How Auto Focus Works #120

How Auto Focus Works #120

Auto-Focus. Generally speaking, your camera's auto-focus system will focus on objects closest to the camera and things with texture and contrast. Within your camera, you will have between two and six different options that will allow you to change the size of the active focus area. This is what I like to call the target area. When you change the target area, you are still using the same algorithm that I just mentioned. Things closest to the camera and things with texture and contrast. Many photographers use a small target area because they believe that using a large target area means that the camera will focus on multiple planes, like the tree in the foreground and the mountains in the background. That's called depth of field, which I covered in detail with a physicist way back in episode 20.

The reason your camera may light up many areas of the frame is because it sees many things on the same plane. The camera can only focus on one plane at a time. The reason I bring this up is because I don't think anyone should be afraid to use a wide target area. But having said that, why change the size of the target area at all? Well, imagine shooting a bird in the trees, if you have a large target area, then the camera will likely focus on the branches long before it focuses on the bird in between. In this situation, you would likely want to use the smallest target area available and make sure that the bird is behind that point. But with some of the new mirrorless cameras, you can now program the camera to automatically find types of objects, like eyes, or faces, or even cars. One thing you will need to be careful about is making sure your focus point lands on something that the camera can actually focus on. If there's no texture or contrast at all, like the blue sky, the camera will have nowhere to go and you will find that it will just move the elements back and forth searching for an object.

The last thing to note about your autofocus system is single versus continuous focus. Single focus is when you press the focus button and it locks the focus on something. And that's it. It's done. It's done until you lift your finger and press the button again. In continuous focus, something Canon likes to call AI focus, the camera will track the object as it moves closer or further from the camera when you keep the focus button held down. Use single focus when you're shooting static objects. Use continuous focus when you're shooting moving objects. Note that I did not say the shutter button. Although out of the box your camera is set up to find focus when you press the shutter button, you can also program a button near your thumb on the back of the camera to do the same thing. This is called back button focus. But these days, some lenses, most notably Sony lenses, will also have a focus button on the lens. A very cool feature that I hope all manufacturers will adopt. And that my friends is focus in a nutshell.