A linear relation between the hydrogen column density ( N _ { H } ) and optical extinction ( A _ { V } ) in the Galaxy has long been observed .
A number of studies found differing results in the slope of this relation .
Here , we utilize the data on 22 supernova remnants that have been observed with the latest generation X-ray observatories and for which optical extinction and/or reddening measurements have been performed and find N _ { H } ( cm ^ { -2 } ) = ( 2.21 \pm 0.09 ) \times 10 ^ { 21 } A _ { V } ( mag ) .
We compare our result with the previous studies and assess any systematic uncertainties that may affect these results .