The modification goes G90_1 to G90_5, the right side of G90_5 is the final shape of modified G90/2.8,
which is mounted on Tomy(BORG) M42 Helicoid (16~26mm), which is again mounted on M42_EOS
adapter with EOS rear lens cap. This helicoid is not thin enough to allow "infinity focus", you may
have to get a thinner helicoid (12~19mm) from Tomy(BORG) in order to achieve infinity focus.

See photos taken by this lens > http://www.pbase.com/kkawakami/cg_sonnar_90mm_f28_mod
See MTF curves of G-Sonnar 90mm lens > http://www.pbase.com/kkawakami/image/71784461
BTW,
Examples by M-mounted G-Biogon 21mm are > http://www.pbase.com/kkawakami/cg_biogon_21mm_f28_mod
Examples by M-mounted G-Planar 45mm + M8 are > http://www.pbase.com/kkawakami/cg_planar_45mm_f20

ようこそGゾナー90mmの私流の改造のページへ。改造後はG90mmの頭の部分はM42マウントのレンズとなります、
このままでは遠景にピントがきませんのでトミーのヘリコイドか同等品が必要になります。遠景撮影が主な人は
12~19mmのヘリコイドが良いでしょう、近接専門ならM42用のベローズでも良いかもしれません。勿論16~26mm
ヘリコイドとM42->F-マウントなどをつかってニコンカメラでも使用可能です、その辺りは自分で工夫してください。