Quote Originally Posted by Dan Fromm View Post
Kanga, there's a rule of thumb to the effect that half of a convertible lens covers roughly the same circle as the complete lens. Check carefully before sacrificing a 16x20 Cibachrome sheet. Arne Croell believes that the RF- series of six elements in four groups double Gauss type process lenses cover 45 degrees. If he's right, and he usually is, don't even bother.

I'm sorry, but there just aren't that many lenses that cover 16x20. If the trick you want to try worked well, it would be well-known.
Hi Dan,
I tried the rear group of my RF3 300mm f/10 on my 4x5in and it required 575mm of extension. On the ground glass at least there appears to be no aberration at a marked aperture of f/32 (effective f/64). If it does not cover 16x20in then it appears I've got a portable 600mm lens (or 800mm if I use the rear group from the RF5) for my 4x5in or an 8x10in. I will let you know how the actual negative looks after I give it a go on the 4x5in.
Cheers Kanga