Dreaded Purple Fringe

A short time ago I was shooting with a Tamron 18-270mm zoom lens on a Canon EOS 40D and discovered something very surprising. When I reviewed the images of some leaves that were backlit against a bright sky I didn’t see any DPF. You know what DPF is, of course: Dreaded Purple Fringe. It usually inhabits the contrasty edges that separate highlight and shadow areas in some digital images. I’d upload an example, but I know you have plenty of your own.

Many people call this violet border “chromatic aberration” but that’s a cryptic and somewhat incomplete description. I checked with a close friend who is an engineer with a camera maker in Japan. He explained the phenomenon this way.

“The cause of the purple fringe (sometimes it is ‘blue fringe’) is due to the magnification (image size) being different for each wave length. Therefore, even when the subject is in focus on the green wavelength, for example, the image size of the other color wavelengths are slightly different. That is why the edge of the image has a blue or purple area. I am talking about chromatic aberration related to magnification (in Japanese, ‘Bairitsu Iro Shuusa’).”

He went on, “One of the major reasons why purple fringe is so noticeable in digital photography is because the customer can easily magnify the image in pixel size to one hundred percent. Comparable viewing conditions would be like printing the image the size of your wall and then viewing it from 10 inches away...”

Better lenses correct for this aberration, so that the purple-banded area is all but invisible.

Earlier I was impressed by the Tamron 18-270mm zoom for a completely different reason: the Vibration Control (also known as image stabilization) is so effective that I was able to shoot at 270mm (430mm equivalent) at 1/15 of a second without visible camera shake. And that’s pretty damn good since normally I can’t even eat a forkful of scrambled eggs without spilling some halfway to my mouth.

Take a look at the images of the yellow leaves. Both were shot handheld at 1/15 of a second. Go buy this lens. Nothing I can write will make you understand how good it is until you try it. It’s available for Nikon and Canon digital SLRs and I’ve seen it advertised in the $599 price range. At least go try one so you can judge for yourself.