Correct camera focus for night time viewing

So the installer setup the surveillance cameras, showed you the CCTV system and everything was as you hoped.

A nice tight image that will identify the suspects perfectly. Well so you thought until you had the night time break in and the pictures were all out of focus

I have heard installers say it is because it is a cheap camera but that is not the case

Have you ever seen this? . There several possible reasons for this occurring.

The first and most likely is because the camera is not actually focused correctly.

A camera is setup in the daytime for 90% of installations but for many applications the action is going to occur at night.
Because during the day we have plenty of light, the cameras lens closes its iris to allow just the right amount of light through. At night the iris will open fully to allow as much light in as possible and this results in a change in the depth of field. The amount of distance the image is in focus for. A day time image uses a small f-stop, which gives a wide depth of field in the order of metres where as a low light scene will have a narrow depth of field due to the larger f-stop of the lens. Often 1 metre or less.

To avoid this problem, when setting up the camera you can either

  • Focus it at night (not too many installers want to do this)
  • Use a neutral density filter (ND filter). This is like putting sunglasses on the camera to reduce the light. In fact sunglasses will work in many cases
  • Increase the camera‚Äôs shutter speed to around 1/5000, which will force the iris to open like night time.

The second is due to an Infra Red cut filter that sites over the image sensor during the day automatically being removed for low light conditions, allowing IR light through.

The filter is part of the optical path and removing it shifts the focus point of the lens.

To avoid this one is a little more costly.

  • Either change the lens to an IR corrected lens
  • Or get a camera with auto back focus on Day/Night switching

Either way you fix it, you should end up with a great image day and night.

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  1. CCTV Training | Lens Focus - September 29, 2011

    […] man accused of stealing a laptop was seen on CCTV footage but due to the camera being out of focus was acquitted. Most of the video surveillance cameras that get installed are setup in normal […]

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