summicron lens and accessory question

[whitetr6]

I am new to rangefinders, especially older equipment. I have a Leica M3 in very good condition, which has with it a Summicron 1:2/50 lens. I have a question about the use of this lens. There is an accessory item, which slides onto the shoe of the lens itself and places optics in front of the rangefinder and viewfinder windows. When in place, the focus ring is unlocked. My question is, is this item necessary to use whenever using this lens, or is it for some special purpose such as close-ups? The second thing is, what is the purpose of the locking mechanism on the focus ring?
Thanks for helping out a newbie...

 

[hans voralberg]

Congrat on owning a Cron DR, here's the link on how to use it.

http://www.google.com/url?sa=t&ct=r...r3_qOChfpexg275Yw&sig2=dL4AKKQ0mEn7v_F0hgnbhg

The eye is only used for close-up

 

[whitetr6]

Hans, thank you very much for that information. It cleared up my questions perfectly.

 

[rover]

Welcome to RFF whitetr6