[LargeFormat] 12" Commercial Ektar - lens board extension

[Clive Warren]

Here's a chance for all you peops with the good mathematical brains 
to help me out!

Have decided to put a 12" f6.3 Commercial Ektar on a 4x4 board for 
use with my MPP 4x5 outfit in the absence of a reasonably priced f9 
300mm lens.

The MPP bellows extend to nearly 12" - I am thinking about making or 
having made an extension tube integral with the lens board to allow 
closer focusing and to make sure that it does focus at infinity. If 
the extension tube is parallel to the lens and is say, 2 or 3" even 
5" how much danger is there of mechanical vignetting from the 
parallel extension tube?

Cheers,
        Clive