A Closer Look at the G3's IR Laser Auto Focus System
by Joshua Ho on May 27, 2014 4:15 PM EST- Posted in
- Smartphones
- LG
- Mobile
- G3
Earlier today LG announced its 2014 flagship: the G3. Based on Qualcomm's Snapdragon 801, the G3 features a 5.5" 2560 x 1440 display and a host of other improvements over last year's G2. Although the front and rear facing cameras don't change in terms of resolution compared to the G2, the imaging system did see some upgrades. Optical Image Stabilization (OIS) is back for the G3, but it now includes stabilizing along the z-axis as well (hence OIS+). The big new feature however is the IR laser rangefinder that aids in improving focus time for the rear camera. LG claims the G3 can grab focus in 276ms. It's clear that the G3's AF system is one of the most interesting that I've seen in smartphone imaging, full stop. While PDAF (Phase Detect Auto Focus) is standard on dSLRs, laser rangefinders are generally rare and from another era.
While some guessed that this system was a method of assisting with auto focus by projecting a grid of lasers that would assist with detecting maximum contrast, LG uses accurate timing of emission vs reflection to determine the distance to a target. Looking at the IR window with an IR sensitive camera, the beam emitted is extremely thin in angle, much more focused than the average laser pointer. This means that the chance of multiple returns is greatly reduced compared to most situations.
LG emphasized the hybrid nature of its system, which explains how the G3 compensates for conditions where the laser auto focus fails. It appears that LG compensates for multiple returns, transparent and reflective surfaces with contrast-based AF detection as needed. LG also stated that the greatest benefit to the laser system is for nearly instant detection of distance to subject for the first two feet, and in cases where contrast detection is used the laser system allows the AF algorithm to automatically skip this distance to speed up focus. LG also says that the greatest benefit is seen in low light. I suspect that in outdoors situations the signal to noise ratio drops significantly. All of these claims will need verification, LG has definitely made a very interesting system for their camera, an interesting trade off between the larger sensor size and PDAF of the GS5 and the OIS+ and laser AF of the G3.
It's not quite clear how the laser AF is implemented in the software though. Based upon some casual inspection there is no clear reference to a laser or IR outside of TV. It is likely that the entire implementation is done through a single camera driver which obscures just how the rangefinder is treated. At any rate, all of this will require further investigation in a full review.
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Guspaz - Tuesday, May 27, 2014 - link
Wait, so the laser rangefinder only works on distances less than two feet? Doesn't that render it rather useless for anything but macro shots?coburn_c - Tuesday, May 27, 2014 - link
I believe he said instant detection of the first two feet. So the camera knows whether it can skip the macro range before it even begins to find focus.coburn_c - Tuesday, May 27, 2014 - link
**Actually it says that exactly, read the article. I don't think it even works in the macro range, it just tells it whether to turn on macro focus or not. I imagine it only work in indoor scenarios, where there isn't enough contrast to do a quick focus and not too much background IR to obscure the beam before it bounces back.Guspaz - Tuesday, May 27, 2014 - link
Except, contrast-based autofocus should be able to tell if the current focus setting is in focus instantly, and chances are that whatever you're taking a photo of is pretty close to the previous thing you took a photo of anyhow. Unless I'm taking macro shots, the camera will never check the macro range in the first place, and even if I am taking macro shots, instantly measuring the focal distance with a laser doesn't obviate the need to physically move the lens.coburn_c - Tuesday, May 27, 2014 - link
Actually it doesn't 'tell instantly'. It moves through the focal range until it finds a best case. The 2ft instant range finding eliminates part of the range it moves through. Again, in plain english in the article and painfully obvious if you've ever watched a macro lens focus.Impulses - Tuesday, May 27, 2014 - link
Plus CDAF never knows which way it needs to move first, which is something this might help with.patrickjchase - Wednesday, May 28, 2014 - link
Contrast-based autofocus can *never* tell if it's in focus instantly, because it can't distinguish:1. A low-contrast scene that is in focus
2. A higher-contrast one that is out of focus
The only way a contrast-based system can distinguish those 2 cases is by "hunting" and taking multiple measurements until it finds a setting that yields maximum contrast.
That's the *entire* reason why phase-based systems are strongly preferred for any application that requires low shutter lag. Phase-based systems can tell how far out of focus they are and in which direction with a single measurement.
patrickjchase - Wednesday, May 28, 2014 - link
I probably should acknowledge that phase-based systems do have pathological cases where what I said isn't true. For a good time, try setting your phase-based SLR to use a single AF sensor, and then aim it such that that sensor is orthogonal to a pattern of regular lines. Doing so causes phase ambiguity (i.e. the camera can't tell 90 from 540 from 810 degrees) and often results in endless hunting.usama_ah - Tuesday, May 27, 2014 - link
Seems that it doesn't "only work" in ranges < 2 ft, but that the biggest advantage ("greatest benefit") is seen in detection of distances within the first two feet. An advantage implies a comparison to another system, in this case the contrast-based AF system. The laser works beyond 2 feet too but it seems success is less likely due to signal-to-noise issues (worse when outdoors? since greatest benefit of laser system is seen indoors). If there is not enough signal (transparent object, too bright to see the signal) or too much signal (due to multiple returns , reflections) then the system can drop to the contrast-AF for focusing, which will assume at first that the object is >2 ft away since usually this laser system can nail the first 2 feet. (but if needed the contrast-AF system can still focus for objects <2ft if for some reason the laser failed there). That was what I got from reading the article.krazyfrog - Wednesday, May 28, 2014 - link
The short distances are where it is needed most. Cameras struggle with focusing on objects that are too close to the lens and often take the longest to focus on closer objects whereas focusing on far away objects is near instantaneous on most cameras. The laser system should help make it faster to focus on closer objects than traditional contrast-based methods.