I have a picture taken with a person in front of a Christmas tree; the lights in the Christmas tree look like small blurry things.
However, I want to make the lights look like stars. Is this possible in Pixelmator?
This is most certainly possible in Pixelmator by using a brush in the form of a star. You can easily create one yourself straight in Pixelmator or you search on the net for an image of a star. Make sure the area around the star is transparent then you can save the star as a png-file and drag and drop that file onto the brushes palette. Now with the brush tool you can click on each blurry christmas light and cover it with the star. Try different blending modes for the brush. It could be you could get better results then.
It is also possible to create a bokeh effect in Pixelmator. The guys at http://pixelmatortutorials.net have had a tutorial about this some time ago. The bokeh effect is done by applying one of the standard filters in Pixelmator.
It's not the focal length that's causing the blur (bokeh), it's the aperture relative to the focal length coupled with the distances of the person to the tree.
Shaped bokeh is generally done by shooting through a paper cutout, not by altering the image after its shot.