Canon patent application offers solution for Live View autofocus issues
By Donald Melanson 
posted Feb 11th 2009 4:33PM
posted Feb 11th 2009 4:33PM
Canon's Live View implementation may not be a deal-breaker for most DSLR users, but there's certainly plenty of room for improvement, and a recent Canon patent application suggests that the company could possibly be aiming to do just that. The short of it is that the company seems to have found a way to overcome the inherent problems of employing so-called phase-difference autofocus in Live View, which is caused in part by the physical limitations of the mirror and the AF sensor (hit up the read link for some more background). Canon's solution, it seems, is to place the phase-difference AF sensor below the image sensor instead of above it, and make use of a semi-transparent mirror that'd allow for simultaneous transfer of light to the image sensor and the AF sensor. The average user would see the changes in the form of three new shooting modes, including a Standard Viewfinder Mode, a Fast AF Live View Mode, and a Full Live View mode, which each shift the position of the mirrors for the task at hand. What's more, while the patent application doesn't address it specifically, it would seem that this same method could also bring autofocus to DSLR video or, at the very least, video recorded via Live View.
Nice. But do we really need live view? We didn't have it when we were still shooting on film and it worked perfectly. I feel live view might only be a must for the new generation of photographers, who have grown up with a digital p&s camera, but not for the ones that made the transition from analog to digital.
hear here, I say.
There are certain shots that are only possible when using Live View to accurately take. Sometimes you want to move the camera to a place where you simply can't place your eye in proper position to use the view finder. In those cases, however rare, Live View is a really nice feature.
I must be honest though, this isn't something I'd be willing to pay that much more for. I'm only a (very dedicated) hobbyist so my price range is very stiff when it comes to this. But if it was only $50 more, or hey if it was free and simply an additional tick box between two cameras (if you're buying new, I'm Canon... to much spent on lenses to even consider the others now) it might have been a big deal to me.
I find live view to be very nice for several things. First, is manual focusing from a tripod, especially at night through a 1.6x finder. Zooming into the image makes sure I nail the focus on shots. Also, it can aid in a way that an angle finder would, but without having to buy a relatively expensive accessory. I use it a lot in photojournalism-type shots with the camera above my head. Live view has helped me make many shots that would have been very difficult - or, at least it's allowed me to take a couple frames instead of half a card's worth to know I nailed the shot.
I don't have to buy Canon products.
I can always use my film camera.
The thing is, I am more than willing to evolve and would enjoy using this from time to time as needed. I have already wished I had this several times in bumping crowd shots shooting overhead.
Where live view is really cool is studio shooting, where you have your camera hooked up to a computer. You drive the camera from the computer, and can zoom in to 1 to 1 pixels on the screen and ensure that you have perfect focus. That is very hard to do with the viewfinder or even the screen on the back.
In the field I have never really had a use for it. If I can't get my eye to the viewfinder I probably can't get a good view of the screen either. In these cases I just shoot "blind" and review the pictures, and if need be shoot some more and delete the bad ones. This wasn't feasible with film cameras.
On the whole I'd rather have live view than not, but I probably wouldn't use it that often.
Is Canon's method all that different from Nikon's method?
I use Live View on my XSI all the time for odd-angle shots. (on a tripod). A couple examples.
http://www.flickr.com/photos/ryfter/3235828098/ - I took a picture of the Moon, and Venus. You will NEVER handhold that long, so it was on the tripod. Because I used Live View, mirror lock was already engaged. I could then move the focus square over the moon, and zoom in to 10x magnification, and get the precise focus.
http://www.flickr.com/photos/ryfter/3221419007/ - Here is another night shot. Not only night, but HDR. Live view let me zoom in on the flag up front, and focus precisely. It then fired off 3 shots in a bracket to give me the HDR images I needed.
http://www.flickr.com/photos/ryfter/3108429165/ - On this one, another night shot, live view let me zoom in on a building, once again, at 10x magnification to get perfect focus. It also let me compare what was in the LCD with the scene in front of me, to help me compose the shot how I wanted it. While it sat on the tirpod, I could look beyond it.
Live view has its uses. Just because it was not used in the film day, does not mean it isn't an evolutionary step forward. JUST like the viewfinder, it is a tool. I do NOT, at ALL advocate holding the camera out a foot in front of you, while you take pictures, like a point and shoot. Use the viewfinder for that. Use Live View when you are on a tripod, or in an angle that doesn't LET you look through the viewfinder. Otherwise, brace the camera to your face, and use the viewfinder.
Wouldn't having a semi-transparent mirror (as opposed to a fully reflective mirror) reduce image quality? I would think allowing light to pass through the mirror would affect the amount of light hitting the sensor and thus reduce clarity. The whole rule of thumb has always been to get as much light on the sensor as possible for the best image, but this seems a bit counter-productive for anyone who actually knows what they're doing (ie. someone who doesn't need Live View for their basic needs.)
The light doesn't hit any mirror en route to the sensor. It comes straight off the lens.
I'd be more worried about a dim viewfinder image, though, which is transmitted to the photographer's eye by the mirror at hand.
If I see that diagram correctly, the light does pass through the mirror during live view, but the mirror probably flips out of the way for the actual photograph, just like in every other DSLR without live view.
So yeah, a semi-transparent mirror might reduce the amount of light hitting the image sensor and/or AF sensor, but your photos should get the full light.
I'm sure that they'll look into boosting sensitivity on the image sensor to compensate, and I assume that won't be too much of a problem. I do wonder about low-light focusing with the AF sensor with the assumed light level reduction off that semi-transparent mirror.
Still, this is a very cool idea. I'd wondered before if this sort of thing could've been done, and apparently I'm not the only one.
Ever heard of the EOS 1-NRS? Canon is simply reusing it's pellicle mirror again.
"...is to place the phase-difference AF sensor below the image sensor instead of above it..."
The AF sensor has always been at the bottom of the mirror box. That's why there is a beam splitter attached to the mirror.
I can't wait for this to be implemented into their new models.
Am I the only one that sees a revolver in the picture??
Nope. Rotate it counter clockwise and it totally looks like a gun.
The G1 does this using contrast detect and the sensor. No extra phase AF module, just the sensor and some fancy algorithms. DPReview found it quite fast and accurate, too.
The G1 is fast when using the two available µ4:3 lenses. It slows down considerably when using conventional 4:3 dSLR lenses (the few that actually support CD autofocus). And 4:3 is a fairly modern system. The basic lens to body communication that Canon uses was designed in the 80s.
@plothole thanks for informing me of that! It seems odd, anyway- any speculation as to why? I don't own a G1, but for obvious reasons I am following the development of CD autofocus and electronic VF. I think that Panasonic has come up with a much simpler system for accurate Live View (among other uses for sensor-only-optics cameras); but it's clear that Canon needs to preserve the excellent AF and optical VF systems it has already developed; so until the industry forces it to, it won't be going the EVF route.
Well when these lenses were designed, they only needed to be told to move a certain distance. Contrast detection complicates things by requiring that the lens repeatably stop and reverse itself. This means you need quicker and finer control over the motor. With µ4:3 they added a couple of pins to the mount, supposedly to speed up commands going to the lens. The motor and controller inside the lens are also presumably optimized for this behavior.
Now a vaguely related line of thought... What if they made a hybrid VF? I remember a story awhile back about transparent OLEDs. Imagine if they put a tiny screen using this technology within the optical path of the viewfinder!
I think most DSLRs will switch to pellicle mirrors (fixed partially reflective mirror) over the next few years. It'll make SLRs quieter and have less camera shake. The only problem is it takes away about 1/3rd of your light. But digital sensors are so sensitive now that they can afford to lose that much with virtually no impact.
http://en.wikipedia.org/wiki/Pellicle_mirror
It will save a lot of money, mechanics and it means no more cleaning your sensor (you will have to clean the mirror though). It also means full-time live view for both the camera user and the sensor. Having the sensor able to see the image all the time means that the camera can do live view, can do AF with the sensor itself (makes the mirror cheaper) and also the camera can have such features as smile shutter that P&S (range finder) cameras have.
Autofocus with the main sensor can already be done on several dSLRs while in liveview mode. The problem is that it's slow. The lenses and interface between them were never designed for the fine control needed for Contrast Detection style autofocus.
Lose 1/3 of the light? No way. This is a battle for F-stops here, man.
Just because the sensors aren't right for it today doesn't mean they never will be. Once companies decide they have to have the main sensor see the image all the time for features like face detection, they'll begin to change their designs so the main sensor can be used in a different mode for focusing than for image preview. And then the second sensor can be removed to save money.
It's not a problem with the sensor. It's a problem with the lens. They weren't designed to go back and forth as contrast detection demands. Only to be told to move to a certain point after the phase detection autofocus sensor had already determined what distance the subject was from the camera.
Perhaps the lenses could be modified. Or the sensor can be modified so that the main (matrix) sensor can provide the information needed to do phase detection autofocus. I would expect the latter, because backwards compatibility with lenses is fairly important to these companies.
Panasonic was able to optimize the µ4:3 mount and lenses for strict contrast detection. So the former certainly isn't impossible.
The phase detection sensor doesn't have the same point of view as the imaging sensor. It actually peers through the lens at an angle... or two angles rather: One from each side of the lens. This way it gets two slightly different images which it can compare. In a sense it works a little like our depth perception.
It's clear that Canon is developing a faster AF system for LiveView (they already stated that months ago at Photokina) as well as improving Movie mode, because they know that those features will have Huge future and demand
LiveView has enormous potential.
Sometimes is impossible to use the viewfinder, or just much easier to use LiveView.
Also the information displayed in LiveView can be a lot more than in the Viewfinder.
Each way to frame and shoot has its own advantages, so they both MUST be available considering the customers have a very wide range of different needs / tasks / jobs
Thinking that a feature "should not" be included in a DSLR just because "it does not belong to the DSLR world" or one doesn't use it, it's a mistake due to having somewhat narrow perspective of other people's needs, the trends of technology, the demand and the companies' business.
This improvement indeed will help a lot the Movie/Video mode in DSLRs, wich is also going to be improved and implemented in future DSLRs, despite some people like it or not.
(if you don't like, just don't use the feature. It's that simple. And if you don't want to pay "more", don't worry, it won't make much difference to today's DSLRs, so the "extra" price won't show up)
LOT OF CUSTOMERS, both Amateurs and Professionals (Photographers and Videographers) can get lot of advantages of such implementations, wich will offer a level of quality and features that are not still available under several thousands dollars (and some are even inexistant yet)
So, GREAT NEWS that Canon is working on it. Hope that Nikon and other companies do the same.
is this similar to Sony's live view version?
Not quite. The a300 series from Sony uses a small video sensor located within the viewfinder chamber to record live view. This means the mirror stays down so it can exploit the phase detection AF at the same time. But there are a few drawbacks:
- It doesn't have the resolution or sensitivity that the main sensor could offer.
- Like the optical viewfinder, the view is not 100%, nor is the depth of field preview completely accurate.
- In order to fit it in the viewfinder chamber they had to shrink the VF itself.
http://en.wikipedia.org/wiki/Canon_Pellix (1965)
BTW,
For Full Fframe camera bodies, they should make them a bit bigger to allow this system fit inside.
Otherwise, it would be TERRIBLE (and Ridiculous) to have Faster LiveView AF in xxD (crop sensor) than in xD (Full Fframe) DSLRs !!
It think this is a great idea. I for one would love a fully autofocusing "movie" mode on my next DSLR. At the moment when I go out I have to take my DSLR (Canon 40D) + lenses AND a video camera as I shoot loads of family/childrens movies as well as good quality stills. The ability to shoot video footage through a decent lens and have full control over things such as depth-of-field etc is something that woud cost thoasands on a professional video camera.
I think this convergance of function is a logical way to go - obviously such technology would not replace a separate cheap video and/or point-and-shoot camera, but for the advanced Amateur who wants access to such techology it would be great!
This system let you have faster autofocus on cost of the quality of the light. less light to the sensor will mean more noise in the eventual video recordings. The other option is slow autofocus and full quality
Sonys system implies a mirror-viewfinder which means you will not see it in the current implementation in higher end models. It also uses a dedicated (smaller) sensor which means you have a different depth of field than what your photo will have. A video would maybe benifit from the smaller camcorder-sensor in terms of speed but suffer in terms of depth of field control.
So all three systems have drawbacks and are not perfect.
The perfect solution would be to make a sensor that does phase detection as well as image capture. if that ever can be combined. Or that computer power increases to a point contrast detection speeds up at par with phase detection.