With my first SLR, I placed my subject in the middle of the frame and turned the focus ring until a feature in the split prism lined up and the microprism collar lost it sparkly look. Today, things are a lot more complex and technological, and the number and variety of different terms used to characterize AF systems is a little bewildering. Hopefully, this article will clear up some of the confusion. Although contrast-detect AF has some advantages, it is substantially slower than phase detect AF, which is the main reason the latter is the primary AF method for dSLRs.
For phase detect AF, the main reflex mirror, which directs most light up to the viewfinder, passes some light through a semi-transparent window to a secondary mirror which directs the light down to the AF sensor at the bottom of the mirror box:.
Before hitting the AF sensor, the light passes through an array of microlenses which separate incoming light to produce a pair of images that fall on each point in the AF sensor. A simple line sensor then measures the distance between those two images to determine if the image is front- or back-focused, and by how much.
In essence, each AF point is operating as a simple rangefinder. That's the simple case, but modern systems are much more complex - and there's a lot of jargon that make comparisons difficult.
What do things like 'high precision' and 'dual-line zigzag arrangement' actually mean? Number of points is self-explanatory. Geometry of points adds a layer of complexity, but both number and basic geometry are usually part of the top-line spec for an AF system e.
Complex geometry includes things like dual-cross points and dual-line zig zag arrangements. More on that later. Globally, accuracy is how 'close to true' the AF system gets, and Canon doesn't offer any specifications for this characteristic.
Note that global accuracy is affected and hopefully, corrected by AF microadjustment. Locally, geometry affects accuracy, because a cross-type point has a better chance of achieving proper focus than a single-orientation point, since it samples features with different orientations.
Precision is repeatability - if you take several shots of the same thing, how close will the focus of any one shot be to all the other shots? Note that depth of focus is related to, but not the same as, depth of field, although the same factors influence both.
Overall sensitivity is how well the AF system performs in low light. The AF sensor is composed of multiple bit line sensors and associated amplifier circuitry - the more amplification within the limits of signal to noise , the less light needed to focus. AF sensitivity is specified as an EV range, and the lower the first number, the better. EV units are 'stops' so the 1D X can achieve AF in half as much light as the previous 1-series bodies.
Sensitivity is used in another context as well, associated with lens aperture e. Speed is another characteristic for which Canon offers no metrics, probably because there are too many variables. But better AF sensors and better processors result in faster AF performance. Coverage area is a very important factor - the broader the area of the frame where there are AF points, the more likely that an AF point will fall on your subject.
More on this later. Of the above characteristics, number, sensitivity, speed, and coverage area are fairly straightforward.
But geometry, accuracy, and precision are more complex, especially because the lens mounted to the camera will change the way in which the AF points operate in terms of geometry, accuracy, and precision. A basic, single line AF point can detect contrast only in one dimension - the dimension 'opposite' to the orientation of the line.
The resulting images are crisp, sharp and highly-detailed every time. Generally, it is advised to choose a higher number of focus points for low-contrast subjects if the camera has difficulty focusing, or for active subjects that are hard to keep framed in a single point. Photographers can increase the number of focus points if the subject is moving unpredictably or occupies a large area of the frame, or when the subject is at the edge of the frame.
For example, the 9-point option is recommended if you want to focus on a specific subject in the frame, such as a single athlete in a group of players, or the eyes of your model in a portrait. Alternatively, a point dynamic-area AF tracks a wider area in your frame; for instance, a figure skater executing a move.
A or point dynamic-area AF may be preferred if a background contains objects with regular, detailed patterns, like a billboard or the stands. This is particularly effective even when tracking subjects with erratic movements that veer toward the corners of the frame. Nikon Asia and its Nikon corporate websites and external third-party partners use cookies to improve our website and enhance your user experience. We use analytic cookies and marketing cookies to learn from user information, personalise advertisements and measure their effectiveness.
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