Forgive me if this is a stupid question as I still don't completely understand the topic. I know that achromatic filters are of much higher quality, but also much higher price. So how usable are normal macro filters like the cheap Vivitar ones or these even cheaper ones from China:
http://www.ebay.com/itm/52mm-52-Closeup-Close-Up-Macro-1-2-4-10-Filter-Kit-/300932412048?pt=Camera_Filters&hash=item4610f83290
Also, what size would be best to avoid distortion at the corners?