I have bought this Symmar 1:5,6/210, convertible to 1:12/370, in excellent condition.

Today, I tried it for the first time, with Polaroid (another problem there, but that's for another question), and everything looks fine in 210mm.

I have understood I have to remove the front part of the lens to use it as a 370, and then I can't focus with my Shen-Hao 4x5 (as I more or less expected). Would a tophat board (or whatever it is called) help here ? I have no idea what extension I need for this lens, I didn't find info anywhere about that.

BUT when I take off the BACK part of the lens, I get an image on the groundglass that looks quite sharp (that one I CAN focus), as far as that can be judged with a loupe, and has a higher magnification then the lens as a whole ... Is there something wrong working that way ? I couldn't test it yet (because of the Polaroid problem), but maybe somebody else has done this before ?

Thanks,
Stefan.