Light Trails

Use Back Button Focus to Pre Focus

Sometimes it’s useful if the shutter button doesn’t focus. Instead you press another button to focus. I’ve used it in the past when shooting street photography (focus on the ground and I know anything a metre away will be in focus), and at night on a tripod when taking long exposure shots of moving lights.

Back button focus is when you don’t use your shutter button to focus the camera. Instead you’ll use a button on the back of your camera. Not every camera can do it but check your manual or use Google to search for your camera name and “back button focus”. Here are two good videos describing why it’s good for sports photography:

Here’s a video I found to get it working on the Canon 6D using the AF-On button. I usually use the Q menu now to disable focusing on the shutter button.

The beauty of back button focusing is that you can focus your camera before the proper shoot and then take as many photographs as you want of the scene, often when the lighting has changed and focusing is impossible. This is especially important at night, but it also lets you shoot faster as the lens is already focused. This may just mean the difference between a great street photo and a missed opportunity.

Finally, here’s a photo I shot last night using back button focus to set up the shot. I could as easily have manually focused the lens but this worked just as well.

Light Trails

Aperture ƒ/4.5
Camera Canon EOS 6D
Focal length 17mm
ISO 100
Shutter speed 10s

An Garda

An Garda

A member of An Garda Síochána directed traffic in Blarney last night when Santa visited and the lights were turned on in the village.

This is a long exposure zoomed shot, of about one second duration with the flash firing at the end of the exposure.

To replicate:

  • Make sure you set your flash to “second curtain”. Use Google to find out how on your brand of flash. Normally the flash will fire at the start of an exposure.
  • Have the lens at it’s widest zoom.
  • Set the camera to one second exposure in shutter priority mode (Tv on Canon cameras, S on others) or use manual mode with an aperture as wide as possible.
  • Line up your subject, hit the shutter button and slowly zoom in.

If you’ve timed it right the flash will fire when you’re zoomed in right. They’ll be lit by the flash and any background lights will appear as streaks going towards them. It’s hard to get right and not overexpose your subject so keep practising!

Aperture ƒ/13
Camera Canon EOS 6D
Focal length 20mm
ISO 100
Shutter speed 1s

Coburg Street Lights

Coburg Street Lights

The lights of Coburg Street and passing cars in a long exposure shot of the street from the traffic lights at the junction with Bridge Street.

This was a 15 second exposure at f/22. The lovely star shape of the street lights is because the opening in the lens was so small (at f/22) and the diaphragms of my lens. It takes a long time to get the exposure but it’s worth it!

This appears to be a beautiful example of Fraunhofer diffraction. It is due to the wave nature of light. The effect depends on the wavelength (that is, the color). It is most pronounced when bright light from a practically infinite distance passes through narrow slits, causing the light to spread perpendicular to the slits. This spreads a point-like beam of light into a pair of streaks.

Using a small aperture creates slit-like situations at the corners formed by adjacent blades. Thus, when you have a combination of relatively intense, pointlike, monochromatic light sources in the image and a narrow aperture, you should see a streak (of the same color) emanating from the points in two directions perpendicular to the blades…

…Finally, length of exposure is related to the occurrence of this effect, as you have observed, but only because exposures with bright points of light are almost always made much longer than needed to record the lights: you’re trying to see the rest of the scene, which is much darker. The brightness of the diffraction streaks decreases so rapidly away from their sources that if you used a sufficiently short exposure to properly expose the lights themselves, the streaks would be practically invisible.

Aperture ƒ/22
Camera Canon EOS 6D
Focal length 19mm
ISO 100
Shutter speed 15s

Light Trails on MacCurtain Street

Light Trails on MacCurtain Street

Ghostly light from passing cars hangs in the air on the corner of MacCurtain Street and Bridge Street in Cork City, Ireland.

This is the second of three light trails or light streaks photos I’ll post here in an unintentional series of long exposure photos. The first was my Light Trails in Blarney photo published yesterday.

Aperture ƒ/22
Camera Canon EOS 6D
Focal length 17mm
ISO 100
Shutter speed 8s