This past holiday season I was finally able to pull off something I’ve been wanting to do for a while, which is to make some random person’s decorations morph into my own creative vision for the sake of a photo. In this case, I’ve turned all the regular lights on a house into snowflakes. Let’s back up a minute and look at what is going on.
When a bright point of light is out of focus in a photo, it doesn’t just get blurry in a linear fashion. Instead it blooms out and takes on a shape. Generally the shape is coming mainly from the aperture of the lens and will often have between six to nine sides, depending on how many blades the aperture has. If you shoot at the widest aperture, the blades don’t even enter the path of light and instead you get a nice big circle.
That is the key to what we are doing here. By using a fast lens, generally f/2.0 or wider, points of light become nice big circles that we can mold into whatever we want… at least, within reason. The basic procedure is rather simple: if you cover the lens with something opaque that has a shape cut out of it, you are effectively replacing the aperture inside the lens, along with the shape it creates.
I had been thinking before about cutting it out by hand, which is a cheap alternative to 3D printing that’s just as effective… but with my freehand stencil cutting ability people would likely have assumed my 5 year old daughter had helped me with it. Enter the easy solution to all of this: 3D printing!
By using a 3D printer you can make very detailed gobos without much effort. I used Blender to make the model for my gobo, starting off with a simple round disc shape. I used the size of my lens hood as the basis, so it would easily line up with the center and I wouldn’t have to worry about light leaking in from behind it.
To make the whole thing really easy on myself, I just searched for free 3D models of snowflakes and found something I liked. Then I did a boolean subtract of the snowflake from my lens hood-sized disc in Blender. There were some central parts of the design that were then free-floating after being subtracted from the disc. Obviously any floating parts won’t really stay where you need them to on a printed object, so I put in some little bits of geometry to connect them to the main mass. You can experiment with how thin those supports are, though even when printing very thin they are still likely to show up in the finished shape.
I printed mine out at about 2mm thick. Could be thinner even, really just needs to be thick enough to be opaque and have enough strength to be handled. Black is the best choice to print in, though it could be painted afterwards as well if you don’t have any black filament. For the lens, I used an 85mm f/1.4, which gives a very pronounced bokeh. Remember that the lens should be shot using it’s widest aperture, closing it down will cut down on usable space out of the gobo. Try to make the design take up around 1/2 to 2/3 the diameter of the gobo. Small designs are the same thing as closing down the aperture, so they’ll make the points of light smaller. However, too big and you’ll be more likely to lose details at the edges.
So that’s it, an easy way to turn a blurry photo of a Christmas tree into interesting graphics. A gobo can be used for more than just shaping bright points of light. It also adds texture to soft areas of the photo, which can be great for adding style to a portrait. Experiment with it and see what kind of effects you can achieve.