what is this and are there any benefit about it?

I'm looking for a new lens to replace my Nikon 70-300mm lens and I saw a few lenses that have this type of lens.

At the moment, I am considering the Sigma 50-500mm lens but not sure what to look for in a lens or if I need something this big. I want something fast and with image stabilizer (not really needed but it is a +). Any recommendations?

 

I think parabolic lens are better but more expensive than spherical lenses.

EDIT: I don't have much photographic knowledge just a good bit about telescopes.

 

http://www.photozone.de/3Technology/lenstec8.htm

 

this lens is VERY heavy. They will give you great range cover but optic quality is not that great compare to Canon 100-400.

 

btw, since you just switch to Canon, go here for review on ALL canon lens:

http://www.the-digital-picture.com/R...s-Reviews.aspx

 

I haven't switch to canon yet. I'm going to wait till I see if I'm going to trqde my car. If not then I would like to see if Nikon comes out with a replacement/upgrade for the D80.

 

D300 is out now.

 

D300.

Or sell me your nikkor lenses.

 

I would like to see a full comparison on dpreview on the D300. They don't have any comparison on it yet. I looked at the D80 and D200 and the Canon 40D looked to have a sharper image. But If I choose to get the Canon, I think I will still keep my Nikon.