I have bought a carl zeiss jena biometar 80mm f2.8 lens with a F-mount adaptor. The adaptor is plain with no imprints on it except for the words "open" with an arrow direction indicator. It maybe the Psix to Nikon adaptor that you mention but I cannot tell. The shop told me that the lens is for a medium format camera and so I figure it could be the Pentacon 6x6 you refer to. I have put this lens on my F5 and it is bright and seems sharper even just looking thru the viewfinder. I will be trying a few rolls of film with it. There are other very good lens eg 135mm, 29mm. I was surprised that these lenses are new. It seems that a factory making Practika cameras are also making these lenses too. As a earlier post suggests, the quality of these lens may not be consistent but I will soon find out. I have not come across the 300 f4 you mention. Confirm it is a carl zeiss or another german made. I would like to know the make as I have a need for a 300mm lens. By the way for those on the digest I do have Nikkors (mostly AF ones) but am trying these new inexpensive manual lens. Manual lens of such good quality is fun to work with and it brought me back to manual photography. Back to basics. The electronic wizardry of modern cameras almost make us forget the art of photo taking. ______________________________________________________ Get Your Private, Free Email at http://www.hotmail.com