Back Focus



The Back Focus in security cameras is accomplished by moving the image sensor in relationship with the lens, which enables differing back focal lengths for the lens. This adjustment is called Back Focus and is important when fitting a zoom (varifocal lens).

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Adjusting back focus is not needed onptz cameras. The lens on a PTZ camera is mechanical and they usually have an auto focus feature build in.

A fixed lens, such as a 4mm lens, may need a slight adjustment which can normally be done by just slightly turning the lens one way or the other. In most cases with a fixed lens camera the lens will already have had the back focus set so no further adjustments will be needed.

On standard full body (box cameras) you have either a C or CS mount. The difference between the two is how far away from the sensor the lens is. A lens will be either a C or a CS mount in this type of camera. Most of these cameras come with adapters so that either type of lens can be put on the camera so you will want to verify that the lens and the adapter match. In some cases you will need to adjust the back focus on these cameras. That is typically done by loosening a small screw at the base of the mount where the lens is attached. This will allow for turning the base slightly in order to get a better focus. Typically though you will be able to get a good focus by adjusting the varifocal lens.

Adjusting a varifocal lens is very easy to do. In the image below you will see a fairly typical varifocal lens in a security camera. The top, or adjustment closest to you if your were looking at the lens, will adjust the focus. It is designated by the letter N (for near) and the infinity symbol (for far). You will need to loosen the set screw in order to adjust it. Best practice is typically to do that after you set the focal length. You will loosen the set screw on the focal length which is designated by W and T. (Wide angle and telephoto) Once you have adjusted for the field of view you want you can then make the adjustment for the focus bringing everything into a sharp clear image. Of course you will want to tighten down the set screws so that your settings remain.

Through the years Security Cameras have been manufactured to make the back focus less of a problem. You even have some cameras today that have an Auto Back Focus feature. Either way it is typically very easy to get the camera in focus so that you are capturing the images that you want.

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