• Craig Colnan

Choosing a Camera Part 1 - Focal Length

This is part one of a series of articles which will help you to understand the basic factors involved in selecting the right camera for your video security requirements.

In this article I will examine the importance of the camera lens focal length, how that affects the image area that the camera sees and why choosing the right lens determines the usefulness of the captured video.

First off, some important terms.

The focal length of a lens is the distance, in millimetres, between the optical centre of the lens and the image sensor when focused at infinity. The optical centre of the lens is the point at which the rays of light, entering the lens, cross.

The Field of View is the area in front of the camera that is captured and is affected by the focal length.

Now, it’s not important to have a technical understanding of focal length when selecting a camera but you do need to understand what different focal lengths will do to the field of view and how this affects what you are trying to achieve with video security.

A typical default focal length on security cameras is 3.6mm. This creates what we refer to as a wide angle view (about 72 degrees), that is, the image will cover a wider area at the expense of magnification detail. This is somewhat similar to what the human eye would see.

At longer focal lengths, the image focus is magnified but with a narrower field of view. Longer focal lengths are used in security cameras when you need to focus the camera on a specific target with a high degree of detail but with a camera that is mounted some distance away. An example might be a camera that focuses on a car park gate where you want to capture number plates from 30 metres away but are less concerned with events or movement leading up to the gate. The issue with using a long focal length is the amount of light that reaches the camera sensor as the camera moves away from the target. That's the focus of a later article. 

The image below shows the effect of different focal lengths on the magnification of the target area. You can see that as the focal length increases, so does the magnification but there is a loss of detail from the scene.

Most security cameras have fixed focal length lenses. That is, the camera is manufactured with the lens that is required to capture the required field of view and this cannot be changed. It’s set and forget so, when ordering fixed lenses, it’s important to understand the image area that you will capture. If you are unsure, check the focal length on your phone or pocket camera, go to the place where you intend to mount the camera and take a photo of the target area. If the result is what you need then you now know what camera lens to order.

For more complex situations, there are also variable focal length cameras. These can be adjusted to any level within the limits specified for the camera. For example, Bluefly’s 5 megapixel varifocal bullet camera can be manually adjusted within a range from 2.8mm through to 12mm. We also have outdoor cameras with focal length ranges from 3.6 to 16mm.

There are other factors which affect the quality or resultant image that can be achieved using video security cameras. In coming weeks I will be publishing articles on several of these factors including aperture and light, the image sensor, and resolution. Together, with considerations such as weatherproof ratings and body type, these factors will contribute to helping you make an informed decision on which camera best suits your individual needs.

If you have any questions or feedback relating to this article of you need help to build your video security solution, please call us or use our contact form to get in touch. We're here to help and we'd love to hear from you. 


© 2019 Bluefly IT Solutions