Does Purple Haze mean more to users of the iPhone 5’s camera than just being a Jimi Hendrix song? According to a number of sources online, the camera on the new Apple phone differs from its predecessors in one significant way: if you take a photo with “lens flare” from an out-of-picture light source, the flare will appear purple rather than white or yellow.
DP Review, one of the most authoritative sites on the web for camera reviews, looked at the iPhone 5 camera and on page 3 of that review comes to the vexed issue: “people began reporting an issue with a mysterious ‘purple haze’ appearing in photos taken with a bright light source just outside of the frame,” its reviewers noted. “There has been a lot of speculation to what may be causing this phenomenon.”
They then go on to consider and reject a number of possibilities: sensor blooming (“No. Blooming happens when the intensity of the light reaching the sensor is so great that there is an overflow of electrons that spill over into adjacent pixels. This is not a satisfactory explanation for the iPhone 5‘s purple haze, since sensor blooming affects the entire imaging field”).
Chromatic aberration (“No, definitely not. Typically visible towards the edges of the frame especially in images taken using wide-angle lenses, CA takes the appearance of a green-and-magenta or blue-and-yellow ‘fringing’ around peripheral scene elements”), infrared sensitivity (“No, it isn’t. This is one of the more outlandish suggestions”), or internal reflections/lens flare (“Almost certainly, yes”).
And indeed, Apple has now put up a support note entitled iPhone: Camera image effects, it says that:
Most small cameras, including those in every generation of iPhone, may exhibit some form of flare at the edge of the frame when capturing an image with out-of-scene light sources. This can happen when a light source is positioned at an angle (usually just outside the field of view) so that it causes a reflection off the surfaces inside the camera module and onto the camera sensor. Moving the camera slightly to change the position at which the bright light is entering the lens, or shielding the lens with your hand, should minimise or eliminate the effect.
But why is it these effects are only now showing up? As DP Review notes: “The primary way that lens manufacturers prevent lens flare is to coat the glass elements in an anti-reflective coating. The iPhone 5’s lens elements are coated, but we don’t know if the coatings are identical to those used in the iPhone 4S’s lens. Maybe the sapphire glass has to be coated differently we don’t know. And we won’t know, unless Apple releases a more in-depth statement about the technology.”
The support note doesn’t really fit that bill, though. Jared Earle, who is a sysadmin by day, photographer by other days, suggests that the purple element of the flare is definitely down to the anti-reflective (AR) coating: “The iPhone 5 uses a new sapphire lens, and the AR used on sapphire is different to the AR used on mineral glass,” he explains. He also offers two photos taken by an iPhone 4S (which has a different lens), in one of which he’s put an AR filter on as well.
And now, the same camera but with a piece of sapphire with mild anti-reflective coating:
But is that all? To find out a little more about the challenges of avoiding lens flare in cameraphones, I spoke to Damian Dinning, who works for Nokia as its lead program manager on imaging experience.
Of course, he points out that he cannot speak about what Apple’s done, though he has seen the discussions about purple haze online. “It could be caused by a number of things. Apple may know exactly which part is the cause.”
Lens flare is a known problem, of course: “In an SLR [single lens reflex, such as the cameras used by professionals] lens it’s much easier to protect against, though it’s still a challenge,” he explains. “Often you’ll see long lenses with lens hoods; and TV cameras have them too.” Those all protect the lens against incident light from areas you’re not pointing the lens at.
But building cameraphone optics turns out to be a hugely complex process, which is full of tradeoffs. You want your phone to be thinner (usually); but thinner means you have less depth in which to put the multiple lenses you can use to filter out effects such as lens flare.
But multiple lenses are needed: “you put in one lens, and then another lens to correct errors such as aberration or soft edges in the first, and then another lens to correct the errors in the previous one,” he explains.
“We prioritise having a wider angle lens, and protect against lens flare in the design with the lens coating. And we also have matt black internal surfaces.” The latter protecting against internal reflection from lens elements which could cause other sorts of flare.
So between DP Review, Apple, Jared Earle, and Nokia, it seems pretty clear what the answer to the iPhone 5’s purple haze is: the combination of the anti-reflective coating and the sapphire lens. What’s also clear: ever since the iPhone 4, people will examine new iPhones minutely for anything that seems slightly different from what went before phone reception, battery life, and of course maps and analyse it to bits.
From: guardian.co.uk – Read more