When buying wide-angle lens adapters, BEWARE that certain lenses labelled as 0.45x or 0.38x or 0.42x may in fact be only around 0.7x or 0.8x.
I have just purchased an "Advance" 0.45x digital super wide macro adapter (52mm thread diameter) ("manufactured by S.Bower, USA in Japan") - all clearly labelled on the box AND on the lens itself - and it is in fact 0.77x.
Checking the UPC bar-code, the item does not exist, but the manufacturer's name tallies. This was bought from a US-based seller of lots of similar items.
I see also in ebay similar lens adapters, with accompanying photographs, and they are clearly NOT 0.45x, but nearer 0.75x.
For example, "Digital" brand, "digital professional lens"; other non-brand / unbranded items regularly appear, too. They are NOT 0.45x ! These particular dodgy items seem to come from China / Hong Kong: I've written to the vendors, telling them that the pictures clearly show they are not the same specification. They have given me the standard brush-off "our technicians say the sepecs are correct", or even, "if you're not happy, don't bid" !
How To Tell
In order to tell the magnification factor, measure an object in the "wide angle" shot, measure the same object in the "normal" shot, then divide these two lengths to get the x0.nn factor. For more accuracy, repeat this, and use as large an image as possible.
Example, an object measures 32mm in the "wide" shot, 45mm in the "normal" shot; 32/45 = 0.71x.
Example 2, an object measures 1.4" in the "wide" shot, 3.3" in the "normal" shot; 1.4/3.3 = 0.42x.
IF A LENS DOESN'T HAVE ACCOMPANYING PICTURES, DON'T BUY IT !
If it does, make sure they are taken with the actual lens, and not generic photos off the internet. Or off the lens box itself - the one I've got is inaccurate. The "normal" shot is merely a cropped, smaller version of the "wide" shot.
If I could trouble you to give feedback, that would be nice - I suspect this guide is "negatived" by persons selling these dodgy lenses, especially from the US and from China, I'm afraid.
Also, if you have anything to add or discuss in regard to this guide, please contact me :)