If you’ve been on the hunt for a new digital SLR Canon lens lately, you may have noticed a range that has a green ring.
For example, Canon’s EF 70-300mm f/4.5-5.6 DO IS USM lens, shown in the image below, has a green ring.
The green rings means it comes from the DO range of digital SLR Canon lenses. The letters DO stands for diffractive optics. I won’t go into the technical details of how these lenses work, as this information is already listed on a wikipedia article about Defractive Optics.
In short, what it means is that the lens contains superior characteristics that allows it to take sharper photographs. Furthermore, DO lenses are smaller and lighter than non DO lenses.
Is a DSLR Canon lens with a green ring (DO) as good as one with a red ring (professional L series)?
Canon DO lenses are not considered as good as their professional L camera lenses, however they are very close. As noted earlier, the main advantage being smaller dimensions and lighter weight.
However, there a few gripes as well. For example, many photographers have noticed problems with lens flare or bullet hole patterns seen within the bokeh from time to time. Some like the bokeh effect, others don’t.
You can read some discussions on defractive optics and view examples of photographs taken with them at the flickr DO IS USM group.