expanding/contracting in the same location to form smooth edges shape.

Hi im a newbie in photoshop. heres my queries. I want to make a a big circle and a smaller circle in the same point of location just like using offset path cmd in adobe illutrator using select modify contract or expand cmd.I selected a circular marquee with the size of 6cm x 6cm and filled it w/a color.while selection is active i contracted it w/ the amount of 4pix and filled it with another dif. color the result is smooth edges, but when i contracted it again by a bigger amount like 24pix it looks distorted.I think there is a technique in doing it. Can you help me guys?Just remember i want to know if contraction or expansion in ps has the same principle with the offset path cmd in ai. because the offset path cmd in ai produces smooth edges even if u enlarge it in any possible amt.

thanks in advance
Sagittarius

I think some of your problem is in Illustrator you are dealing with Vector which is smooth at any size. Where as in PS you are dealing with raster graphics, which means you have to contend with pixels.
It sounds like you know the method of contracting and expanding.
Sometimes you can mess with the bluring and trimming to smooth out any unwanted jaggies.
Someone else may know a better answer....

Instead of using the circular marquee tool, use a vector shape instead (U on Keyboard)