Set rmagick image depth?

I am trying to set the image depth to 8 as it is defaulting to 16 bits:

@img = Magick::Image.from_blob(data) { self.depth=8 }.first

That is what I have now but the self.depth=8 doesn’t seem to be changing
anything. Is that right? I shouldn’t have to set
–with-quantum-depth=8 when I installed ImageMagick (or is it for
Rmagick), should I?

Any ideas would be greatly appreciated! Thanks.