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Glossary: Zoom Lens

Varifocal Lens

There are many IP cameras available today which have a zoom lens and this allows the camera to enlarge areas of a scene so that more detail can be seen from distant objects. This is not to be confused with digital zoom, which is another feature entirely.

Technically, the way that the lens achieves a level of magnification is by adjusting its focal length. On zoom lenses this can be done mechanically using the camera’s controls so images can be zoomed from a remote computer once the camera has been fixed in position. Mechanical zoom cameras will usually have an auto-focus feature to allow it to refocus once it has been moved. The other type of zoom lens is a varifocal lens and these in general have a fixed, manual zoom as they are normally found on static cameras. Once a varifocal lens has been set to the required zoom (and manually focussed) it cannot be adjusted without physically adjusting it at the camera.

Focal Length Vs Viewing Angle

Simply put, the viewing angle of the camera will become narrower when you zoom-in. This corresponds to the horizontal and vertical viewing angles. As you zoom-in (telephoto) you will start to narrow your field of view as you magnify one particular area of interest. It is important to check, especially with fixed varifocal lens cameras, that the lens will give you the required shot before you purchase.

Zoom Ratio

Most zoom lenses present their zoom capabilities by using a number such as 3x or 10x etc. This is known as the zoom ratio. It is determined by calculating the ratio between the cameras focal length at the telephoto (zoomed-in) position and the focal length at the cameras wide-angle (zoomed-out) position. IP cameras with varifocal lenses will typically have a zoom ratio of about 3x but some of the high-end PTZ dome cameras can have as much as 30x/35x zoom giving them the ability to capture distant subjects in extremely high detail.

Published on August 5th, 2008 by Greg

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