Setting Up The Back-Focus


A good lens is designed to keep your subject focused throughout the whole of its zoom range. For cameras with detachable lenses however, it is important that the back-focus has been correctly adjusted.

If you zoom in on your subject, focus, then zoom out and you discover that the picture looses sharpness, your back focus has not been properly set up or has been changed during production.

The back-focus setting

focus chart small

The flange focal-length adjustment ring on the zoom lens controls the back-focus lens, which makes sure that your subject stays focussed while zooming. This setting is known as the back-focus setting, and it should be correctly set up when preparing your camera, or when changing lenses.

Correctly setting up the flange focal-length

To correctly adjust the flange focal-length adjustment ring, download the back-focus chart you see on the right. It is called a 'Siemens star', or more commonly the back-focus chart. Print it out on normal paper and pin in to a wall somewhere in your house.

The procedure to set up the back-focus is as follows:

  • Step 1: put your camera on a tripod and point it camera directly on a focus chart, about 10 meters away. Make sure the iris is fully open so the depth of field is minimized, and focusing will be more precise.
  • Step 2: zoom in so you see the center of the chart in your viewfinder. Focus the picture as best as possible while looking through the viewfinder.
  • Step 3: zoom out again and check if the picture is still in focus. If the picture is still sharp, your flange focal-length adjustment ring is set up properly, you are done.
  • If the chart is out of focus, correct focus by adjusting the flange focal-length adjustment ring and repeat steps 2 and 3 until the picture stays sharp in the complete zoom range.

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About the author
Joseph de Meij

I am Joseph de Meij, 25 years old and I am a freelance cameraman who loves to travel. I'm currently trying to find different ways to overcome my toxic career syndrome and find out what my goals are in life.

This article was first published at www.askthecameraman.net.

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