What does a camera’s zoom lens specification really mean and how do I know if it will work for my application?
I am frequently asked to calculate if a zoom lens will work for a specific application. Perhaps someone is shooting from the back of a sanctuary and looking to project a life-sized image of a worship leader in an overflow room. Or, maybe they want to make sure they can get a close-up on musicians’ hands.
So, how do you figure out how much zoom is required?
Let’s use the example of making a worship leader look life-size in an overflow room to help figure this out. Given a shooting distance of 150’, if we are trying to make a 6’ tall person appear 6’ tall on a 9’ screen, we are shooting the entire 9’ high field, of which the worship leader is occupying 6’ (the target size.)
We then need the manufacturers spec sheet for the proposed camera and a lens calculator. The spec sheet we’ll look at in this example is the Panasonic AW-HE120 PTZ camera.
- Sensor Size. This is the size of the device in the camera that turns light into an electronic video signal. In this spec sheet, we see this camera is a 1/3” sensor with Full-HD resolution.
- Shortest Focal Length. Focal length is the distance from the lens to the sensor. The AW-HE120’s shortest focal length is 4.5mm
- Zoom. Zoom is created by changing the focal length; increasing the distance between the lens and the sensor. Sometimes this is expressed in terms of a range of available focal lengths or as a multiplier of the shortest focal length. Increasing the focal length reduces the field of view (how wide of angle you can see) thereby increasing the apparent size of the image. The AW-HE120 shows the range of 4.5 to 90mm. 90mm is twenty times longer than 4.5mm. So this camera has a 20X Zoom.
Armed with the above information, you can use an online lens calculator, such as one provided by Fujinon.
Under the “field of view” tab, enter the subject distance (150’), camera format (1/3” JVC/Panasonic/Sony), Field of view (9’ high). If you only know the height, it will automatically calculate the width and vise versa. Press “calculate” and it shows a required focal length of 48.9mm. This is well within the 4.5-90mm zoom range of the camera.
This same process can be used in most any scenario by plugging in the appropriate variables. Be aware, that the more an image is zoomed in, the more small structural and other vibrations appear as large vibrations in an image. In these cases, proper isolation of the mounting becomes important.
While there are other factors beyond zoom range that determine if a camera is suitable for an application, the preceding information should help address one of the basic criteria.