Quote Originally Posted by dynachrome View Post
A 50mm lens which has its own focusing helicoid and which goes from 1:1 or 1:2 to infinity is more correctly referred to as a purpose built macro lens. A true macro lens is one which has no helicoid, which is optimized for a limited range of magnification and which has an RMS thread mount. The Zeiss Luminars and Leitz Photars and Macro Summars are the best known of this type. In some cases the shorter lenses were in RMS mount while the longer ones were in Leica 39mm mount. This is true of the Macro Nikkors, not to be confused with the oddly named Micro Nikkors.
Canon MP-E 65/2.8, anyone?

There's another artificial but somewhat useful and widely adopted dichotomy: photography at magnifications <= 1:1 is closeup photography, at magnifications > 1:1 is photomacrography. If you buy this, lenses for use above 1:1 are macro lenses, regardless of mounting hardware.

All lenses are optimized for relatively limited ranges of magnifications. Some types, usually but not always fairly symmetrical, hold their optimizations well, if not perfectly, at all distances.

Funny, the handful of 100/6.3 Luminars I've used are not in M39x26tpi and neither is my 100/6.3 Neupolar, a somewhat better lens.

Moral of this post? If you're going to lay down the law, be right.