Focus Testing

Reader query: Would you please be able to tell me where I can get an SLR camera/lens tested for optical quality. I am having focus problems and need to know if the problem is the equipment or me – while there is still some warranty.

Margaret’s reply: I’m not aware of any specialist testing facilities for camera lenses in Australia. You can conduct some basic tests yourself if you have a suitable target.

The focus target should cover an area that is larger than the camera’s central focusing point. It should also be perfectly flat and parallel to the camera’s focal plane. Lighting should be bright and even (outside in the shade during the day is best). Follow these steps:
1. Mount the camera on a sturdy tripod. Camera-to-subject distance should be no less than 50 times the focal length of the lens, ie: for a 50 mm lens, that would be at least 2.5 meters.
2. Set the camera to its default ISO (ISO 100 or ISO 200).
3. Use aperture priority AE and set the lens to its maximum (widest) aperture.
4. Set the camera to single-shot autofocus.
5. Select the centre focusing point.
6. If the lens or body has a stabiliser, turn it off.
7. Trigger the shutter with a remote control or the camera’s self-timer.
8. Before each exposure, manually set the focus on the lens to infinity before allowing the camera to autofocus the target. Make at least three exposures of the target.
9. Examine the resulting set of images on your computer monitor at 100% pixel magnification.

If you’re interested in finding out how much the focus is out of whack (assuming it is), repeat the above with the test target tilted slightly; first with the top towards the camera and then with the top tilted away from the camera.

If you detect anomalies, return the equipment to the shop you purchased it from, along with prints showing the results of your tests.

If you don’t have facilities that allow you to go through these steps, you could try consulting a camera repair specialist (you’ll find them listed in the phone directory). They have optical test benches and should be able to quickly tell you whether the camera is performing correctly. (Expect to be charged for this service.)