I tried a few experiments to determine how to get decent Bokeh Balls for Christmas photos. Especially with portraits, I find it hard to find the optimal location where to place the subject relative to the background Christmas lights.
In this experiment I decided to set the camera up at about 4 meters from the Christmas lights I want as background bokeh balls. I chose the 4 meters as a reasonable distance achievable in a standard living room with a lit Christmas tree in one corner or other lit decorations.
Here is the setup
Below a series of images taken at variable distances from the subject with a 35 mm lens at maximum aperture f/1.8.
First I focused on the black curtain (4 m) with the Christmas lights hanging right in front of it. Notice that the wires of the Christmas lights show while the bust is slightly out of focus.
Next I focused on the bust itself at 2.5 m from the camera. Notice how the Christmas light wires are beginning to get fuzzy and the lights are showing a bit of Bokeh.
The following images are taken closer by moving the bust closer to the camera at 0.5 m intervals while leaving the distance to the Christmas lights at 4 m.
Here is a side by side comparison of the 35 mm lens experiment. Cropped as close as possible for equal comparison.
I did the same experiment with a 70 mm lens. In this case I did only four distance changes, as the lens depth of field at extreme close-up is too shallow for portraits. However, comparing the results of the 35 mm f/1.8 at 0.5 meter distance was equivalent to the 70 mm f/2.8 at 1 m distance. I have cropped the images to compare the subjects. The result is that I will use my 35 mm this Christmas, as I get a much wider field. I think the sweet spot is 0.5 to 1 meter to the subject and 4 meters to the Christmas lights for appealing Bokeh.
Below the field of view comparison of the 35 mm f/1.8 lens at 0.5 m from the subject and the 70 mm f/2.8 lens at 1 m distance.