With the pervasiveness of Live View modes from DSLR makers, it is only a matter of time before similar technology brings a “movie mode” to DSLRs. While the ability to record video is a common feature among point & shoot cameras, technological challenges make the incorporation of a video recording more difficult in DSLRs. A recently published patent application by inventor Hiroshi Terada may change all of this. The patent addresses many of the technological hurdles that have prevented incorporation of a movie mode into DSLRs.
As we all know, DSLRs are designed for optimal performance in capturing still images – and DSLR manufacturers have truly raised the bar over the past couple of years. Accordingly, DSLRs are specialist tools that have been optimized to have a very narrow focus tolerance and an ever-increasing auto-focus speed. These features are not quite conducive to smooth video capture. Additionally, the field-of-view changes, albeit slightly, during auto-focus operation. Finally, fast and accurate hand-held auto-focus is dependent up accurate phase-difference AF evaluation, which requires a mirror to reflect the image to the AF sensor.
As you can see, getting live image to the image sensor and capturing smooth, in-focus video seems difficult to achieve without sacrificing some still image capture properties of DSLRs. These obstacles, among others, are what Mr. Terada attempts to overcome in his patent application.
Terada’s patent calls for 2 different AF functions – one for still captures and one for movie mode. In the preferred embodiment of his invention, the focusing tolerance is doubled in movie mode so that there is a deeper range of distances that are “in-focus” according to the AF sensor.
This is because if the focusing accuracy is set high, the photographing lens is driven fractionally as mentioned above to cause a hunting phenomenon or the like, resulting in poor usability.
Terada’s patent also calls for achieving the same results using other focusing methods as well.
The present invention is applicable to contrast AF for performing AF by extracting high frequency components from the subject image signal output from the image pickup device. In this case, a threshold value considered to be in focus in the contrast AF system can vary between still image shooting and moving object shooting, i.e., the threshold value range for moving object shooting can be set wider than that for still image shooting.
In addition to splitting the AF tolerance between still image and movie modes, Terada’s patent calls for two different focusing speeds. This is necessary to keep the camera competitive with current AF performance for still images that we have all grown to expect from our DSLRs and, at the same time, have smooth AF during the movie mode. The patent application describes the movie mode AF speed as follows:
After completion of setting the in-focus determination value, the low-speed lens driving mode is set as the lens drive mode . . . . As mentioned above, if the lens is driven in the high-speed lens driving mode during moving image shooting, driving and stopping are repeated fractionally to increase the possibility of causing hunting. Therefore, in the embodiment, the low-speed driving with less acceleration and deceleration cycles is performed during moving image shooting . . . . Since the lens is driven toward the in-focus point smoothly without a hitch, a reasonable picture without a sense of discomfort can be obtained during moving image playback.
As noted above, the field-of-view changes slightly during auto-focus on DSLRs. This would result is a jumpy/erratic video image when the focus point changes during filming. Terada’s solution is to implement a cropping function during the movie mode. Terada’s cropping function allows users to zoom (zooming is still performed by the standard manual adjustment ring on the lens), but catches the FOV changes that occur during AF adjustments.
[C]ropping is performed based on the angle-of-view variation . . . during moving image recording unless zooming is performed, so that the angle of view can be maintained constant even if the in-focus position of the subject changes.
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Further, in the embodiment, image cropping is performed according to the angle-of-view variation of the photographing lens . . . . Therefore, even if the in-focus position of the subject changes during moving image shooting, a constant angle of view can be maintained and hence an image without a sense of discomfort can be obtained. In the embodiment, this cropping is performed only in the moving image shooting operation, but cropping can also be performed during live view display in the still image shooting mode.
We’ve examined the auto-focus obstacles that Terada’s patent addresses; however, the big issue is what to do with the mirror in the DSLR. The mirror is necessary to reflect the light that passes through the lens to the AF sensor. In traditional still image capture, once AF is acquired and the shutter button is fully depressed, the mirror rotates out of the path of the light to the image sensor and the shutter opens for the set duration – exposing the sensor to light. In order to operate at 30 frames per second (the video capture speed called for in Terada’s patent), the mirror would have to bounce up and down 3 times faster than the rocket-fast Canon EOS 1D Mark III. Clearly, that’s not a desirable (or quiet) way to do video on a DSLR.
The patent calls for a mirror that remains in the path of light. It’s a special mirror though. It reflects 30% of the light and transmits 70% of the light, allowing the light to make its way to the image sensor without moving anywhere.
The benefits of this semi-translucent mirror are two-fold. First, the reflected light allows AF to occur via the traditional phase-detection method. Second, live view (during still shooting) and video (during movie mode) can co-exist with fast AF due the transmittal of light through the mirror. Live view “still” shooting allows the mirror to retract from the light path upon image capture. Due to the reduced quality necessary for video capture, the transmitted light is used for movie mode and the light reflected via the mirror is used for continuous AF.
Further, in the embodiment, since the target image quality is different between still image shooting and moving image shooting, the position of the mirror member . . . is changed. In other words, since still image shooting typically requires a high image quality, the mirror member . . . is retracted from the shooting optical path to prevent degradation of image quality caused by the mirror member . . . . On the other hand, since moving image shooting does not require the high quality for still image shooting, the mirror member . . . remains in the shooting optical path to eliminate the time required for inserting and retracting the mirror in order to reduce the time lag to the start of shooting.
Further, in the embodiment, the mirror member . . . is inserted into the shooting optical path upon activation of the camera to reflect part of the subject light beam into the distance measurement/light metering sensor . . . . Therefore, when the release button . . . is pressed halfway to turn . . . on during live view display, light metering and distance measurement can be performed immediately and conveniently in parallel with the live view display.
In the embodiment, the CCD . . . as the image pickup device receives light transmitted through the mirror member . . ., while the distance measurement/light metering sensor . . . receives light reflected by mirror member . . . . However, on the contrary, the digital camera can be configured such that the CCD . . . receives reflected light and the distance measurement/light metering sensor . . . receives transmitted light.
This has seemed inevitable to me for some time now. Given that all of the major DSLR players feature some form of live view in at least part of their camera lines, I expect that a manufacturer will release a DSLR with a movie mode in the near future, perhaps this year. Personally, I expect this to be a consumer-grade DSLR, which would fit the market for such a feature. Additionally, I can see how this feature would be useful among photojournalists in today’s web-intensive media world.
What do you think about have a “movie mode” on your DSLR? Do you want it now!? Do you hate the idea? Would you ever use it?
Source: Pat. App. No. US 2008/0030594.