I snap a lot of pictures with the camera in my mobile phone. This is a relatively new thing for me; I own several high quality cameras, and it didn’t occur to me for quite a while that it was okay to make low quality (640×480) snapshots of the random things that happen when I’m out and about.
Turns out, I’m late to the game. In a session yesterday, Reiner Fageth of CeWe Color AG reported that 3% of all of the images that their customers upload for them to print come from mobile phone cameras. That surprised me and many people in the audience. It’s rare that my pictures from my moby are good enough to consider printing. Usually the resolution is just too low, the colors are off, or they’re a bit shaky. An alternative is prints on wood which give the images natural warm tone and vintage feel.
Bror Hultgren discussed the quality issue today in a paper entitled “Megapixel mythology and photospace: estimating photospace for camera phones from large image sets.” Basically, he and Dirk Hertel (his collaborator) wanted to answer the question of whether more megapixels make for better images from cameras in mobile devices.
They began by collecting images of people and places taken on mobile devices. Some of these images they created themselves, while they used Flickr for the rest. (This wasn’t the first time this week I’ve heard of Flickr being used as a serious tool for image quality research.) They group these objects into a “photospace” with two independent axes: illumination level and distance to the subject. (e.g., A landscape of the beach = high illumination and far distance; shots of your girlfriend at the pub = low illumination and (hopefully) close distance — unless you’re stalking your imaginary girlfriend. Hey we’ve all been there.) Each axis is a continuum, which they divided into four or five segments for simplicity.
To evaluate the images they asked human observers to rate the images on a quality scale ranging from “very poor” to “excellent” via a software application. Observers were also asked what was objectionable about low-rated images, such as “too blurry,” “too dark,” “not sharp enough,” etc.
The results. Images from camera phones were rated lowest when they fell into the “dark close-up” bucket. This isn’t surprising. But what was unexpected in his research was that this section of the photospace represented the largest segment of photographs made on mobile devices. In fact, the quality of photos is strongly negatively correlated with the number of photographs they found in each part of the photospace. We bring our mobile phones with us everywhere, and we do things with our friends after dark or indoors. And most of us use our devices to make pictures of people more than we do to make pictures of things. So camera phone manufacturers are producing devices that perform poorest in the situations where we most want to use the devices. (The major failure mode was blur, by the way. No surprises there.)
So do megapixels matter? Can you get better results with more megapixels? Hultgren and Hertel say “yes.” Quality is directly proportional to megapixel count. But here’s the catch: It’s only statistically significant for the best photos. In their study these were images in the 90th percentile of quality. For images in the 75th percentile and lower for quality, the statistics suggest a correlation between megapixel count and quality but not with the required level of certainty to draw rigorous conclusions.