Photoshop’s Info Pallet provides useful… info… about the values in specific areas of your image. Using the Eyedropper tool you can run the cursor over the image to see how the RGB ( or CMYK or Black) values fall. More powerfully, using the Color Sampler tool (the Eye Dropper with cross-hairs) you can lay down points in the image and see how the values change as you apply corrections. This is especially useful, for instance, to insure that highlights don’t blow out and shadows don’t block up as you make edits.
Unfortunately there is a bug in CS4 that causes the info Color Sampler points to read out different values depending upon what layer you’ve targeted. When a pixel-bearing layer is highlighted (targeted) in the Layers Panel you’ll get one value. Target a Levels Adjustment layer to make a change and the value will be different.
Note that an Image Layer is targeted in the Layers Pallet and the RGB values listed for Sample Point #1.
The only change here is that an Adjustment layer was selected (targeted) in the Layers Panel. No changes were made to that adjustment layer. Note the values for Sample Point #1.
So it appears that we are unable to both measure the value and alter it at the same time. Tricky.
And it’s worse than this.
I’ve opened the same image in CS3 and found different values for the Color Samplers than CS4 indicates. (Thanks to Ray Prevost for discovering the discrepancy between versions)
So, will your highlights hold a dot when the images gets to press? Who knows.