on based on contrast in the subject. The contrast is created by lines in the subject – the line between the pupil & the iris, for example, or between a pink flower & a green leaf. Using a cross-type focus point, the camera can look for contrast along both horizontal & vertical lines.
In addition, this center focus point is extra sensitive at apertures wider than f/2.8.
Above and below that square are vertical line sensitive points. They can only look for contrast along vertical lines. The focus point rectangles you might see in your viewfinder are horizontal to allow more width for the camera to find a vertical line. If that’s not confusing, I don’t know what is.
The remaining focus points look for horizontal lines to focus on.
The f/5.6 notation for these points means that they work on all lenses that are at least f/5.6 or wider at their largest size.
In comparison, the Nikon D810 (for example) has 51 focus points. 15 are cross type.
What does all this horizontal & vertical stuff mean? If you are trying to focus on a flagpole in the distance, you will only be able to focus with the 3 focus points in the center, because they are the only points that are sensitive to vertical lines. On the other hand, if you were trying to focus on the horizon, you would need to use any of the points except for those two vertical line sensitive ones.
If you’ve ever had trouble achieving focus with one point, but can use a different focus point with no problem, this difference in focus point types probably explains why.
To apply this to your photography, if you have trouble focusing with a given point, you can either change the orientation of your camera to make your focus points work in opposite directions, or change to a focus point with a different orientation.
The “How Do Cameras AutoFocus” lesson is a dense one. Read it a time or two if you need to, and make sure to ask questions on anything that is not clicking by posting a comment below. I LOVE your questions!
- What to Shoot: Tiny.
- How to Shoot: On manual mode.
- Hashtag: #Guided365, #Day112Guided365
- Include with Post: Your settings. How many focus points does your camera have, and what type are they (cross, horizontal or vertical)?
- Carry forward from this assignment:>Cameras focus for distance, not subjects. And, be aware of the type of focus point you are trying to focus with – sometimes you are better off by changing points.