Canon patents a 1.4x extender that helps with contrast AF during video shooting
posted Monday, July 7, 2014 at 8:51 AM EST
When shooting video with your DSLR, you have to either set the focus manually, or you can use autofocus. If you choose the latter, many cameras will resort to contrast detection autofocus for the simple reason that video is being shot in live-view mode, and no light can be directed to the phase detection sensor.
Not all lenses are equally well suited to be used with contrast detection AF, though, and despite a slow focusing speed (which should not be much of an issue for video as it can be for still photography) some lenses will cause a "wobble" of the image when being focused during video capture. This "wobble," also called focus breathing, mainly appears in lenses that use internal focusing.
While many prime lenses, especially those of the olden days, move the complete lens assembly forward or backward in order to focus, lenses with inner focusing move only one or a few lens elements on the inside. The downside to this is that with the moving of the focusing lenses, the focal length is being altered slightly in addition to the focus point.
This becomes apparent when focusing during live view, and it can be a nuisance when shooting video. In order to fix this issue for some of its lenses, Canon has patented a 1.4x teleconverter that will, according to the patent description, help reduce "wobbling" or focus breathing when using some DSLR lenses with contrast AF during video shooting.
As far as we understand the machine translation of the Japanese text, instead of focusing the lens itself, the focusing will happen in the teleconverter. We assume that the converter is constructed in such a way that there appears no visible "wobbling" in the video footage while using contrast detection autofocus.
As interesting as this patent is, we wonder how many people are using a 400mm telephoto lens plus a 1.4x extender for their videography -- except for wildlife photographers specializing in video, maybe. In addition, the trend clearly goes towards implementing phase detection pixels on the camera's main sensor, as Canon has done with the EOS 70D. Using a camera that is capable of phase detection AF during live-view, the issues that the patented teleconverter fixes shouldn't even appear to begin with.
(via Canon Watch)