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