CCTV BASICS: LENS PART 3

Understanding lens

From Part 1 and Part 2 you learned how to choose focal length, how to check sensor size and choose proper lens for each sensor and resolution of the camera. 

Now it's time for another part - aperture of the lens. 

It's all about the light

In optics, an aperture is a hole or an opening through which light travels. To be more specific, the aperture and focal length of CCTV lens (or any lens actually) determine the cone angle of a bundle of light rays that come to a focus on a sensor. 

[1] How light rays travel without "collimator" (aperture)

[1] How light rays are "arranged" to focus image on a sensor using aperture.

So the bigger the hole - the more light gets to the camera sensor, the size of the aperture is described by "F" Number. In photography it's written as (for example) "f/2.0", in CCTV more common is this format "F2.0"

[2] Aperture in lens - hole size

Confusing part - bigger the F, smaller the hole.

[2] Aperture size - bigger "F", smaller hole

The "F" number is the ratio of the diameter of the aperture to the focal length of the lens. For example, if we had a lens that had a focal length of 50mm, and the aperture had a diameter of 25mm, then the ratio would be 1/2. The F value would be F2,0.

Same as 100mm lens with 50mm aperture. 

Fun fakt: The pupil of the eye is its aperture in optics nomenclature,  the iris is the diaphragm that serves as the aperture stop. The entrance pupil is typically about 4 mm in diameter,

although it can range from 2 mm (F8.3) in a brightly lit place to 8 mm (F2.1) in the dark. 

So how much light hits the sensor for each aperture size? Here is quick table that will show you, that every time we jump one F-Stop higher, we receive 50% of light. 

Most common apertures and amount of light they pass

What if you have two bullet cameras, one with F1.4 lens and other with F1.2. We know that F1.2 will provide you better low light image, but how much better it is comparing to F1.4?

Here you go! Another Excel calculator that will help you :) 

We need to go deeper

So we want the smallest possible number, right? 

Well, sometimes yes.... but... 

If an aperture is narrow (small hole), then highly collimated rays are admitted, resulting in a sharp focus at the image plane - bigger field of view is sharp and focused, but image is "darker" because of limited light getting to the sensor.

A wide aperture admits uncollimated rays, resulting in a sharp focus only on small region of an image - but the sensor gets more light.
This is called "depth of field" 

[2] Depht of field

So, big aperture is not always a good thing. If you need to secure long corridor you should think twice before you use F1.2 lens on, a camera.

Vari-focal lens

Also remember, when you have vari-focal lens, and you check aperture value in datasheet - this is the value for widest angle! So for example in PTZ camera, with 30x zoom you have F1.6 on the begging, but as you zoom in, you can get F6! And thats only 7% of light you started! So make sure that you use proper IR iluminator with this PTZ. or there is plenty of light on scene. 

Aperture in Dahua 30x zoom PTZ camera

That's it, for now.
You know the basics. Of course this is only tip of the ice berg. There are still some other thing that you need to consider - constant aperture lenses, T-value of lens, distortion, aberration, etc. but now you know the basics of basic stuff.
If you have any questions, something is not clear - ask, leave a comment. Here or on my facebook page:
www.facebook.com/CCTVnerd

[1] https://en.wikipedia.org/wiki/Collimated_light
[2] https://en.wikipedia.org/wiki/Aperture