Extension tube for Hasselblad 150mm F4 CFT



First of all , the figure 21 stands for the amount of extension in mm (millimeter) .
So , the extension ring 21 has an extension of 21 mm .

There were actually 3 series of extension rings for the V-SYSTEM .

The first serie were 3 rings with 10 , 21 and 55 mm .
Then the second serie has tubes of 8 , 16 , 32 and 56 mm .
The third serie is 16E , 32E and 56E , where E stands for electric (electronic) contacts they have to transmit the aperture value to the V-SYSTEM models 202FA , 203FE and 205TCC(FCC) .
The ring sizes of 8mm and 10mm can not be used as the first ring (if you want to use more than one ring) on any 2000 or 200 V-SYSTEM model .

I found the 8mm or 10mm ring as my favorite rings , when using the SONNAR 150mm for portraits . The 21mm ring gives you more "close up" but also less DOF , which can be wanted , when taking portraits .

The ring , you give in your link , seems to be in a very good condition .