How to match the camera lens every day?

Speaking of the focal length of the lens, everyone may have heard of it, but ask another question: Do you know how to count it? do not know? It doesn't matter, Xiaobian teaches you to get new skills quickly!

Focal length refers to the distance from the optical center of the lens to the focus of the light when the parallel light is incident. That is, the distance from the center of the lens to the imaging plane such as the film or CCD. Simply put, the focal length is the distance between the focus and the center point of the mirror.

Know the focal length, it is not much to say, directly on the hard goods:

Formula 1: f=wL/W

Formula 2: f=HL/h

Where f is the focal length of the lens, w is the width of the image (the image is imaged on the ccd target surface), h is the image height (the image is imaged on the ccd target surface) field of view (intake scene) Height, W is the width of the subject, L is the distance from the subject to the lens, and H is the height of the subject.

CCD target surface size unit: mm

Ok, now let's take an example. Use a 1/3" CCD camera to view a 10cm wide 30cm wide license plate (requires license plate display to occupy 1/2 of the monitor screen width), which requires a lens with a large focal length. ?

By the formula: f=wL/W

Therefore: focal length f = 4.8mm × 10 × 1000 × 1/2 / 300M = 80mm

An optical system with a short focal length has better ability to concentrate light than a long focal length optical system. However, when the focal length value is calculated, there is often a lens without a corresponding focal length. Don't worry, the more common ones are: 8mm, 15mm, 24mm, 28mm, 35mm, 50mm, 85mm, 105mm, 135mm, 200mm, 400mm, 600mm, 1200mm, etc. If there is no corresponding focal length lens is very normal, you can choose the most similar model according to the product catalog, generally choose a smaller than the calculated value. This perspective will be larger.