Today it was pointed out that desaturating an RGB image doesn't give you accurate values, but converting to grayscale does. All these years I thought it was the same thing. Thanks, Tom.
...and the more idealized version. Because cantaloupes should look happy.
Dave Guertin (of Creature Box fame) kindly wrote in confirming the grayscale trick :
"Saw your recent blog post and thought you may get a kick out of a little photoshop tweak if you haven’t stumbled across it yet. Under 'View -- Proof Setup -- Custom' you can set the device to simulate to Gray Gamma 1.8. Now, whenever you hit 'cntrl+y' you get a realtime look at your values without the need to make any conversions. Hit 'cntrl+y' again and you’re back to color. You can also make adjustments in this mode."