I have two canon kit lenses which came with my 1100D. While experimenting casually with my lenses, I came across something that I find peculiar. When I am focused at an object at a certain distance on my 18-55mm lens set at 55mm, I get a set of shallow depths-of-field as I move the focus ring. On the other hand, if I use the 55-250mm lens at the same distance and same focal length of 55mm (thus viewing the object at the same scale), then my depths-of-field are much wider and the amount of defocus in the foreground and background is lesser than the 18-55mm lens.
I imagine this must be because of the flexible focus of the lenses and the fact that one lens is at it's telephoto end while the other is at it's wide-angle end. I would like to understand this better though and would be glad if someone could explain the phenomenon or point me to the correct reference.
Thanks in anticipation!