How to automatically figuring out the optimal resolution for a pdf file?

Hi,

I'm trying to converting a pdf without explicitly specify the density
as Acrobat can automatically figure out the best resolution. But it
seems that convert can not do so correctly, it use a far too low
resolution. I'm wondering if there is a way to automatically determine
the best resolution. (Let is assume the pdf was created by converting
from a tif file. the optimal resolution is just the resolution of the
original tif file.)

convert image.pdf +adjoin image_%02d.tif


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Regards,
Peng