Make a photo into a sketch

Kind of a fun trick in photoshop... this has been done many ways many times... but I will give my take on it here. Open an image and make a copy with command J.
Hit the layer style - Color Dodge and then hit command I (for invert)
  Sometimes you will think your image has disappeared here... keep going.
Then on that same layer, select gaussian blur and go medium on that blur.
Come back to layer and choose to desaturate with the hue/saturation slider.
  That's generally where you would be done.

But if you want to try something fun... try motion blur instead of gaussian.  It will give you darker lines.  In addition, you can create a layer from visible layers (as layer 3 in the example is).  And you can then try giving it an effect, like accented edges or something else from the effects menu.

One more thing I tried was to create a layer mask and give the mask a gradient so that I could see through to my original as shown below.  Enjoy. 

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