Diffuser Halo Problem

[Creatavet]

I am shooting with a 20D, 28-135 IS lens and a Cokin Diffuser 1 filter. I get beautiful soft facial features, but when my subject is wearing white (as many do for outdoor portraits), I'm getting a "halo" effect around the white shirt. It is very annoying against a dark green background (also common for outdoor nature type portrait). Help! Any suggestions. My friend shot side by side with her Nikon D70 2.8 lens with the same filter and did not get the same effect... :(

[Keiffer]

What Fstop are you at? try stopping it down some. This worked for me when I was shooting the moon, I got alot of Aberation and stopping it down helped eliminate it.

[Creatavet]

I was shooting at 5.6. It was pretty decent lighting too. Any ideas?