I have tried a bunch of triple convertibles (B&L, Wollensak, Zeiss) on both 7X17 and 12X20. My experience is that the performance is not worth the trouble with the single elements. Performance on the corners is dismal, even with single elements in the 20-28" range. Same is true of the big modern convertible Symmars, say the 360mm. Great lens with both elements but indivudlaly the elements won't cover even 7X17.
Originally Posted by Markus Albertz
In my opinion you would get vastly superior performance with apo lenses in the 20" to 35" category.