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I'm getting very different gray tones from two different ways of reducing saturation:
Why are they different? Shouldn't both be the same as setting the Saturation to 0 in HSB color space on all the pixels?
Here's the original image for comparison:
Left – uses a layer with blend mode set to Saturation. The layer's color has 0 saturation (e.g. white or black fill), and layer is 100% opacity.
The layer you are using to set the blend mode must have a value. The fill value of that layer is calculated to adjust saturations on the lower layer, using the top layer as a "baseline".
Right – uses an Adjustment Layer (or Image > Adjustments > Hue/Saturation gives same result).
There is no additional layer fill value added. It is merely the removal of Hue on the layer(s) below.
In short, using 2 layers is a calculation between both layers - there's additional data when using 2 layers. Using an adjustment layer is a calculation using what is below the adjustment layer only (no additional data).
Beyond this, I think it would require Adobe to explain the math behind the calculations.
