How to use ximage -e parameter?

Apologies for the very basic question, but I am attempting to set the exposure in ximage similar to how I would set it in rvu (i.e. e 2) by doing:

$ ximage -e 2 foo.hdr

Unfortunately it returns

Usage: ximage [-di disp][[-ge] spec][-b][-m][-d][-f][-c nclrs][-e spec][-g gamcor][-s][-ospec][-t intvl] hdr ..

What am I doing wrong? -e auto works, and so does -e human, I’d like to adjust the exposure just like in rvu.

I guess the man page doesn’t spell this out as clearly as it ought. You need a +/- in front of the integer power of two you give. If you want an exposure bump of 2.0, you would use “ximage -e +1”. If you want a two-stop bump, use “ximage -e +2”. To darken the exposure by a similar amount, “ximage -e -2”. There are also the special values “auto” and “human” that invoke tone-mapping features similar to pcond.

Thanks @Greg_Ward! Yes, the man page wasn’t quite clear on this but now it makes sense!