problem with 400d

I have the 400d with the Sigma 18-200 OS and sometimes I get a dark circle in the edge. Here is an example:
http://www.flickr.com …s/ilanjshazan/1​398374989/
Why is that and someone knows the solution?

Vignetting maybe?

Great image!

Yep, definitely looks like vignetting.

You can fix this in PP. You can either adjust this in the RAW file (e.g. under "Lens" in ACR) or in Photoshop under Filter >> Distort >> Lens Correction.

Alternatively, when you are taking the shot, try moving away from the extremes of the zoom range as the vignetting will be worse at the 18mm and 200mm focal lengths.

Finally, you can also try to use vignetting creatively to pull the attention to the centre of the photo and isolate the subject (see the top right image on the wiki link above).

I tried the PP. Great job! Thanks.
I think I need to know more when it appears with my lense. is there a way to know exactly the limits or I'll have to try alone?

ilan wrote in post #3962781
I tried the PP. Great job! Thanks.
I think I need to know more when it appears with my lense. is there a way to know exactly the limits or I'll have to try alone?

As a rule of thumb the vignetting will be worse at the focal length limits (i.e. at 18mm and 200mm) and worse when at the widest aperture.

qtaran111 wrote in post #3963578
As a rule of thumb the vignetting will be worse at the focal length limits (i.e. at 18mm and 200mm) and worse when at the widest aperture.

With zooms, it tends to be the widest focal lengths. My Tamron will get lots of vignetting on my 5D wide open, dark light source, and at 28mm. I think it's because there's a wider angle of light that's being recorded: more chance to get shadows from the lens barrel.

As Mike's link illustrates, vignetting is more apparent with larger sensor sizes, wider focal lengths, fully open apertures, and mechanical vignetting from the metal rings on filters/poorly designed lens hoods. I think amount of light has an effect, to an extent too. Basically, with my 5D, I'll always try to go for a smaller aperture unless I think it will be easy to PP out.

With your particular picture, vignetting is not bad at all! One of the good things about a 1.6x crop camera!!

Thanks a lot all for your answers. I was so afraid I had a problem that I ran to the store to complain and they didn't know what to say. It looks to me a bit strange that not a cheap lence (here it's arond 500 US$) has this kind of problem (never had that with my old film cameras), don't you think? as I see now it's quite a long range that I have this thing and not only in the xtreme as some of you said (not sure about that but it looks like). Thanks again all.
ilan