do you mean this
http://cgi.ebay.co.uk/New-52mm-2x-2-0x-Tele-Telephoto-Converter-Lens-Black-/170554647647?pt=UK_Lenses_Filters_Lenses&hash=item27b5d9645f#ht_3216wt_1139
if so my girlfriend has one it make a little difference on the 18-55 lens i would imagine on a bigger lens it would be better but cant say for sure.