Surveying the scene to select the correct lens for the required viewing angle?

[Phil]

It's important to select the right CCTV lens to cover the scene that you want to monitor. In CCTV terms we call this the field of view, and more specifically we usually talk about the horizontal field of view.

Describing the horizontal field of view in degrees of angle works well but can be hard for us to visualise or interpret.

Here are a couple of tips which you can use when surveying a scene which you wish to monitor with a camera to assess the required field of view in degrees.

30° Tip

1. Stand with you back to the wall at the point where you want to mount the camera and look out at the scene you'd like to cover.
2. Make a V's-up sign with your first and second finger.
3. Turn your hand around so that the back of your hand is now facing you - in a 'Victory-V' sign.
4. Now lay your hand flat so that your fingers point away from you.
5. Raise your hand so that your knuckles are just below your eye and 'sight' over your knuckles looking along the V formed by your two fingers.
6. This horizontal 'V' gives you a pretty close approximation to a 30° horizontal viewing angle.
7. You can of course 'pan' left and right with your new finger sights and work out multiples of 30° - just pick a landmark in your scene and move finger to finger against that point.
8. Of course, you can also approximate half your V to be 15°.

10° Tip

1. Make a fist.
2. Stretch your arm out, with the back of your hand flat and your fist pointing at the scene.
3. If you sight along the top of your arm and over the top of your fist, the distance between the outer edge of your left knuckle and your right knuckle is approximately ten degrees.
4. You can test your body calibration by checking that three of the 10° fists fit nicely into one of your 30° Vs (I can visualise people at home trying this now)

Finally to say - don't try to cover the world with a single camera.
If the scene is wide then the height will be correspondingly tall (4:3 or 16:9 the camera / viewing format is fixed).

Finally, finally - the JVSG lens calculator does a really good job of helping you to work out lens angles and even illustrates the resultant views for you.

Hope that helps, any queries please feel free to ask ...

 

[Stanislav]

See also freeware CCTV Design Lens Calculator . In addition to usual Lens Calculator features this calculator offers several new unique tools:

Calculating camera field of view in 3D, taking into account camera height and tilt angle.
Calculating spatial resolution (pixel density) at any distances or calculating distances of specified spatial resolutions,
Simulating images from cameras using prepared photos or pictures.
Simulating field of view and resolution of megapixel cameras up to hundreds of megapixels using Picture-in-Picture technology.
This free Lens Calculator is based on professional CCTV Design software - VideoCAD.

 

[Alex Smith]

You have to use cctv lens selection tool. It helps you to determine the correct lens for the required viewing angle.