OK........let's go back to making a selection of an area or more specifically a hard edge. The idea is to not change the clone stamp brush softness or hardness, it's to let the selection define the edge.
Let me explain again to see if this makes better sense.
Here we have a selection
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If we use the Brush Tool to paint within the selection, then the brush strokes are constrained to a defined border.
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The same holds true for the Clone Stamp Tool.
Selection....
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Cloning within the selection. Mainly I want you to see how the edge of the curb will be defined without altering the Clone Stamp Tool's setting.
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Here's the defined area (on a layer by itself).
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Selection off (Cmd/Cntrl + D)
Note the defined edge of the curb.
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Now here's the part I neglected to tell you about.
You can then invert the selection using Shift + Cmd/Cntrl + I
Now you can work on the other side of the selection (curb).
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This depends on the size and resolution of the image I'm doing the work on. So zooming in and then backing out is exactly what I do. But I never work too zoomed in or close.