Canon wants to patent a movable sensor assembly that promises better movie AF
posted Monday, February 29, 2016 at 3:54 PM EDT
We've mentioned Japanese tech blog Egami more than a few times over the years on our news page, and today we note that they've uncovered yet another interesting Canon patent application. This time, it's continuous autofocus using contrast detection which the company seeks to improve with an interesting sensor shift-based solution. Specifically, the application suggests that the patent, if granted, would be used to bolster movie autofocus.
According to Egami, Canon's patent application 2016-20982 isn't the first to suggest moving the sensor assembly instead of the lens' focus group to perform contrast-detection autofocus. It's potentially an attractive idea if, as Canon does, you have a lot of legacy lenses that aren't designed with a suitably lightweight focus group to allow for the constant start/stop motion of contrast detection, leading to slow focus adjustment.
Instead of putting phase-detect AF pixels on the image sensor, the sensor assembly is shifted in line with the optical axis of the lens, determining the distance and direction the focus drive will need to be moved, instead of moving the focus group in the lens. Once you have the information as to how far and which way to move focus, the lens' focus group can then be moved in one swift step.
The downside to the technique is that the sensor assembly itself has a fair bit of mass, and moving it back and forth will shake the camera body, as described in Newton's third law: "Every action has an equal and opposite reaction". Clearly, it wouldn't be desirable to have the camera body vibrate during focus adjustments, either for the user's comfort or for the stability of the camera itself while trying to record a movie.
Canon's solution, as posited in the patent application? Add counterweights which counteract the weight of the sensor assembly itself being moved. Of course, it wouldn't be desirable to increase the overall depth of the camera body, and for that reason Canon suggests placing these counterweights on either side of the image sensor, keeping depth to a minimum. It's an interesting idea, and one which could offer an alternative to either the use of on-sensor phase detection autofocus pixels, or slower lens-based contrast detection autofocus.
Might we one day see Canon touting "in-body CDAF" for its historic lenses, in much the same way some of its rivals now promise in-body stabilization for older glass? Perhaps, but it's important to remember that patent applications are just that: Canon's application isn't guaranteed to be accepted by the Japanese patent office, and even if it is, that doesn't necessarily mean the idea will appear in retail products.
(via Canon Rumors)