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Lens format is the focal plane size of the lens. You can use 1/2" format lens for smaller image sensors, such as 1/2.7" or 1/3" or 1/4", but not the other way around. Or else, you will end up seeing part of the lens barrel shadows at the corners of the image especially in wide angle mode.
[M, L]Mega pixel lens / Lens resolution
Optics play a very important role in cameras. And good lenses are critical to build a good surveillance system. When we talk about resolutions, we often talk about image sensor resolutions but not the one for the lenses. Your image is going to be only as good and sharp as the capability of the lens that you use in your camera. Mega pixel lenses can "resolve" mega pixel resolutions on its focal plane.
You can use analog camera lenses on mega pixel cameras. You will see the images, but they will never be mega pixel sharp, but blurry no matter how hard you want to focus it because the lens you are using can not resolve more than about D1 resolution, not mega pixel resolutions.
Infrared LEDs are used to illuminate low light scenes. Typically they are 850nm (visible to most human), and non visible 920nm.
If you are using IR illumination in your scene, make sure you use an IR corrected lens on your camera, if not you will experience focus drift if your lens is not IR corrected and when only the IR LED lights are used. IR corrected lens allows the IR lights to focus on the same focal plane as the visible lights.
[D]Digital Day and Night
Instead of ICR, it uses a IR pass filter which passes the visible spectrum of lights as well. It works great in artificial light conditions, such as indoor applications, but when you have light sources that emits IR spectrum, such as incandescent light blurs, halogen or sun light, the color can be slightly distorted.
Because it is less complex to build the camera without ICR, it is more affordable than True Day and Night cameras.
[I]ICR (Infrared Cut-filter Removal)
Infrared Cut-filter Removal is a mechanism used in True Day and Night cameras. Camera sensors are sensitive to a wider spectrum of lights the spectrum of visible lights. So it can see what you can not see. During the day time, it puts the IR cut filter between the lens and the image sensor in True Day and Night cameras. Without this filter, the sensors will see the non visible lights, and may distort the colors on the scene or blur the scene. Also, Frequently Infrared LED lighting is used to assist to low light conditions. So the IR cut filter is removed during the night time so that the IR LED light source can be used to sense the scene in IR spectrum. Because human eyes can not see the IR, the scene is rendered in black and white.