Bring out the twinkle in city lights simply by changing your aperture.
A cityscape at night with lights blazing away like stars in the sky can be a scintillating show, and when captured in a photo, it can dazzle the eye. To create this twinkling effect in your after-dark cityscapes, thread a star filter from B+W, Hoya, Lee, or Tiffen in front of your lens. The better ones are variable, and let you pick the size and direction of the exploding burst.
But what if you happen upon such a scene without a star filter? Don't worry. You'll find help in an always-available tool: the iris diaphragm of your lens.
You're well aware that changing the lens aperture can pay off in better exposure by controlling how much light reaches the film or image sensor. You also know that dialing down the aperture can increase the zone of sharpness (i.e., depth of field) in your pictures. While all this imaging power might seem like enough, the aperture has several other imaging tricks up its sleeve -- including the ability to increase the apparent sparkle of spectral highlights.
It's very simple: Setting smaller apertures will create and exaggerate the star-like radiating shafts of light that can appear to emanate from small points of light in a scene.
What you're seeing is the pentagonal, hexagonal, heptagonal, or octagonal shape of the actual lens opening as it's created by the five to eight leaves that make up the diaphragm of the lens. An eight-bladed diaphragm can create an eight-sided star shape. As you stop the aperture down, the tiny points of light in your photos come progressively into focus -- recreating, as they become sharper, the physical shape of the lens opening.
Will this work with any lens? No. As you might imagine, apertures formed from arc-shaped leaves produce round openings, so they don't reproduce spectral highlights as star shapes, regardless of how much you stop the lens down.
And here's a bonus tip: The more you magnify or enlarge these small points by increasing output (print) size, the more dramatic they'll appear.