This is a fun tip for making interesting pictures with Christmas lights in the background.
Before you take your picture, you are going to need a cutout. Take a piece of black paper and cut out a square a little larger than the front of your lens. Cut a shape out of the square. This cutout is what your camera is going to "see" through. In the examples below, I've used a cross shape, a Mickey head, a star, and a butterfly.
1) Set up your camera (preferably on a tripod, but you can do this handheld with an assistant) to take a picture of your Christmas tree or some scene with Christmas lights in the background.
2) I like to use manual exposure mode and set up my exposure to underexpose a little (about 1 stop). It will probably work in auto mode, but most cameras have a tendency to overexpose those bright little lights. The main thing is to use as wide an aperture (f-number) as possible for this shot.
3) Put your subject somewhere between you and the tree. You don't want your subject too close to the tree because you want your subject to be in focus and the lights to be out-of-focus.
4) Focus on your subject.
5) You may want to add some light to your subject so that they aren't too dark. I bounced a flash off the ceiling for my pictures. You could also just turn on some bright lights in the room.
5) Hold your cutout in front of your lens. You want it as close to your lens as possible. If you have a lens hood on the front of your lens, take it off so that you can hold your cutout right on the front of your lens.
6) Take your picture.
Here are my sample shots. Pardon the subject. My usual models were all busy when I took these, so I had to use a timer mode and do modelling duty.
