Converting a Photo to a Pencil Drawing

First open the image you would like to convert into a pencil sketch and erase the background:

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Next press CTRL+J to make a duplicate the layer. Then press CTRL+U and lower the saturation to reduce the complexity of the image:

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Press CTRL+J again and make the image a negative by pressing CTRL+I:

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Then make the blending mode of the new layer to Color Dodge:

You won't be able to see anything, so to view the lines or strokes apply a Gaussian Blur at the top layer as according to your needs:

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Your final result will look something like this:

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If you really want to beat out Leonardo, try tricks like painting only some parts of the top layer with a new color. Or, color just the lips or eyes of a portrait by selecting the area and applying some hue with CTRL+U.