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A patent application filed by Canon was recently published by the USPTO (App. No. 12/188,385) that may give us some insight as to how Canon will solve the abysmal autofocus performance of Canon’s Live View mode. If you are unfamiliar about how Canon’s Live View AF system currently works (or doesn’t work), you can read my prior rant in Photography Bay’s Canon Rebel XSi Review. For some insight on how Live View AF should work, see the Sony A350 Review.
Nothing works faster for DSLR autofocus systems than the tried and true phase-difference method. If you don’t know what phase-difference AF means, the you can read about it on Wikipedia; however, it’s not essential to understand all the technical details to get excited about Canon’s new patent.
The fundamental problem with making phase-difference autofocus work in a Live View mode is that the AF sensor receives light from the mirror, which also reflects light up into the pentaprism for the optical viewfinder. When you press the shutter button, the mirror flips up and you get a brief moment of black out in the view finder while the shutter opens and exposes the image sensor to the light from the lens.
When you think about this process, which I have overly simplified for illustration purposes, the problem with Live View mode and fast autofocus is apparent. When you set your Canon DSLR to Live View mode, the mirror flips up and the shutter is opened to directly expose the sensor to light from the lens. The only difference here is that the image is not yet recorded, but rather displayed on an LCD screen for you to frame, focus and then finally capture your image. Without the mirror in place to reflect light to the AF sensor, you cannot obtain fast and accurate phase-difference autofocusing. Accordingly, Canon has been forced to suffer (as have the rest of us) with the sluggish contract detection autofocus in Live View mode.
Canon’s US Patent Application No. 12/188,385 claims to have solved this predicament by putting [Updated: Thanks to Micah for pointing out this error in the comments.] the phase-difference AF sensor below the image sensor rather than above it. Additionally, Canon has plans to put a semi-transparent mirror in place a different configuration to allow simultaneous transfer of light to the image sensor and the AF sensor.
The novel part of the patent above seems to be that they are using a mirror that is partially transmissive in a different configuration for live view. It looks like the mirror flips in the opposite direction for live view and the AF sensor is positioned optimally for this, with the secondary mirror’s placement higher up the main mirror compared to the standard position of center. [Ed. See Micah's comments below.]
New Shooting Modes
If the camera design in this new patent application ever comes to fruition, we will get 3 shooting modes out of it:
1. Standard Viewfinder Mode: In this mode the main mirror will be in its normal position, but light will pass through it to a sub-mirror, which will in turn reflect down to the phase-difference AF sensor.
2. Fast AF Live View Mode (shown in the top image): The smaller sub-mirror will move out of the way and the main mirror will rotate to a different angle to take advantage of its translucent properties, which will allow light to pass through to the image sensor, while at the same time reflecting a portion of the light to the phase-difference AF sensor. This is what we all want - fast, accurate autofocus during Live View.
3. Full Live View Mode: Both mirrors are rotated/retracted out of the sensor path to allow full exposure of the sensor while observing Live View. This will likely come into play when shooting video or when zooming in during macro or still life shooting for critical manual focus.
When Will We See It?
Who knows if we will even see it. The patent application was filed August 8, 2008, so that’s fairly recent in patent world. Canon files a lot of patents though and many inventions never see the light of day in the form that they are embodied in the original patent application.
This method makes a lot of sense though. Canon needs to make a Live View system that works with fast AF. If the light splitting mirror is really the cake and icing that this patent touts it as, then Canon could reinvigorate its waning EOS Rebel line at PMA 2009 with a feature like this. We will know in a couple of weeks if we’re going to see this feature in a first-half 2009 camera from Canon. Otherwise, we’ll all be playing the waiting game.
Unfortunately, I have no idea how close Canon is to putting this feature into a market-ready camera. Only the guys at Canon know this - and they aren’t talking.
source
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