Passive vs. Active
Autofocus systems are broadly categorized as "active" or "passive".
Active autofocus systems do not use the light coming through the lens to determine correct focus, instead they have a separate mechanism for determining the distance to the subject. Some active autofocus systems use infrared light from two separate sensors to triangulate the position of the subject, others have sonar to "echo-locate" the subject like a bat. Pretty slick, huh? Well, active auto-focus is not commonly used in modern cameras.
Instead, most cameras now use "passive" autofocus. All this means is that the camera analyzes the image coming through the lens to determine correct focus. There are two ways this can be done, "phase detection" and "contrast measurement".
Phase Detection vs. Contrast Measurement
In phase detection, the camera uses mirrors and prisms to split the light coming through the lens into two similar images. Because of the way the prisms are positioned in the camera, the images are focused slightly differently from each other. The camera then compares the images to calculate the correct distance to the subject and focuses the lens. Phase detection tends to be faster than contrast measurement and is the most common autofocus method used in DSLR cameras.
In contrast measurement, the camera analyzes the boundaries between light and dark areas of the image to determine correct focus. While the image is out of focus, the boundary between a light and dark area will appear as a gradient (i.e. gradual change in brightness from one pixel to another, low contrast). When the image is in focus, the boundary will be sharp (i.e. quick change in brightness from one pixel to another, high contrast). The camera adjusts the focus until the image has the highest possible contrast, which corresponds to the edges in the image being in focus.
This is the method used in digital point and shoot cameras, most video cameras, and DSLR's when using the "Live View" or "Movie" modes. Contrast detection is traditionally much slower than phase detection, although this technology is quickly improving.
Some Light Please?
As I mentioned above, almost all digital cameras use some form of passive autofocus. Since passive autofocus relies on the light coming through the lens, the autofocus system can really struggle in dark environments. To overcome this, many cameras have an AF-assist lamp, a LED that turns on whenever autofocus is engaged in a low-light situation. However, autofocus still does best when the subject is brightly lit. Autofocus can also struggle if your subject is strongly backlit (the sun is directly behind them), as it will struggle with the comparably dim light on your intended subject.
Not So Good Up Close
Autofocus also has trouble with very nearby subjects. This is more of a concern for "macro" or close-up lenses on DSLRs, and also for point and shoot cameras which are sometimes able to focus from only inches away. I couldn't tell you exactly what causes this problem, but it is a common and predictable problem for macro photography. This is a great time to use manual focus!
Better Than You or I
Despite its limitations, autofocus generally focuses more accurately and consistently than people do, especially when it is used properly. Most modern cameras are not optimized for manual focusing, and unless you have a big, bright viewfinder and excellent eyesight, you're probably better off using autofocus like the rest of us.
Up next, how to use autofocus properly!
No comments:
Post a Comment