Does anyone have any experience of these? http://www.jessops.com/Store/s30152/0/F ... lse&comp=n Basically thinking about getting one of these before I can save up some money to get a good macro lens. How good are they as a cheap alternative for a true macro lens? Also how do you work out the magnification change with zoom lenses? So for example what magnification would you have using a 250mm lens? Thanks for any help with this.