Zoom in close so you can work precisely, but choose Window→Arrange→New Window for [filename]. Choose Window→Arrange→Tile All Vertically and keep that second window zoomed out and off to the side so you can monitor your progress while you work.
Keep a copy of the original image open for reference. You can make a copy of the file with the Image→Duplicate command or by clicking the left button at the bottom of the History panel. As you make changes to the original image, refer to the duplicate (the original filename appended with copy). If you're not happy with the previous change, you can undo it.
Here are some tips for working effectively with the Clone Stamp tool:-
Work on a separate layer. Before cloning, click the New Layer button at the bottom of the Layers panel and set the Sample menu to All Layers in the Options bar. By cloning to the new layer, you protect yourself from irreversible errors (you can always erase part of the upper layer or delete it), and you can show/hide your work layer to check progress.
If the image already has multiple layers and you want to clone from only one, hide the other layers in the Layers panel by clicking the eyeball icons in the left column.
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If a color or pattern is uniform, choose a source near the area you want to clone. Option+click /Alt+click the area in the image you want to clone. If, for example, you’re removing a power line in a beautiful blue sky, clone from right above and below the power line so that you get the best possible color match. For delicate jobs or larger items, you can clone by halves — clone half from one side and the other half from the other side.
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To avoid a recognizable pattern, choose a source that's far from the area you want to clone. You can clone from a variety of places to avoid creating any recognizable replicas of nearby flowers or rocks. You should, however, try to clone from areas that are approximately the same distance from the lens as the area over which you’re cloning. If you clone from the far distance into the foreground, you’ll have a recognizable size mismatch and perhaps a focal difference, as well.
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To copy areas or objects, use Aligned. By using the Aligned option, the relationship between the point from which you sample and the point to which you clone remains constant when you release the mouse button. To pick a new source point, Option+click/Alt+click elsewhere in the image.
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To repeat a pattern or texture, don’t use Aligned. If you have a specific object, texture, or pattern that you want to replicate in more than one area, you can clear the Aligned check box on the Options bar. Every time you release the mouse button, the source point returns to the exact spot where you Option+clicked/Alt+clicked. You can copy the same part of the image into as many different places as you choose.
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You can vary the tool’s opacity and blending mode. Generally speaking, when you want to hide something in the image, use the Normal blending mode and 100% opacity. However, you can also clone with other blending modes and reduce opacity to subdue rather than hide and, of course, for fun special effects.
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Adjust your brush size on the fly. Pressing the left and right brackets keys (to the right of P on the standard English keyboard) decreases and increases the brush diameter without having to open the Brushes panel.
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Check the brush’s hardness and spacing settings. To get the smoothest result for general cloning, reduce the brush’s Hardness setting to about 25%, allowing edges to blend. There are times, however, when you’ll need a more distinct edge to the brush, but you’ll rarely need to clone with a brush set harder than perhaps 90%.
In the full-size Brushes panel, you can generally set the Spacing (in Brush Tip Shape) to 1% for cloning to ensure the edge is as smooth as possible.
The Spot Healing Brush works much like the Healing Brush to repair and replace texture. However, instead of designating a source point by Option+clicking/Alt+clicking, the Spot Healing Brush samples from the immediate surrounding area, which makes it perfect for repairing little irregularities in an area of rather consistent texture.
You can also clone from another image. Open two images and tile them vertically (Window Arrange Tile All Vertically). Option+click/Alt-click the image you want to clone (the source) and drag inside the image you want to clone the pixels to.