The Green lens, or 70-300mm DO-IS, is built to the same quality as any "L" series lens. So consider "DO" to be equal to "L" in all materials/construction.

But.. the DO is much smaller than one might expect for a 300mm zoom with a f4.5 aperture. This makes it a great travel lens -- eg. it was in my camera bag when we visited you folks in 2008.

The way it gets to be small, is by using a very special fresnel lens (DO = Diffractive Optic) as the main element. This is an element with zillions of finely etched concentric rings, similar to the flat magnifying "lenses" that many of us keep in our wallets or purses. Instead of bending the light like a normal lens, it instead uses a diffraction effect to achieve a similar thing in less space/weight.

The downside, is bad bokeh. Instead of a nice, beautifully blurred background, some of the circular blurs have bulls-eye centers. This is a problem for some photographs, and non-existent in others.

I'll see if I can find some examples of it from our Italy-2008 photos.. later.

Cheers