ただし details

Examples

provided that; with the following exception — attaches a condition or limit to the statement just made

見学けんがくは自由じゆうです。ただし、写真撮影しゃしんさつえいはご遠慮えんりょください。

kengaku wa jiyuu desu. tadashi, shashin satsuei wa go enryo kudasai.

You are free to look around. Please refrain from taking photographs, however. formal

会費かいひは一人ひとり千円せんえん。ただし、学生がくせいは半額はんがく。

kaihi wa hitori sen'en. tadashi, gakusei wa hangaku.

The fee is a thousand yen per person. Students, however, pay half. neutral

明日あしたの試合しあいは予定よていどおり行おこないます。ただし、雨あめがひどい場合ばあいは中止ちゅうしです。

ashita no shiai wa yotei doori okonaimasu. tadashi, ame ga hidoi baai wa chuushi desu.

Tomorrow's match will go ahead as scheduled. That said, it will be called off if the rain is heavy. neutral

しかし
しかし sets the next sentence against the previous one and may overturn it outright; ただし leaves the previous sentence standing in full and only carves out the case it does not reach (入場にゅうじょうは無料むりょうです。ただし、予約よやくが必要ひつようです) — swap しかし in and the free entry starts sounding cancelled.
でも
でも answers back, so it opens a reply to somebody else's turn; ただし is notice-language attaching a proviso to the writer's OWN preceding sentence, which is why it fills signs, rules and terms of service and almost never starts a spoken objection.

ただ
ただし fences off an exception to what was just said (ただし、雨天うてんは中止ちゅうし); this ただ measures an amount instead, so it qualifies nothing that came before it and needs no previous sentence to attach to.