fisheye numbers

[rollinfive0w]

.3x or .55x etc. what do these numbers mean

and is this a pretty decent one for the price, and wht is with the decision to have a flat glass ontop of the curvature one? protection??

http://www.bhphotovideo.com/c/produ...FEAD_XLH_0HD_FEAD_XLH_0_3x_Ultra_Fisheye.html


thank you

pieces