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It sounds like you want a lot of working distance between you and your subject.
While on full frame I prefer 100mm over 180mm, since that 100mm is a more useful focal length
for my all-around shooting, I think on full frame you would be most happy with the EF 180/3.5L,
which has a great reputation. However, you are shooting on a cropper, so you should at least
try a 100 or 100L macro to see if you like this focal length. If not, go for the 180L.
This is what I am worried about, I am thinking maybe 180mm is too long for 15ft x 15ft room. My requirement is too fill the frame [ full frame ] with my subject.