Sekonic has the retractable hemisphere primarily for convenience, when needing to measure the relative intensity of light sources (without turning of the lights off). But Sekonic does offer an accessory specifically for meauring light falling upon flat art.
[edit: Uh-oh, the L758 does not offer that flat disk option!]
Essentially you can somewhat mimic the retracted hemisphere simply by shielding the meter hemisphere from the unmeasured light source simply with your hand as a gobo!
I recently learned that the Sekonic L358 (and, after reading its owner manual, the L758) cannot be set for 1/3EV increments of shutter speed while also supporting 0.1EV increments of f/stop...the f/stop assumes the 1/3EV increment as well. Not terrific if you have 0.1EV precision on studio light power, since it would always round to the nearest 1/3EV. This is not merely speculation, but one member on another forum bought and returned the Sekonic meter for that very reason.
Last edited by wiltw; 09-02-2012 at 06:48 PM. Click to view previous post history.