Can you recommand me a software for doing this job? I shoot using Sigma 8mm lens and I would like to be able to obtain a normal image out of that. The camera is canon 550D.
Use a Lens Profile
If you're post-processing with Adobe software or software that understands Adobe's lens profiles, the easiest way to correct for the Sigma 8mm circular fisheye is to simply use a lens profile. According to the Sigma website, both Sigma's 4.5 and 8mm circular fisheye lenses have profiles in Adobe's databases. Simply enable lens profile correction in Adobe Camera RAW (ACR) or Lightroom's Develop module, and the image will be defished.
Use Panorama Tools via PTLens
The Panorama Tools open source suite of command line tools includes the capability of lens correction--basically building a profile on the fly. However, since not everyone is a command-line jockey, there are also a number of Panorama Tools GUI front-ends to consider that can do the same thing, such as Hugin or PTGui. But there is one PT frontend that is dedicated solely to the task of lens correction, which is PTLens. This may be easier to use and suss than going into the overkill of using Hugin or PTGui off-label, as both those apps are centered around panorama stitching.
Another popular tool for defishing is the commercial Fisheye-Hemi Photoshop plug-in. It's using a proprietary remapping that is not simple rectilinear defishing, which gives results some find more pleasing.
I use GIMP 2.8 to correct pictures with lens distortion (Fisheye) effect. This is a free and very extensive software, easy to use. Click in file on open and add the photo that you want to correct. In Filters, click on distortion and choose lens distortion. In "General" you can set the degree of correction with the slider and check in "example" click "OK" in file "Export as" Here I choose as jpeg. Done Available for the following OS: GNU / Linux / OS X / Microsoft Windows all. GIMP - The GNU Image Manipulation Program www.gimp.org/
Ken Rockwell wrote a review of DxO Optics that illustrates converting fisheye to rectilinear photos. The most impressive examples are the kitchen ones about 3/4 of the way down the page. I've used DxO and it's pretty good at this as long as you have a body/lens combination DxO has profiled.
Not a cheap solution, but certainly worth considering if you need the ultra-wide angle, but don't care for the distortion.
There are many other less comprehensive posts and of course DxO's own site. Google "DxO +architecture +fisheye".