Exclusive: Fujifilm's phase detection system explained
With the announcement of the Fujifilm F300 EXR and Z800 EXR coming on a day that also saw four other cameras being launched, it would be easy to overlook their most radical feature. Because, with the latest version of its EXR sensor, Fujifilm has achieved something that's been hoped for but not previously brought to market - through-the-lens phase detection autofocus on a compact camera. The company is claiming the system enables focus times as fast as 0.158 sec. We got some more details of the system from Hitoshi Yamashita, manager of the company's Technical Support Group.
What's so special about Phase Detection AF?
All current compact and mirrorless cameras use contrast detection AF, where the lens is racked back and forwards until the camera finds the position that gives the greatest contrast (which signifies being in focus). Their lenses tend to be designed with very light focusing elements that are fast to move so that this process of trial-and-error can be conducted as quickly as possible. Intelligent processing also attempts to minimize the need to hunt too far for focus but there is still some hunting to be done.
Phase detection, the primary focus method used in most DSLRs tends to be faster but works very differently. To understand why what Fujifilm is trying to do is so desirable, it's first necessary to understand how it works.
The easiest way of understanding phase detection is to think about what would happen if you only considered the light entering the far left and far right sides of a lens as it focuses an image.
|In this simplified schematic, you can see what happens to the image cast by the light passing through the left (blue dotted line) and right (red dotted lines) sides of the lens. |
When in focus, the light from both sides of the lens converges to create a focused image. However, when not in focus, the images projected by two sides of the lens do not overlap (they are out of phase with one another).
Of course this is a massively simplified diagram with a single, vertical straight line as the subject (and no inversion of the image as it passes through the lens). The point is that we can derive information about focus if we can separately view light coming from opposite sides of the lens.
| How does a phase detection sensor 'see'? |
And we don't need the whole image to do this. Think about a strip of pixels taken from the sensor in the previous diagram. If you could make one such strip that receives light only from the left hand side of the lens and another that 'looks' only to the right-hand side of the lens, then you have enough information to find focus.
By comparing images from just these two strips it's possible to work out not only how far but also in which direction the lens needs to be moved to bring them into phase.
As you can see, the key thing required for phase detection is the ability to capture light only from certain parts of the lens so that (at least) two distinct images can be formed and compared. This information makes it possible to calculate exactly where the lens needs to be moved to, without any of the trial-and-error of contrast detection AF.
It also makes continuous AF easier, since the system can quickly calculate a new focusing distance (and even assess the subject's rate of movement), whereas the contrast detect system has to hunt again, during which time the subject is still moving, meaning the point of highest contrast will keep changing.
Traditionally, phase detection in DSLRs has been conducted by a dedicated AF sensor. The main mirror on DSLRs is slightly translucent which allows some light through to a second mirror and down onto this sensor. This light is split by a series of prisms and lenses in front of the sensor so that different lines of pixels on the sensor receive light from different parts of the lens.
As soon as you see this, you can understand why conventional compact cameras (and all the current mirrorless cameras) have to use contrast detection autofocus. Their sensor elements (sensels) are designed specifically to catch light from all directions and all areas of the lens at once. As such they have no way of separating the incoming light into distinct images. Even with the fastest, lightest lens elements, the trial-and-error nature of contrast detection tends to make it slower.
How does Fujifilm's system work?
Fujifilm's system provides a seemingly simple solution - masking-off half of a sensel means it only receives light from one side of the lens. By creating strips of these sensels, half 'looking' one way, half 'looking' the other, the camera gets the two distinct images necessary for phase detection AF.
'The structure is equivalent to the two light paths divided by two dedicated lenses in conventional phase detection AF device in DSLR's,' says Mr Yamashita.
Although he wouldn't be drawn on the exact arrangement he does give some detail: 'The AF sensels are only arranged in the center area of a CCD, so when phase detection AF is activated the AF point is fixed to the center of an image.'
Of course this means some sensels are receiving 50% less light than their neighbours. Yamashita suggests it need not be a big problem: 'we use several tens of thousands of pixels in the center area of a CCD, which is a very small number of pixels compared to the 12 megapixels used for imaging.' And, he says, they don't simply go to waste when taking pictures: 'sometimes they are used to compose image data and sometimes not, depending on the situation.'
In low light where there isn't enough light to allow phase detection (since the focus sensels are only receiving half the available light) or when face detection is needed, requiring the camera to focus away from the center point, the camera switches back to contrast detection AF. As such the company is billing the system as 'Hybrid AF,' but that shouldn't disguise what is a potentially significant development for compact cameras. We'll be interested to see how the promise of this system works when we get a chance to subject it to testing.
|Forever Stalled by Domenick Creaco|
from The End of the Road
|Lost, But Not Forever by Domenick Creaco|
from Lost and found
The GoPro Fusion is designed to make it easy to capture 360-degree video and stills. We took it out recently on a typically hot Seattle summer day to see what it can do.
We've got our hands on a full-production Nikon Z7 camera and have updated our gallery with additional samples.
A new Kickstarter campaign seeks funding for Chroma Chrono, a programmable RGB camera flash that emits multiple colors during long exposures.
Think Tank Photo has launched a new lineup of six dual-access, water-resistant protective lens cases it calls Lens Case Duo.
Canon and Nikon finally entered the full-frame mirrorless market this summer with the brand-new RF and Z mounts. Now that we've had some time with the cameras, we wanted to revisit our earlier predictions and take stock.
The devices' camera specs look pretty much identical to last year's iPhone X but under the hood a number of important improvements have been made.
Blackmagic Design has announced the public beta of its new Blackmagic RAW video codec. The company says the new format combines the benefits of shooting Raw video with the ease of use and smaller file sizes usually associated with non-Raw video files.
Serif, the company behind the Affinity suite, has announced the latest update for its mobile Photoshop competitor Affinity Photo for iPad.
The Atomos Ninja V external video recorder and monitor will be ready to ship at the end of this month. The 5.2in Ninja V is designed to provide a smaller option, while still offering many of the features of the larger 7-inch models.
Having shot with the camera, spoken to Canon and read the tea leaves, here's what DPR Technical Editor Richard Butler thinks the EOS R tells us about Canon and the RF's mount's future.
After last week's teaser, lighting manufacturer Profoto has announced its 'small big' new product. The B10 is designed to be used as studio flash head but in a very small body, and has a powerful continuous light source for videographers as well.
Konseen has launched Photo Studio, a new light box tent large enough to photograph people, as well as objects.
Seagate has introduced new high-capacity hard drives for Network Attached Storage (NAS) devices: the 14TB IronWolf and 14TB IronWolf Pro HDDs.
The case was first announced earlier this year as a Kickstarter campaign and comes with a range of features aimed at iPhone photographers.
Manfrotto has introduced a new two-in-one tripod to its Befree lineup. Called the Befree 2N1, this new addition is both a tripod and monopod in one and is available with both of Manfrotto's locking mechanisms.
This new high dynamic range editing software comes with an AI-powered Quantum HDR Engine for improved photo merging.
Apple has unveiled the next generation of its iPhone X in the form of three variants: the 5.8" iPhone XS and 6.5" iPhone XS Max with OLED screens, and the 6.1" iPhone XR with an LCD and single rear camera.
Ahead of the launch of the CamRanger II the company has announced a mini version of its wireless remote control system that it says has a longer range than the original in a body half the size.
Lens manufacturer Sigma has announced a trio of fast cinema lenses for full-frame camera systems, that it says will also be available in the future in the LPL mount for Arri’s large format camera system.
LumaPod is a a new tripod being funded on Kickstarter that takes just four seconds to set up and uses patented tension technology to keep your shots steady in a compact design.
X-Rite ColorChecker Video XL is an oversized color target for wide-angle, long distance, and aerial shooting.
ExperimentalOptics has launched a Kickstarter campaign to fund its second lens design, a 35mm F2.7 lens it claims is the world's 'smallest fastest pancake lens.'
The new XF 8-16mm F2.8 R LM WR and XF 200mm F2 R LM OIS WR are aimed at enthusiasts and professionals, and add considerable versatility to Fujifilm's growing XF lens lineup. We've been taking a look.
The Getty family is working to regain control of stock photo agency Getty Images, according to multiple reports published late last week.
The Phoneslinger line, a modular bag system for mobile photographers, has been launched on the Indiegogo crowdfunding platform.
CamRanger has announced the impending arrival of its CamRanger 2 wireless tethering and trigger system, complete with redesigned apps, updated wireless features, and support for select Sony and Fujifilm systems.
As well as high-resolution stills, the new Nikon Z7 also shoots 4K video and 120p HD video. We recently spent two days with director Chris Hershman, shooting a music video on the Z7 for pop artist Emily Blue.
London’s National Portrait Gallery has released the shortlist for its annual Taylor Wessing Photographic Portrait Prize, ahead of the winner being announced in October.
Lighting manufacturer Profoto is teasing users with a video clip that includes frames of a new light it will announce later this month. The launch as billed as ‘something big’ for ‘something small’.