Lights, Camera, ACTION !!!
Sizing up the iPhone 4 for shutterbugs
When Apple announced the iPhone 4 at WWDC 2010, however, the company ticked off nearly every box on the wishlist: more megapixels (though as ever, there are people for whom no amount is ever enough), better low-light sensitivity, an LED flash, a wider angle of view, 720p HD video recording, a front-facing camera, and more. An iPhone 4 isn’t going to replace a DSLR or a high-end point-and-shoot, but for all but the most devoted pixel-peepers, the hardware is capable of some very nice images.
Let’s start with a look at the image sensor itself. The iPhone 3GS has a 3.1MP, 1/4″ CMOS sensor from OmniVision. That sensor has 1.75µm pitch individual pixels. Rumors coming out of Taiwan late last year suggested that OmniVision would supply Apple with 5MP sensors for the next iPhone, and we speculated that Apple would use one of OmniVision’s 1/4″, 5MP sensors as a drop-in replacement.
The increase in pixels alone means more detail and potentially sharper images, especially when printed. 3.1MP images can usually be printed up to 5 x 7″ at “photo-quality,” while 5MP images can be printed at about 8 x 10″—big enough to be framed and put on the wall. To be fair, though, something like 99 percent of the photos taken with an iPhone are going to end up on Twitter, Facebook, Flickr, or on the iPhone’s 3.5″ screen—in many cases, the extra resolution won’t be readily apparent.
Backside illuminated pixels
Packing more pixels onto a given sensor size makes them smaller, so a 1/4″ 5MP sensor has 1.4µm pitch pixels. All other things being equal, smaller pixels means diminishing light sensitivity, but OmniVision had begun implementing a sensor design technique called backside illumination. This process technique flips the order of the photodiodes—usually on the bottom—and wiring—usually on top—making sure more of the light striking the sensor actually hits the photodiode itself. Using this technique, OmniVision’s 1/4″ 5MP sensor with BSI was about as sensitive as its 1/4″ 3.1MP sensor without it.
During the recent WWDC keynote, however, Steve Jobs announced that the camera in the iPhone 4 would still use a 1.75µm pixel pitch combined with backside illumination, giving an even bigger boost in light sensitivity. In fact, judging from the specs published by OmniVision, the combination will be nearly twice as sensitive as previous iPhone models. Images shot in low-light situations—like having some post-WWDC drinks at a nearby watering hole, or even grabbing an atmospheric sunset—should look much better than they do now.
We spoke to Dr Eric Fossum, an expert in CMOS image sensor technology, about the importance of backside illumination for digital cameras. “Backside illumination was essentially devised for the mobile market—as sensors continued to shrink, something was needed to increase the sensitivity,” he told Ars. Fossum explained that while the BSI concept and working sensors have been around for some time—it is widely used in CCD sensors for specialized scientific applications, for instance—the technique is essentially state of the art for CMOS sensors. Companies like OmniVision, Aptina, Sony, and Canon have just recently begun using the technology due to silicon process techniques that have made BSI possible for mass-produced, consumer-oriented sensors.
The technology is absolutely critical for the tiny 1.4µm and even newer 1.1µm pitch sensors that are finding their way into more and more mobile electronics, Fossum explained. “At 1.75µm, BSI is not a huge advantage, but it certainly helps.”
Another consequence of the decision to keep the larger pixel pitch is that the sensor also has to be physically larger. Instead of a 1/4″ sensor, the iPhone 4 has a 1/3.2″ sensor. However, the lens is still the same 3.85mm focal length as that of the iPhone 3GS. The net effect is that the iPhone 4 has a wider angle of view—closer to that of a 28mm on a 35mm-format camera, versus the equivalent of about 35mm on older iPhone models.
Those who shoot landscapes, architecture, or groups of friends in tight interior spaces will appreciate the wider angle of view the most. Those of you trying to take pictures of your favorite band on stage from 12 rows back won’t gain anything, though the extra megapixels allowed Apple to add a digital zoom feature. Digital zoom is really just a clever marketing term for “cropping,” however, so you won’t be getting any better pictures than if you cropped them yourself on a computer.
Some pixels are more equal than others
After Apple announced the hardware specs at WWDC, we saw some readers quickly lament Apple’s decision not to jam in an 8MP or higher sensor that some of the competition is sporting—after all, if 5MP is better than 3.1MP, then 8MP (or even 12MP) is even better, right?
Unfortunately, not all pixels are created equal, and it’s worth considering the impact that more megapixels would have on the iPhone’s design as well as the resulting image quality. For comparison’s sake, OmniVision offers an 8MP sensor in the 1/3.2″ size; since Apple already chose a 1/3.2″ sensor for the iPhone 4, there’s no difference there. However, it would require a larger and/or more expensive lens to resolve enough detail to take advantage of those additional 3 million pixels—not an easy feat since Apple shaved off 24 percent of the size of the iPhone 3GS. Further, an 8MP sensor has 1.4µm pixel pitch, with sensitivity below that of the iPhone 3GS.
Going 8MP or higher might look more impressive on a spec sheet, but the resulting images would be more noisy and less sharp. Consider the HTC EVO 4G, considered to be very competitive with the iPhone 4: Gizmodo noted that its camera does well in bright light, but becomes a “crapshoot” once away from the power of the Daystar. Throwing more megapixels into a mobile phone just isn’t a good solution, and from a photographer’s point of view, Apple has chosen a wise balance between size, cost, and image quality.
The camera hardware also has two qualities that have remained constant through all iPhone models: an electronic rolling shutter and a fixed f/2.8 aperture. An electronic rolling shutter scans each line of a sensor in succession as opposed to a mechanical shutter. Usually this happens fast enough to produce a nice still image, but it can result in some really strange effects due to subject or camera motion. The fixed aperture isn’t much of an issue since the tiny sensor and wide angle of view result in a relatively large depth of field. However, MEMS-based shutters and adjustable apertures for mobile camera modules are just now coming to market, and could offer improvements in motion capture and exposure control in the future.
Another oft-requested feature addition that iPhone 4 brings is the inclusion of an LED-based “flash.” This isn’t a flash in the traditional sense of the term—there’s no xenon flash tube and no waiting for its capacitors to charge. Instead, there is a relatively bright white LED that can light up in low-light situations. For the most dimly-lit interiors, this LED should be sufficient to light up subjects that are close-by—a typical head-and-shoulders portrait, for instance.
There are some downsides to the LED flashes that I’ve experienced on other mobile phones, however. Typically, the LED is generally of the “cool white” variety, meaning the light output is slightly bluish. In most low-light situations, this contrasts sharply with the warm color temperature of indoor lighting, and can result in bluish faces on your subjects. The quality of the lighting leaves a lot to be desired, too. Typically, subjects are either still too dim, or are washed out by the LED. We’ll have to wait until we get our hands on a shipping unit to really evaluate it, but our hope it that it can be turned off by the user—if the quality of light sucks, we’d rather do without it.
HD video and more
With the updated sensor, Apple also bumped the video recording capability from a rather pedestrian SD resolution to full 30fps 720p HD. The OmniVision sensor is capable of 1080p at 30fps, but that could tax the iPhone’s processing hardware and could quickly eat away at the built-in flash storage. 720p is more than adequate for most uses, and could for many users replace a separate device like a Flip.
The sensor can also record 720p at up to 60fps—especially cool when played back at 30fps for a slow-motion effect—though we have seen no indication that the built-in recording software can take advantage of this feature of the hardware. It’s possible Apple could expose this capability to developers via new media APIs, though we haven’t yet seen any indication this is happening, either.
Beyond the HD resolution increase, though, two feature additions are there to improve the footage captured with an iPhone 4. Apple added tap-to-focus for video recording, and like still image capture, it will adjust focus and autoexposure for the subject in the area tapped. Also, the LED flash (called a “torch” in video parlance) can be turned on as an additional light source (cue the amateur TMZ paparazzi footage).
The Camera app still has the same basic editing features introduced in iOS 3, but for iPhone 4, Apple is also offering the $4.99 iMovie for iPhone. Along with transitions, titles, themes, and more, iMovie for iPhone can edit and output 720p footage for a complete handheld HD studio that fits in your pocket. Because it requires the A4 processor and copious amounts of RAM for the realtime effects and 720p video rendering, iMovie only works on iPhone 4.
Apple also added a long-awaited front-facing camera to enable FaceTime video calls. Apple wouldn’t comment on the specs beyond what is listed on Apple’s website, which says the front-facing camera is capable of “VGA-quality photos and video at up to 30 frames per second.” It is likely run-of-the-mill, commodity VGA webcam hardware mated to a fairly wide-angle fixed-focus lens, judging from the FaceTime demo during Jobs’ keynote. The specs say it’s capable of stills, but you won’t likely be using these beyond the occasional Facebook or Twitter avatar update. Unlike the rear-facing camera, there’s no LED flash, though the 3.5″ display can serve as an additional light source (much as it does when using an iSight on a MacBook or MacBook Pro).
There are some missing features I’d like to see that Apple could easily add via software. One is some kind of exposure control. Full manual control isn’t necessary, but it would be nice if there was an exposure compensation control to tweak the autoexposure in tricky lighting. Another is improved white balance; though manual white balance might be overkill, a handful of common presets would be welcome. Also, the main camera sensor includes the ability to use 2 x 2 pixel “binning,” meaning it treats four pixels as one. This can improve images shot in very low light in exchange for lower resolution—personally, I think better exposed 2MP images beat a murky, noisy 5MP image any day.
In all, the iPhone 4 offers a lot of photo and video power for the average user—some may even find it enough to consider eschewing a standalone point-and-shoot still camera or compact camcorder. Those with more serious imaging needs won’t be ditching a DSLR or 3-CCD video camera anytime soon, but there are a few welcome improvements for casual use. Judging from the samples Apple has provided so far, the improved camera hardware looks like it will be a solid performer.