In article <4751b2$t2m@barnacle.iol.ie>, derekh@iol.ie says...
>A friend gave me an old Nikon EM to let me get back into photography.
>Its semi automatic on aperture.
>There are three settings ; AUTO, B and M90.
>I have absolutely no idea what M90 does ! (..)

B and M90 are shutter speeds that will work without a battery.
M90 is the top shutter speed that will synchronize with electronic
flash (the highest speed in the regular speed sequence that will synch.
is 1/60th).
Hope this helps.