Why is Micro Four Thirds autofocus much faster than DSLR autofocus in live view?
Asked 1/2/2014
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Both DSLR live view and Micro Four Thirds cameras can autofocus using the main imaging sensor rather than a separate phase-detect AF module. If they both use sensor-based autofocus, why are many Micro Four Thirds cameras much faster, while DSLR live-view AF is often noticeably slower? Is the difference mainly due to the lenses, the autofocus method, or the camera design?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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Lens motors used for phase detect autofocus are optimized for a fast one-shot seek at a precise point and they go there with a good precision (with the chance of slight mistakes though); in layman terms they obey the command "go there, now!" in the fastest possible way. This because DSLR autofocus works by precalculating - using phase detect sensors - the distance and direction the lens focus mechanism should be moved.
Lens motors used for contrast detect autofocus, on the other hand are optimized for a succession of micro corrections of movements with very high precision and little inertia so to change direction quickly; in layman terms they obey the command "forward.. no no now a little backward.. no not this much now forward.. ok!" in the fastest possible way. This because focusing using the sensor works by moving the focus mechanism of the lens, see if contrast improves or worse and correct its action accordingly.
Note that there are also cameras using a mix of both technologies (e.g. Nikon 1), I don't know how the lens motors are built for those cameras.
Originally by user15580. Source · Licensed CC BY-SA 4.0
user15580
12y ago
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The main reason is that many DSLRs were originally designed around separate phase-detect autofocus, while Micro Four Thirds cameras were designed from the start for on-sensor autofocus.
With DSLR viewfinder AF, phase detect can calculate both how far and in which direction to move focus, so the lens motor is often tuned for a fast jump to a target position. In live view, many older DSLRs switch to contrast-detect AF on the main sensor, which works by hunting back and forth in small steps until contrast is maximized. That is inherently more dependent on fast feedback, quick reversals, and fine motor control.
Micro Four Thirds systems were heavily optimized for this style of focusing in both hardware and software, and their lenses were often designed to make many small, precise focus movements quickly. DSLR live-view AF was often added later and was less optimized, so it feels much slower.
A further factor is that some cameras use on-sensor phase-detect pixels or other dedicated focus-sensing features, which can speed things up beyond pure contrast detection.
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