Focusing your scene correctly is one of the most important elements of being a photographer. If your capture misses the focus, it doesn’t matter if you nailed composition, lighting, or choice of subject.
Modern digital cameras use two different types of autofocus; phase or contrast detection. Let’s run down what they mean, how they work, and which could be better for you.
[Cover photo - Trumpeter by Craig Hull]
Focusing
All focusing happens within the lens. The array of convex and concave-shaped optical glass elements collect and converge the light coming from the scene. Achieving a sharp focus happens when the convergence point hits the light-sensitive medium. If the light hits before or after, it results in a blurred, and useless image.
The first autofocus came with the Konica C35 AF but wasn’t a standard option until the 80s. Now, a click of a button focuses the light and ensures our scene is sharp. This is through motorized elements (tiny motors in the lens) as part of a ‘focusing group’. They focus on two methods; Phase Detection and Contrast Detection.
How does the camera know, I hear you ask. With film photography, the photographer or technician could use a loupe to view the grains of the film to know. With modern, digital photography, the lenses make tiny adjustments. It moves backward and forwards quickly, comparing the scene until one or all elements are sharp.
Phase Detection
If you own a DSLR, such as the Canon Mark IV, chances are that your camera is using phase detection autofocus. Light enters the lens, hitting a mirror that sends most of the light to the viewfinder. A small amount of light is separated and diverted to the autofocus sensor. This is where the magic happens.
The light that hits the autofocus sensor comes in pairs, used to compare the light from the scene. The computer inside the camera looks at both. It commands the focusing group to move forward or backward, assessing if the focus is sharper or stronger. When the two images match, it is a focused scene and time to take a shot.
Phase Detection by Cmglee
Contrast Detection
Contrast detection is the focus mode used by mirrorless, smartphones, and point-and-shoot cameras – DSLRs in live view. Any camera that doesn’t use a mirror will use this focusing type. Instead of comparing two shots of the same scene, contrast detection looks at the light intensity entering the lens from many angles.
This type of autofocus requires more evaluation. The camera’s computer doesn’t know immediately if it can already see the greatest intensity of light. The focus group needs to hunt to find the highest intensity of light, thus, the strongest focus.
Which is Better – Phase Detection or Contrast Detection?
As the technology of cameras improves, both types of autofocus will become better, faster, and with more focal points. Contrast detection seems to be more accurate for still subjects and less expensive. The problem is time; it requires more for the focusing group and computer to find that sweet focus spot.
Cameras with contrast detection have many more focal points. This allows you to capture well-focused shots of even minute elements in your scene. The biggest benefit is having both, such as the Nikon Z7 with 693 phase and 425 contrast detection points. That way you can focus on landscapes and sports with the same system.