Only a few days after the iPhone 5 became available, people began reporting an issue with a mysterious 'purple haze' appearing in photos taken with a bright light source just outside of the frame. There has been a lot of speculation to what may be causing this phenomenon. Some theories revolve around sensor blooming and chromatic abberation, some speculation centers on the new sapphire glass element in front of the lens.
|iPhone 5 - (Outdoors - mid-day sun)||iPhone 4S - (Outdoors - mid-day sun)|
|iPhone 5 - (Studio, artificial light)||iPhone 4S - (Studio, artificial light)|
The so-called 'purple haze' issue is most obvious when the adjacent area to the flare is quite dark, which is a scenario that is actually quite common in low light situations where you may have a single bright lamp in an otherwise dark room. It can also be triggered by shooting towords the sun, or with any particularly bright light source at or just beyond the edge of the frame. So what's causing it? Here are some of the various explanations we've seen bandied around online.
Is it 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 adjcent pixels. This is not a satisfactory explanation for the iPhone 5's purple haze, since sensor blooming affects the entire imaging field, meaning that you'd see it associated with a bright highlight positioned anywhere in the image, not just towards the edges of the frame, as we're seeing in pictures from the iPhone 5.
Look at the bright light at the top of the frame in this picture (not taken on an iPhone) - that's blooming.
Blooming is sometimes accompanied by vertical or horizontal purple lines, caused by electrons cascading between pixels during read out. Cameras with CCD sensors are more prone to blooming (the camera in this example is CCD) than the CMOS sensors found in the iPhone 4S and 5, and you're more likely to see it in video footage than stills.
Is it chromatic abberation?
No, definitely not. Typically visible towards the edges of the frame especially in images taken using wideangle lenses, CA takes the appearance of a green-and-magenta or blue-and-yellow 'fringing' around peripheral scene elements. It looks nothing like the iPhone 5's purple haze.
Is it because the iPhone 5's sensor is really sensitive to infrared light, like the Leica M8?
No, it isn't. This is one of the more outlandish suggestions that we've seen proposed, but still...
Long-time readers of this site will remember that the Leica M8's sensor was exceptionally sensitive to IR light which among other consequences meant that it had a nasty habit of turning black man-made fabrics purple. Leica ultimately 'fixed' the issue by sending M8 owners screw-in IR filters to attach to their lenses.
Is it internal reflections / Lens flare?
Almost certainly, yes. The most likely cause of the iPhone 5's purple haze is probably lens flare and internal reflections in the camera lens assembly. All lenses are succeptable to lens flare to some degree, and as you can see from the images at the top of this page, the iPhone 4S isn't immune either (ditto the iPhone 4 and competitive smartphones from other manufacturers).
But in our shooting we've found that it's a little more noticeable on the iPhone 5. So why is that? It's unlikely that the flare is solely due to the much-vaunted inclusion of a sapphire glass lens cover (although the refractive index of the sapphire glass is different to conventional optical glass, so it could be a contributing factor). Our money is on it being caused by a combination of different things, none of which, alone, is unique to the iPhone 5.
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.
Really, our advice is not to worry. Just do what you should do anyway, and avoid putting bright lights near the edge of the frame when shooting.